seegongsik
Grade 6 (age 11-12)

grade 6 Levers & Pulleys

Levers and pulleys are tools that help us move heavy things with a small force. A lever is a bar resting on a fulcrum; the closer the fulcrum is to the load, the longer the effort side becomes, and the less force is needed to lift the load. A fixed pulley changes the direction you pull, and a movable pulley halves the force needed, but you have to pull twice as much rope. Move the fulcrum to see how much the force goes down or up, then pick each of the three pulley setups to compare the size and direction of the force.

How a lever works

30%
💡 What is a lever?
①A lever is a bar resting on a fulcrum, used to lift a load.
②The force needed depends on the ratio of the distance from the fulcrum to the load and the distance from the fulcrum to where you push.
③The closer the fulcrum is to the load, the smaller the force. With distances of 30 : 70, as in the starting picture, you need about 0.43 times the load's weight.
④In exchange, your end of the bar has to move a longer distance.
⑤Seesaws, bottle openers, scissors and nail clippers all use levers.

Kinds of pulleys and how they work

🔧 Fixed pulley
①A fixed pulley is attached to something like a ceiling and does not move.
②Pulling the rope down lifts the load up, so it changes the direction of the pull.
③It does not reduce the force: to lift a 10 kg load you still pull with 10 kg of force.
④The pulley at the top of a flagpole is a fixed pulley.

What happens when you use less force

Levers around us
Seesaw / bottle opener / scissors / tongs / nail clippers
The fulcrum, the place you push and the load sit in different spots on each tool.
Pulleys around us
Flagpole (fixed) / crane (combined) / elevator (pulleys and cables)
They make it easy to lift heavy things or pull in a handy direction.
🏗️ What happens when you use less force
①Levers and pulleys can reduce the force you need.
②In exchange, the part you push or pull has to move further.
③If the force is halved, the distance doubles, so force times distance stays the same.
④A tool does not do work for free; it trades force for distance.

Summary

Force and distance
Force × distance moved = work (unchanged even with a tool)
A tool reduces the force but increases the distance you move.
🎯 Key points
①Lever: putting the fulcrum near the load lets you lift it with a small force.
②Fixed pulley: changes only the direction of the pull, not its size.
③Movable pulley: halves the force, but you pull twice as much rope.
④Combined pulley: reduces the force and changes its direction.
⑤Even with a tool, force times distance stays the same.

Examples and Unit Test

Example 1
If you lift a load with one movable pulley, how do the force needed and the length of rope you pull change?
1
A movable pulley moves with the load, and two strands of rope share the weight, so the force is halved.
2
In exchange, you have to pull twice as much rope as the height the load rises.
▸ The force is halved, and you pull twice as much rope.
Pick the movable pulley in the second picture: a 10 kg load is lifted with 5 kg of force.
Example 2
A tool halves the force needed to lift a load. What happens to force times distance, and to the distance you have to move?
1
Even with a tool, force times distance does not change.
2
The force is halved, so the distance doubles and the product stays the same.
▸ Force times distance stays the same, and the distance doubles.
Save on force and you spend more distance. A tool simply trades one for the other.
Unit-test style
Which statement about levers and pulleys is correct?
①A fixed pulley reduces the size of the force.
②A movable pulley halves the force needed.
③With a lever, the further the fulcrum is from the load, the smaller the force needed.
④Using a tool makes force times distance smaller.
⑤Using a pulley does not change how far you pull.
▸ ② A movable pulley halves the force needed.
1
A movable pulley moves with the load and halves the force, but you pull twice as much rope.
2
A fixed pulley only changes direction (①), a lever needs less force when the fulcrum is near the load (③), force times distance stays the same (④), and less force means pulling further (⑤).
← Previous
Combustion & Fire Extinguishing
Next →
Human Body Systems

Last updated