A seesaw is a lever: a stiff bar that turns on a pivot. A light child can balance a heavy adult, if they sit at the right places. The rule is a proportion, and it is the same trade of force for distance that every simple machine makes.
Key idea
A lever balances when force × distance from the pivot is the same on both sides: $$F_1\,d_1 = F_2\,d_2$$ Force times distance is the turning effect (later called torque).
Why it works
Whether a seesaw tips depends on both the weight and how far out it sits. A weight far from the pivot turns the bar more than the same weight close in, just as a long wrench turns a bolt more easily than a short one. When the turning effects on the two sides match, neither side wins, and the lever balances.
Worked example: Where to sit
A 40 kg child sits 1.5 m from a seesaw's pivot. Where must a 30 kg child sit to balance?
- Balance: $40 \times 1.5 = 30 \times d$. (The weights are both mass × 9.8, so you can use the masses.)
- $60 = 30\,d$, so $d = 2$ m.
Answer: 2 m from the pivot, on the other side
Worked example: A crowbar
A crowbar's pivot is 0.1 m from a heavy rock and 1.2 m from your hand. The rock pushes back with 600 N. What force must you apply?
- Balance: $F \times 1.2 = 600 \times 0.1 = 60$.
- $F = 60 \div 1.2 = 50$ N. The long handle gives a mechanical advantage of 12.
Answer: 50 N
Common mistake
Thinking the heavier side of a seesaw always goes down. A 60 kg adult 0.5 m from the pivot ($30$) loses to a 20 kg child 2 m out ($40$). Check: compare weight × distance on each side, not weight alone.
For balance in motion, try stacking a chain of books on a rolling cart in the Science Lab: Book Cart.