Question

A boy of mass 40kg and a girl of mass 30kg play on a see-saw of negligible weight. If the boy sits 270 cm from the pivot of the see-saw, where must the girl sit to make it balance.

Please show the steps....I will follow you if you do.

Answer 1

Hi there!

To solve, we can use a summation of torques.

For the see-saw to balance, the sum of torques must equal 0, so:

∑τ = 0

Recall that:

τ = rFsinθ

r = distance from pivot (meters)

F = force (N)

In this case, the forces are the weight of the children. Also, we can leave the distance in centimeters for this type of problem, but others do require a conversion to meters.

Multiply each mass by 10 (g ≈ 10 m/s²) to get the weight:

Στ = 270(400) - x(300) = 0

270(400) = 300x

Solve for 'x', the distance in cm away from the fulcrum:

108000/300 = 360 cm