How to play Seesaw
- Choose a seesaw and read the masses and their signed distances from the pivot on the card.
- Work out each torque, weight × distance, and add them on each side.
- Type the distance, mass, or force that makes the two sides equal, and press Let go.
- Watch the torque bars: equal bars keep the plank level; otherwise it tips toward the larger torque.
What keeps a seesaw level?
A force can turn an object about a pivot. Its turning effect, the torque or moment, is the force times its perpendicular distance from the pivot: M = F·d. A weight m·g at distance d gives M = m·g·d. An object at rest does not start turning when the torques cancel: ΣM = 0, so the counter-clockwise torques equal the clockwise torques. That is why a light child far out can balance a heavy child close in, why a plank's own weight matters when the pivot is not at its centre, and why levers such as a wheelbarrow let a small force lift a large load.
A focused learning model
The plank is rigid and children are point weights at fixed distances. The pivot's friction holds the plank level while the net torque is at most 2% of the larger side's torque. Otherwise the plank turns from rest, θ = ½·(ΣM ÷ I)·t², until its lower end touches the ground, found exactly. Torques are counter-clockwise positive. Plays in real time.