Levers and torque

How it works

A weight pulls on the lever with the force F = m·g. Its turning effect is the torque M = F·d, where d is the lever arm, the distance from the fulcrum. The lever balances when the torques turning it left add up to the torques turning it right. That is why 2 kg at 2 m balances 4 kg at 1 m: each side has a torque of 39.24 N·m.

M = m·g·dbalance: ΣM left = ΣM right

Try it yourself

1. In the starting experiment, 2 kg sits 2 m left of the fulcrum and 4 kg sits 1 m to the right. The torque panel shows 39.24 N·m on both sides.
2. Change the heavier weight to 6 kg. Predict where the 2 kg weight must go: balance happens 3 m from the fulcrum, at the 0 m mark.
3. Add another weight and use “Find the balance point”. The app finds the position where the torques cancel.
4. Set “Beam mass” to 5 kg and move the fulcrum off centre. The beam adds its own torque, because its weight acts at its middle.
5. Switch the gravitational acceleration to the Moon: the torques shrink, but the balance stays. Balance does not depend on g.

Model limitations

The model calculates torques for a rigid horizontal lever without friction. The tilt only shows which side is heavier; it does not simulate real rotation.

Related experiments

• Double pendulum
• Projectile motion
• Gravity and orbits