Balanced and Unbalanced Forces: a full term of science

Ten ready-to-teach lessons for Year 7 Physical sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards, a results sheet or exit tickets, an assessment kit and worked answers.

Start here: five minutes to Monday

1. Skim the term at a glance on the next page.
2. Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards, a results sheet or tickets.
3. Gather the few items under “You need” on the plan. Most come from the science prep room.
4. Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
5. Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.

For any Year 7 science teacher

The pack suits a science specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: a force is a push or a pull with a size and a direction. When the forces on an object balance, its motion does not change: it stays still, or keeps moving at a steady speed in a straight line. When they do not, the net force changes the motion, and the more mass an object has, the smaller the change from the same force. Gravity pulls every object towards the centre of the Earth; that pull is its weight.

Two ways to run each lesson

Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 4 needs old CDs and balloons, Lesson 6 needs a shoe for each group (old ones from home are ideal), and Lessons 7 and 8 need trolleys, or toy cars, and a ramp.

Safety and care in one look

Force meters are not stretched past their scale or let go while stretched. Masses hang over a tray, never over feet. Trolleys run along the bench into a barrier, with someone ready to catch them. Check for latex allergies before the balloon lesson. Parachutes are dropped by a student standing on the floor, never on a chair or a table.

The term at a glance

One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.

How the sequence builds

Lessons 1 to 3 build the tools: measuring forces in newtons, drawing them as arrows, and adding them to a net force. Lesson 4 uses them to explain why things keep moving, and Lessons 5 and 6 study two forces that act on almost everything: gravity and friction. Lesson 7 relates the change in motion to mass, Lessons 8 and 9 apply it to seatbelts and parachutes, and Lesson 10 brings it together.

Curriculum links (Australian Curriculum V9)

The whole term teaches the Science Understanding descriptor AC9S7U04 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:

Assessment in this pack

• Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 7 voice, with working.
• The assessment sheet near the front has a class observation checklist and a three-level rubric.
• Lesson 10 is the summative pair: the forces through a bike ride, plus the “Show what we know” check.

Materials for the whole term

One gathering session covers all ten lessons. Most of the kit is in a science prep room: force meters, masses and trolleys. A few things are worth collecting ahead: old CDs and pop-top lids for Lesson 4, an old shoe for each group for Lesson 6, and plastic shopping bags for Lesson 9.

The one-trip list

• From the prep room: force meters (0 to 10 N, 0 to 20 N and 0 to 50 N), hooks with 100 g masses, 0.5 kg and 1 kg masses, trolleys, pulleys and stopwatches.
• From the classroom: rulers, metre rulers, masking tape, sticky tape, scissors, string, graph paper and plasticine.
• To collect: old CDs, pop-top drink-bottle lids, balloons, an old shoe per group, plastic shopping bags and metal washers.
• From the hardware shop: a sheet of sandpaper, a plastic sheet and a carpet offcut per group.

Safety and care in one look

• Force meters are not stretched past the end of their scale, and are never let go while stretched.
• Masses hang over a tray, never over feet; trolleys run along the bench into a barrier, with someone ready to catch them.
• Check for latex allergies before Lesson 4; glue is used by the teacher or set the day before.
• Parachutes are dropped by a student standing on the floor, never on a chair or a table.

Assessment without extra work

The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the bike-ride task, in which students draw and explain the forces at each stage of a ride, and the final check. This sheet is the place to note what you see along the way.

The three levels

Class observation checklist

A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.

Word wall cards

Cut out the cards and build the wall as the words arrive: force, newton and non-contact force in Lesson 1; force diagram and tension in Lesson 2; net force, balanced and unbalanced in Lesson 3; inertia in Lesson 4; mass, weight and gravity in Lesson 5; friction in Lesson 6; acceleration in Lesson 7; air resistance in Lesson 9.

Dear families

This term in science, Year 7 studies forces: the pushes and pulls that start things moving, stop them, and change their speed or direction. Students learn to measure forces in newtons, draw them as arrows, and add them up to find out whether they balance. They find out why the same push moves a light object more than a heavy one, and how gravity and friction act on almost everything around us.

Along the way your student will build a balloon hovercraft from an old CD, work out what they would weigh on the Moon and on Jupiter, test the grip of a shoe on four surfaces, crash a trolley carrying a plasticine passenger with and without a seatbelt, and design a parachute.

Try this at home

• Push a shopping trolley when it is empty and when it is full. Which is harder to get moving, and to stop?
• On a bus or in a car, notice which way you lean when it brakes or turns. Why?
• Drop a flat sheet of paper and a scrunched-up sheet at the same time. Which lands first, and why?
• Find three places where friction helps you, and one where it gets in the way.

What to ask your scientist

• What is the difference between mass and weight?
• Why do we need seatbelts?
• Why does a parachute fall slowly?
• What does “balanced forces” mean?

A small note on care: in the trolley lessons only plasticine passengers crash, and in the parachute lesson students drop their parachutes while standing on the floor.

Warm regards,

The Year 7 science team

Printed from the free seegongsik Balanced and Unbalanced Forces teaching pack · seegongsik.com/au/y7/physical/AC9S7U04/pack

Pushes, pulls and newtons

A force is a push or a pull. Every force has a size, measured in newtons (N), and a direction. Contact forces act where things touch: a push, the pull of a rope (tension), friction, the support from the floor, the push of the air. Non-contact forces act across a gap: gravity, magnetism, and the pull of a rubbed balloon. Groups measure the forces in five everyday jobs with force meters, draw them as arrows, and sort twelve forces into contact and non-contact.

We are learning to

• use a force meter to measure a force in newtons,
• tell contact forces from non-contact forces,
• show a force as an arrow with a size and a direction.

Success criteria

• I can read a force meter to the nearest half newton, using the meter that suits the job.
• I can sort a force as contact or non-contact, and draw it as an arrow to scale.

You need

• for each group: two force meters (0 to 10 N and 0 to 50 N), a pencil case, a 1 L bottle of water and a school bag,
• the worksheet (next sheet), one per student, and the force cards (third sheet), one set per pair.

Safety: never stretch a force meter past the end of its scale, and never let it go while it is stretched.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the meters and the five jobs in Session A, and the board, the sort and Part D in Session B.

Watch for these ideas

• “Force is measured in kilograms.” Kilograms measure mass, the amount of matter; force is measured in newtons.
• “Only living things push or pull.” The floor pushes up on a desk, and a rope pulls a sled.
• “Gravity needs air to work.” Non-contact forces act across empty space: gravity holds the Moon in its orbit.

Make it easier, make it bigger

• Easier: measure three jobs, and sort the cards before the measuring.
• Bigger: explain why the 0 to 50 N meter is a poor choice for the pencil case, even though it can hold it.

Answers and look-fors

• Part A, example class: lift a pencil case 3 N; lift a 1 L bottle of water 10 N; pull open the classroom door 8 N; drag a school bag across the floor 15 N; pull open a desk drawer 6 N. Every job is a pull through the force meter, a contact force.
• Part B: arrows at 1 cm for every 2 N, for example 10 N as 5 cm, each pointing the way the force acts and labelled with its size.
• Part C: contact A, C, D, F, H, I, K; non-contact B, E, G, J, L.
• Part D: 1. The bottle’s weight, the pull of gravity on it: about 10 N. 2. Kilograms measure mass, the amount of matter in something; a force is a push or a pull, measured in newtons.
• Bigger: its scale is spread over 50 N, so a 3 N reading moves the pointer only a little and cannot be read as precisely.

Pushes, pulls and newtons

A. Five jobs

B. Arrows to scale

C. Contact or non-contact?

D. Questions

Contact or non-contact?

Cut out the cards. Lay down the two grey heading cards and sort each force.

Teacher note: 7 cards are contact forces and 5 are non-contact.

Force diagrams

A force diagram shows an object as a box with one arrow for each force on it: drawn from the object, pointing the way the force acts, its length to scale and labelled with its size. Things at rest still have forces on them. A book on a table is pulled down by its weight and pushed up by the table (the support force), and the two balance. Students measure weight and tension with a force meter, draw force diagrams to scale for four things at rest, and see the push of a surface in a bending ruler.

We are learning to

• draw force diagrams with labelled arrows to scale,
• identify the forces on things at rest and show that they balance,
• measure weight and tension with a force meter.

Success criteria

• My force diagrams have one labelled arrow for each force, drawn from the object and to scale.
• I can explain, with two forces, why a book on a table does not fall.

You need

• for each group: a force meter, a hook with 100 g masses, two books and a 30 cm plastic ruler; rulers and sharp pencils,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Safety: hang the masses over the desk, not over feet, and add them one at a time.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the meter, the board and the worked example in Session A, and the four diagrams, the ruler and the ticket in Session B.

Watch for these ideas

• “A book on a table has no forces on it.” Two forces act on it, and they balance.
• “A table cannot push; it is not alive.” A surface pushes back when it is squashed or bent, just like the ruler.
• Arrows drawn from the hand or the floor. Every arrow starts on the object the force acts on.

Make it easier, make it bigger

• Easier: draw the book and the school bag only, both at 1 cm for every 10 N.
• Bigger: you press down on the 2 kg book with 15 N. Draw its force diagram now.

Answers and look-fors

• Part A, example class: about 2.9 N. The mass is still, so the meter’s pull up (tension) balances its weight down.
• Part B: a 2 kg book on a table: weight 20 N down and support 20 N up, each 2 cm at 1 cm for every 10 N; a 5 kg school bag hanging from a hook: weight 50 N down and tension 50 N up, each 5 cm at 1 cm for every 10 N; a 300 kg boat floating on a lake: weight 3000 N down and buoyancy 3000 N up, each 3 cm at 1 cm for every 1000 N; a 60 kg student standing on the floor: weight 600 N down and support 600 N up, each 3 cm at 1 cm for every 200 N.
• Part C: 1. It bends down, more with more mass. 2. The bent ruler pushes back up on the mass, like a squashed spring. 3. A table bends too, far too little to see, and pushes up with a support force equal to the weight; the forces balance, so the book stays still.
• Bigger: weight 20 N down and your push 15 N down, so the table pushes up with 35 N.
• Exit ticket: 1. About 30 N down (weight) and 30 N up (support), the same length. 2. Tension, in the cord. 3. The forces on it balance, so its motion does not change.

Force diagrams

A. Weight and tension

B. Four things at rest

C. The bending ruler

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Net force

When several forces act on one object, we add them, taking direction into account: forces the same way add, and opposite forces take away from each other. What is left is the net force. If it is zero, the forces are balanced and the motion does not change. If it is not, the forces are unbalanced and the object speeds up, slows down or changes direction, the way the net force points. Groups pull a trolley two ways with force meters, add the forces on twelve diagram cards, and sort the cards into balanced and unbalanced.

We are learning to

• add forces along a line to find the net force and its direction,
• decide whether the forces on an object are balanced or unbalanced,
• predict how the net force will change an object’s motion.

Success criteria

• I can find the size and the direction of the net force on any of the cards.
• I can explain how a balanced object can still be moving.

You need

• for each group: two force meters and a trolley,
• the worksheet (next sheet), one per student, and the diagram cards (third sheet), one set per pair.

Safety: pull the meters gently along the bench, with the trolley kept away from the edge.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the trolley tug and the board in Session A, and the twelve diagrams and Part C in Session B.

Watch for these ideas

• “The biggest force wins, so the others do not matter.” Every force counts; the net force is what is left after adding them all.
• “Balanced means not moving.” A balanced object can be moving at a steady speed, like the cards with the trolley, the boat and the parachutist.
• Adding an upward force to a sideways one. Only forces along the same line add or cancel directly.

Make it easier, make it bigger

• Easier: start with the cards that have only two forces, and leave cards D, F, K until last.
• Bigger: the lift on card H has a mass of 800 kg. Use a = F ÷ m to find how quickly its speed changes.

Answers and look-fors

• Part A, example tries: 4 and 4: zero, the trolley stays still; 3 and 5: 2 N right, the trolley moves right; 6 and 2: 4 N left, the trolley moves left.
• Part B: A 50 N right, B zero, C 60 N right, D 2 N left, E zero, F 400 N right, G zero, H 1000 N up, I 25 N left, J zero, K 150 N left, L zero. Balanced: B, E, G, J, L; unbalanced: A, C, D, F, H, I, K.
• Part C: 1. Add the forces the same way, then take away the force the other way: 200 + 300 − 100 = 400 N to the right. 2. The only force points against its motion, so the ball slows down. 3. Balanced forces do not stop motion; they keep it the same, so the parachutist falls at a steady speed.
• Bigger: 1000 ÷ 800 = 1.25 m/s², upwards.

Net force

A. Trolley tug

No trolley? Use the example tries: 4 N left and 4 N right; 3 N left and 5 N right; 6 N left and 2 N right.

B. Twelve diagrams

In each box write the net force with its direction (or zero), then B for balanced or U for unbalanced.

C. Questions

Balanced or unbalanced?

Cut out the cards. Lay down the two grey heading cards, find the net force on each card, and sort it.

Teacher note: 5 cards are balanced and 7 unbalanced.

Keep on moving

With no net force, a moving object keeps moving at the same speed in a straight line, and a still object stays still. This is inertia, described in Isaac Newton’s first law of motion. On Earth, moving things usually slow down because friction and air resistance act against them; take those away and they keep going. A balloon hovercraft floats on a thin cushion of air, so the friction almost vanishes and it glides a long way. The space probe Voyager 1, launched in 1977, has coasted out beyond the planets for decades with no engine pushing it along, because in space almost nothing slows it down.

We are learning to

• explain inertia: a still object stays still and a moving object keeps moving unless a net force acts,
• explain why moving things on Earth usually slow down,
• compare results to judge the effect of removing friction.

Success criteria

• I can explain the coin-and-card trick with the word inertia.
• I can use the glide results to explain why the hovercraft keeps moving for so long.

You need

• for each group: an old CD with a pop-top drink-bottle lid glued over its hole (glue the day before), a balloon, a plastic cup, a playing card and a coin; a smooth floor or a long table; a metre ruler,
• the worksheet (next sheet), one per student, and the build sheet (third sheet), one per group.

Safety: check for latex allergies first; blow balloons up only partly and never share them.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the coin and card and build the hovercraft in Session A; test it, show the board and explain in Session B.

Watch for these ideas

• “A moving thing needs a force to keep it moving.” A force is needed only to change the motion; on Earth a push keeps a trolley going only because it has to cancel friction.
• “Things naturally slow down and stop.” They slow down because friction and air resistance act on them.
• “The card pulled the coin down into the cup.” The card was pulled away too fast to drag the coin; with nothing under it, gravity pulled it down.

Make it easier, make it bigger

• Easier: run the hovercraft as a class demonstration, and fill Part B together.
• Bigger: Voyager 1 travels at about 17 km every second. Is any force still acting on it? What does that force do to its speed?

Answers and look-fors

• Part A: the coin drops into the cup. It has inertia: it stays still unless a net force acts, and the card slid away too quickly for friction to drag it along.
• Part B, example class: a CD on its own 30 cm; a CD hovercraft with the balloon empty 35 cm; a CD hovercraft with the balloon blowing 240 cm. With the air cushion the glide was about 8 times as long.
• Part C: 1. The air cushion keeps the CD off the floor, so there is almost no friction; with almost no net force against it, its speed falls very slowly. 2. A small friction or air-resistance arrow backwards; the weight down balanced by the push of the air up. 3. In space there is almost no air or friction to act against Voyager 1, so with no net force it keeps moving.
• Bigger: yes, the Sun’s gravity still pulls back on it, slowing it very gradually.

Keep on moving

A. The coin and the card

B. How far does it glide?

C. Explain it

Build a balloon hovercraft

Steps

1. Check the pop-top lid is glued firmly over the hole in the middle of the CD, with no gaps, and push the lid closed.
2. Blow up the balloon partly, twist its neck so no air escapes, and stretch the neck over the closed lid.
3. Set the CD flat on a smooth floor or a long table, then pull the lid open.
4. Give the CD one gentle flick and measure how far it glides, from where it started to where it stops.
5. Do the same flick with the CD on its own and with the balloon empty. Try to make every flick the same.

The coin and the card

1. Lay the playing card over the top of the plastic cup and put the coin in the middle of the card.
2. Flick the edge of the card sharply sideways with a finger, and watch the coin.
3. Now push the card away slowly instead. What changes?

Check for latex allergies before handing out balloons. Do not share balloons.

Gravity and weight

Gravity pulls every object towards the centre of the Earth, and the size of that pull is the object’s weight, a force measured in newtons. Mass is the amount of matter, measured in kilograms, and it does not change from place to place; weight does. Near the Earth’s surface each kilogram weighs about 9.8 N. On the Moon gravity is about a sixth as strong, so the same mass weighs much less. Groups graph weight against mass for hanging masses, find the pull on each kilogram, and work out what a student would weigh on the Moon, Mars and Jupiter.

We are learning to

• tell mass from weight,
• graph weight against mass and describe the relationship,
• calculate weight from mass on different worlds.

Success criteria

• My graph has labelled axes, a sensible scale and a line of best fit, and I can find the weight of each kilogram from it.
• I can calculate a weight from a mass, and explain why the mass stays the same on the Moon.

You need

• for each group: a force meter (0 to 10 N), a hook and five 100 g masses, a tray; rulers,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Safety: hang the masses over the tray, adding one at a time, and keep feet clear.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Hang, read and graph in Session A, and do the board, other worlds and the ticket in Session B.

Watch for these ideas

• “Mass and weight are the same thing.” Mass, in kilograms, is the amount of matter and stays the same; weight, in newtons, is the pull of gravity on it.
• “Heavier things fall faster.” Without air in the way, everything falls together: on the Moon a hammer and a feather landed at the same moment.
• “There is no gravity on the Moon.” The Moon pulls with about a sixth of the Earth’s gravity; that is why astronauts could walk there.

Make it easier, make it bigger

• Easier: use 10 N for each kilogram in Part C.
• Bigger: on which world would a 50 kg student weigh about the same as a 20 kg dog does on Earth?

Answers and look-fors

• Part A, example class: 0.1 kg 1.0 N; 0.2 kg 2.0 N; 0.3 kg 2.9 N; 0.4 kg 3.9 N; 0.5 kg 4.9 N.
• Part B: a straight line through zero; weight per kilogram = 4.9 ÷ 0.5 = about 9.8 N.
• Part C, a 50 kg student: Earth 50 × 9.8 = 490 N; Moon 50 × 1.6 = 80 N; Mars 50 × 3.7 = 185 N; Jupiter 50 × 24.8 = 1240 N. The mass is 50 kg everywhere.
• Part D: 1. Weight is in proportion to mass: double the mass, double the weight. 2. Still 80 kg; the amount of matter does not change, only the pull on it. 3. On the Moon there is no air to push back on the feather, so the only force on each is gravity, which makes every object fall the same way.
• Bigger: the dog weighs 20 × 9.8 = 196 N on Earth, so Mars (185 N) is closest.
• Exit ticket: 1. 5 × 9.8 = 49 N. 2. Still 5 kg. 3. There is no air on the Moon, so nothing slows the feather down.

Gravity and weight

A. Hang and read

B. Graph it

C. Weight on other worlds

D. Questions

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Friction

Friction acts where two surfaces rub, against the way they slide. It depends on how rough the surfaces are and how hard they press together. To measure it, pull an object at a steady speed with a force meter: at a steady speed the forces are balanced, so the pull equals the friction. Groups drag a shoe loaded with a 1 kg mass across four surfaces in a fair test, average three tries on each, and graph the results. Friction can help, as in brakes and grip, or be a nuisance, wearing parts out and wasting energy.

We are learning to

• measure friction by pulling at a steady speed,
• plan and run a fair test, repeating and averaging,
• explain when friction is useful and when it is a problem.

Success criteria

• I can name the variable we changed, the one we measured and three we kept the same.
• I can explain why the pull at a steady speed equals the friction.

You need

• for each group: a force meter (0 to 20 N), a shoe with a 1 kg mass inside, and four surfaces: the desk top, a sheet of sandpaper, a plastic sheet and a carpet square, each taped flat,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Plan and test in Session A, and average, graph and explain in Session B.

Watch for these ideas

• “Friction only acts on moving things.” Friction also stops things starting to slide, like the shoe before the meter overcomes it.
• “Smooth surfaces have no friction.” Every real surface has some; ice and oil reduce it a lot.
• Reading the meter while the shoe speeds up. The reading equals the friction only at a steady speed, when the forces balance.

Make it easier, make it bigger

• Easier: test two surfaces, the sandpaper and the plastic sheet.
• Bigger: predict what will happen to the friction on one surface if you add a second 1 kg mass, then test it.

Answers and look-fors

• Part A: changed, the surface; measured, the pull at a steady speed (the friction); kept the same, the shoe, the load, the force meter, the direction of the pull and the speed.
• Part B, example class averages: desk top 4.7 N; sandpaper 10.7 N; plastic sheet 2.3 N; carpet 7.0 N.
• Part C: four bars, from most friction to least: sandpaper, carpet, desk top, plastic sheet.
• Part D: 1. At a steady speed the forces are balanced, so the pull is exactly as big as the friction. 2. Sandpaper: its rough grains catch on the sole. 3. Wanted, for example brakes, shoe soles and car tyres; a problem, for example wear in bike chains and engines, which are oiled to reduce it.
• Bigger: the friction roughly doubles, because the shoe presses twice as hard on the surface.
• Exit ticket: 1. At a steady speed the pull equals the friction. 2. Plastic sheet. 3. Any sensible pair, such as grip on a running track and wear on a skateboard’s wheels.

Friction

A. Plan the fair test

B. Results

C. Bar graph of the averages

D. Questions

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Mass and acceleration

An unbalanced force changes an object’s motion, and how quickly depends on its mass. With the same net force, twice the mass gives half the acceleration: acceleration = force ÷ mass, or a = F ÷ m, with the force in newtons, the mass in kilograms and the acceleration in metres per second, every second (m/s²). Groups pull a trolley with the same falling mass, add load, and time how long it takes to travel 1 m from rest.

We are learning to

• describe how the mass of an object changes the effect of the same force,
• use a = F ÷ m to calculate an acceleration,
• time a motion carefully, repeat it and average.

Success criteria

• I can use our results to describe how the time changed as the mass went up, and say what stayed the same.
• I can calculate an acceleration from a force and a mass, with units.

You need

• for each group: a trolley (about 0.5 kg), two 0.5 kg masses, string, a 100 g hanging mass, a pulley clamped to the bench edge (or a smooth edge), masking tape, a metre ruler, a stopwatch and a cushion or tray under the falling mass,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Safety: one student catches the trolley before the pulley, and the hanging mass falls only a short way onto the cushion.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the board and the trolley runs in Session A, and the pattern, Part C and the ticket in Session B.

Watch for these ideas

• “A heavier trolley needs a force just to keep it moving.” A force is needed to change its motion; more mass means a smaller change from the same force.
• “If it is heavy enough, the force does nothing.” Any net force gives some acceleration, however large the mass.
• “The trolley moves at a steady speed.” It speeds up the whole way, because the net force keeps acting.

Make it easier, make it bigger

• Easier: time two loads only, empty and 1 kg.
• Bigger: the moving mass is the trolley, its load and the hanging mass, and the pull is about 1 N. Use a = F ÷ m to predict the acceleration for each load, and check it against the times.

Answers and look-fors

• Part A, example class averages: no load 1.10 s; 0.5 kg load 1.50 s; 1 kg load 1.81 s.
• Part B: 1. The more mass, the longer it took to travel 1 m, so the smaller the acceleration. 2. The pulling force (the same 100 g hanging mass), the distance and the trolley. 3. The same force shared over more mass gives a smaller acceleration: a = F ÷ m.
• Part C: 2 ÷ 1 = 2 m/s²; 2 ÷ 2 = 1 m/s²; 6 ÷ 3 = 2 m/s²; 10 ÷ 4 = 2.5 m/s².
• Bigger: the pull is 0.1 × 9.8 = about 0.98 N; predicted 1.63 m/s² for 0.6 kg, 0.89 m/s² for 1.1 kg, 0.61 m/s² for 1.6 kg. From the times (a = 2 × distance ÷ time²) the example class got 1.65, 0.89, 0.61 m/s²: a close match.
• Exit ticket: 1. 6 ÷ 2 = 3 m/s². 2. 6 ÷ 4 = 1.5 m/s². 3. The same force is shared over more mass, so it speeds up more slowly.

Mass and acceleration

A. Trolley runs

The same 100 g hanging mass pulls the trolley every time. Time from rest to the 1 m line.

B. The pattern

C. Work it out: a = F ÷ m

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Seatbelts and stopping

In a crash, the car is stopped by a large force from whatever it hits, but the people inside keep moving forward, because of their inertia, until a force stops them too. A seatbelt supplies that force across the strong bones of the hips and chest, and it stretches a little, so the stopping takes a little longer and the force is smaller. In 1970 Victoria made wearing seatbelts compulsory, the first law of its kind in the world. Groups crash a trolley carrying a plasticine passenger into a barrier from three ramp heights, with and without a rubber-band seatbelt.

We are learning to

• explain, with inertia, what happens to a passenger when a car stops suddenly,
• explain how a seatbelt applies the force that stops the passenger,
• use a model to test an idea safely.

Success criteria

• I can explain why the loose passenger flew forward, without saying it was pushed forward.
• I can draw the force diagram for a belted passenger at the moment of the crash.

You need

• for each group: a trolley, a board and a few books to make a ramp, a heavy book as a barrier, a lump of plasticine, a rubber band and a ruler,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Safety: the trolley runs along the floor or a bench into the barrier; nobody stands in its path, and only the plasticine passenger crashes.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Run the crash tests in Session A, and the board, Parts B and C and the ticket in Session B.

Watch for these ideas

• “The crash throws the passenger forward.” No force pushes the passenger forward; the passenger keeps moving while the car stops.
• “Seatbelts only matter at high speed.” At any speed an unbelted passenger keeps moving until something stops them.
• “A stiff belt that does not stretch would be safer.” A belt that stretches a little stops you more gradually, with a smaller force.

Make it easier, make it bigger

• Easier: test one ramp height, with and without the belt.
• Bigger: the front of a modern car is built to crumple in a crash. Why does that make it safer for the people inside, even though the car is wrecked?

Answers and look-fors

• Part A, example class: 10 cm ramp, loose 12 cm, belted 0 cm; 20 cm ramp, loose 25 cm, belted 0 cm; 30 cm ramp, loose 41 cm, belted 1 cm.
• Part B: 1. The barrier stopped the trolley, but no force acted on the loose passenger, so it kept moving forward: inertia. 2. The rubber band pulled back on the passenger and stopped it with the trolley. 3. A higher ramp gives a faster trolley, so the passenger has more motion to lose: at 30 cm it flew 41 cm. 4. One arrow backwards from the belt; the weight down balanced by the seat up.
• Part C: 1. Everyone in Victoria had to wear a seatbelt, the first such law in the world. 2. Knowing the science did not make people buckle up; a law, and campaigns that explained it, changed what people did. 3. They make the stopping take longer, so the force on the people is smaller.
• Bigger: the crumpling front takes longer to stop, so the people slow down more gradually and feel a smaller force.
• Exit ticket: 1. Inertia: nothing has acted on them yet, so they keep moving. 2. The pull of the seatbelt. 3. It stops you more gradually, so the force on you is smaller.

Seatbelts and stopping

A. Crash tests

B. Explain the results

C. A law that saved lives

In 1970 Victoria became the first place in the world to make wearing a seatbelt compulsory, and the other states soon followed. Modern cars add airbags and crumple zones, which make the stopping take a little longer.

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Parachutes

A falling object speeds up until the air pushing up on it, air resistance, grows to match its weight. Then the forces balance and it falls at a steady speed, its terminal speed. A parachute’s wide canopy pushes against a lot of air, so the balance comes at a low speed. A skydiver falling belly-down reaches about 200 km/h; with the parachute open, the steady speed drops to a gentle landing speed. Groups test three parachute sizes carrying the same washer, drop each from 1.8 m, and graph canopy area against fall time.

We are learning to

• explain a steady fall with balanced forces,
• plan and run a fair test, repeating and averaging,
• graph results and describe the relationship.

Success criteria

• I can draw the forces on a parachute just after it is let go and when it falls steadily.
• I can use our graph to describe how the canopy area changes the fall time.

You need

• for each group: plastic shopping bags, scissors, string, sticky tape, a metal washer, a metre ruler and a stopwatch; a wall marked at 1.8 m,
• the worksheet (next sheet), one per student, and the build sheet (third sheet), one per group.

Safety: the dropper stands on the floor, never on a chair or a table; drop the parachutes away from people’s faces.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Plan and build in Session A, and drop, graph and explain in Session B.

Watch for these ideas

• “If it is moving, the forces cannot be balanced.” Balanced forces keep the speed steady; they do not stop the motion.
• “Air resistance is the same at every speed.” It grows as the object falls faster, until it matches the weight.
• “A parachute switches gravity off.” Gravity still pulls; the air pushes up just as hard.

Make it easier, make it bigger

• Easier: test two sizes, 20 cm and 40 cm.
• Bigger: hang a second washer on the 30 cm parachute. Predict, then test: faster or slower, and why?

Answers and look-fors

• Part A: changed, the canopy size; measured, the fall time from 1.8 m; kept the same, the washer, the string length, the drop height and how it is let go.
• Part B, example class: 20 cm sides, 400 cm², 1.2 s; 30 cm sides, 900 cm², 1.7 s; 40 cm sides, 1600 cm², 2.3 s.
• Part C: the points rise from left to right: the bigger the canopy, the longer the fall.
• Part D: 1. Just after letting go: the weight arrow down, longer than a small air-resistance arrow up; falling steadily: two equal arrows. 2. Bigger canopies fall more slowly, because they meet more air, so the air resistance matches the weight at a lower speed. 3. 1.8 ÷ 2.3 = about 0.78 m/s.
• Bigger: faster: the heavier load needs more air resistance to balance it, and that comes only at a higher speed.

Parachutes

A. Plan the fair test

B. Results

C. Graph it

D. Explain it

Build a parachute

1. Cut three squares from a plastic bag, with sides of 20, 30, 40 cm. Measure carefully: the size is the variable you are testing.
2. Cut four strings, each 30 cm long, for each parachute.
3. Tape one string firmly to each corner of the canopy.
4. Tie the four loose ends together to the washer, so the washer hangs evenly below the middle.
5. Hold the parachute by the top of the canopy at the 1.8 m mark, with the strings and washer hanging free, and let go. Start the stopwatch as you let go and stop it when the washer touches the floor.
6. Drop each parachute three times. If one drop tangles or drifts into a wall, do it again.

Stand on the floor to drop the parachutes, never on a chair or a table. Keep the drop zone clear of faces.

Show what we know

The last lesson brings the term together. Students follow the forces through a bike ride, drawing a force diagram for each of four stages, finding the net force and saying what it does to the motion. Then each student sits the final check on their own, and the class finishes with the unit quiz on the board.

We are learning to

• draw force diagrams and find the net force in a new situation,
• relate the net force to the change in motion,
• show what we know about forces on our own.

Success criteria

• My four force diagrams have labelled arrows to scale, and each net force matches the motion.
• I can back each answer on the final check with a force diagram, a calculation or a reason.

You need

• the bike-ride task (next sheet) and the final check (third sheet), one each per student; rulers,
• the interactive unit open for the closing quiz.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the bike ride and the partner check in Session A, and the final check and the class quiz in Session B.

Watch for these ideas

• “When cruising, the pedal force must be bigger, or the bike would stop.” Ask: what do balanced forces do to a moving bike?
• “At the lights there are no forces.” Ask: what stops the rider sinking into the road?
• Force arrows of the same length for unequal forces. Ask: what scale did you choose?

Make it easier, make it bigger

• Easier: give a scale of 1 cm for every 20 N for stages 1 to 3, and 1 cm for every 200 N for stage 4.
• Bigger: the rider and the bike have a mass of 80 kg. Find the acceleration at each stage.

Answers and marking guide

• Task: starting off, net force 100 N forwards: the bike speeds up; cruising, net force zero, balanced: the bike keeps a steady speed; braking, net force 150 N backwards: the bike slows down; waiting at the lights, net force zero, balanced: the bike stays still.
• Task, last questions: 1. 100 ÷ 80 = 1.25 m/s² while starting off. 2. Inertia: the brakes slow the bike, but the rider’s body keeps moving forwards until the handlebars or pedals push back on it.
• Check: 1. gravity. 2. 30 − 12 = 18 N to the right; the box speeds up to the right. 3. balanced. 4. 70 × 1.6 = 112 N; the mass is still 70 kg.
• Check: 5. a third as big. 6. At a steady speed the forces are balanced, so the pull is exactly as big as the friction. 7. In a crash the car stops but the people keep moving, by inertia, until a force stops them; a seatbelt supplies that force safely, across the hips and chest. 8. With the canopy open, the air resistance is suddenly bigger than the weight, so the net force points up and the skydiver slows; as the speed falls, the air resistance shrinks until it equals the weight, and the skydiver falls at a steady speed.
• Bigger: starting off 1.25 m/s² forwards; cruising 0; braking 1.88 m/s² backwards (slowing); at the lights 0.
• Scoring: 1 mark each for 1, 3 and 5; 2 marks each for 2, 4, 6, 7 and 8. The rubric near the front of the pack turns the task and the check into a level.

The forces through a bike ride

For each stage, draw a force diagram to a scale you choose, find the net force, and say what happens to the motion.

Show what we know

Show what you know about forces and motion. Read each item, then circle or write.

Teacher note: the marking guide is on this lesson’s plan; the rubric near the front of the pack turns the score into a level.