Mon Prof Augmenté
Lifting a Boulder With a Plank
Age 12 · Physics
A plank resting on a steel wedge lets a small push at one end raise a heavy boulder at the other end. The farther your hand is from the pivot, the less force you need.
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In the picture
- The granite boulder: the load, resting on the short arm of the plank.
- The short arm of the plank: the stretch between the load and the pivot; the shorter it is, the bigger the force delivered to the load.
- The steel triangle: the pivot, the fixed point the plank turns around.
- The long arm of the plank: the stretch between the pivot and the hand; a longer arm needs a smaller push.
- The gloved hand: the effort pressing down, travelling a long way to raise the boulder a little.
Examples
- A see-saw: a light child sitting far from the middle can lift a heavier friend sitting close to it.
- A wheelbarrow: the wheel is the pivot, and the long handles lift a heavy load with a small pull.
Key takeaways
- Force times distance from the pivot is equal on both sides, so a longer arm needs less force.
- You trade distance for force: your hand moves far while the load rises only a little.
Quiz: Lifting a Boulder With a Plank (Age 12)
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You are lifting a heavy rock with a plank resting on a pivot. Where should you push to need the least force?
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A lever lets you lift a heavy load with a small push. What do you give up in return?
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You slide the pivot closer to the rock, keeping your hand at the far end of the plank. What happens to the push you need?
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