ACARA v9 CONTENT DESCRIPTION “identify how forces can be exerted by one object on another and investigate the effect of frictional, gravitational and magnetic forces on the motion of objects”
Builds on noticing that pushes and pulls make things move and stop. Here we name those pushes and pulls as forces, and look at three of them, friction, gravity and magnetism, and how each one changes the way objects move.
A force is one object acting on another
A force is a push or a pull. It always comes from one object acting on another object. When you push a box, your hand is the object giving the force and the box is the object that feels it. The box moves in the same direction as the push. A pull works the same way, just toward you instead of away. Forces are how objects make other objects start moving, stop or change direction.
A push moves the box
A force is one thing pushing or pulling another. Push the box and watch it move that way.
A force is a push or a pull from one object onto another. When you push the box, your hand is the object giving the force, and the box is the object that feels it. The box moves in the same direction as the push. With no push, the box just stays where it is.
Friction is a force that slows things down
When two surfaces rub against each other, they make a force called friction. Friction always pushes back against the way something is sliding, so it slows the object down. A rough surface makes a lot of friction, so a block stops quickly. A smooth surface makes less friction, so the same block slides much further. Friction is why a ball rolling across grass stops sooner than one rolling across ice.
Friction slows things down
Give the block the same push on each surface. Friction from a rough floor stops it sooner.
Friction is a force that pushes back when two surfaces rub together. A rough floor grips the block hard, so it stops after a short distance. A smooth floor grips less, so the same push sends the block sliding much further before it stops.
Gravity is a force that pulls things down
Gravity is a pulling force from the Earth. It acts on everything around us and always pulls toward the ground. That is why a ball you let go of falls down instead of floating, and why things feel heavy when you lift them. You cannot see gravity, but you can see what it does every time something drops.
Gravity pulls things down
Switch gravity on and off. With gravity the ball falls to the ground; without it the ball floats.
Gravity is a pulling force from the Earth that acts on everything around us. When gravity is on, it pulls the ball straight down until it reaches the ground. If you imagine gravity switched off, nothing pulls the ball down, so it just floats in the air.
Magnets pull and push across a gap
A magnet can act on certain objects without even touching them. It attracts iron objects, pulling them toward it across a gap. Two magnets can attract or push each other away, depending on which ends face. When the same poles face, the magnets push apart. The magnetic force is stronger when the objects are close and weaker when they are far apart.
Magnets pull and push without touching
Choose attract or repel, and near or far. A magnet acts on things across a gap.
A magnet can pull or push other things across a gap, without touching them. It attracts iron objects, pulling them toward it. Two magnets with the same pole facing push each other away instead. The force is stronger when the objects are near and weaker when they are far apart.
Forces change the motion of objects
Friction, gravity and magnetism are all forces, and a force can change how an object moves. It can make a still object start moving, make a moving object speed up or slow down, or make it change direction. A rolling ball slows down because friction pushes back, and it speeds up rolling down a ramp because gravity pulls it. Whenever motion changes, a force is at work.
Forces change how things move
Step through the story. Watch friction slow the ball and a ramp let gravity speed it up.
Forces change motion. The ball starts rolling at a steady speed. Friction is a force that pushes back, so the ball slows down. On a ramp, gravity pulls the ball downhill, so it speeds up. Each force adds or takes away from the motion, which is why how the ball moves keeps changing.
Why this matters
Forces are everywhere you look. Friction lets your shoes grip the ground and your bike brakes work, gravity keeps you on the Earth and brings a ball back down, and magnets hold notes on a fridge and run inside motors. Knowing how forces push, pull and change motion helps you make sense of why things move the way they do, and gets you ready for more science about energy and machines later on.
Quick self-check
1. You push a box to the right. What happens to the box?
2. You give a block the same push on a rough floor and a smooth floor. Where does it stop sooner?
3. Why does a ball you let go of fall to the ground?
4. You hold a bar magnet near an iron paperclip. What does the magnet do to it?
5. A rolling ball runs onto a rough patch and slows down. What does this show?