Question

Tina has a mass of 25 kg and sits on a seesaw, 1.8 m from the axis of rotation. Jack has a mass of 35 kg and wishes to sit on the other end. Where should he sit if he wishes the net torque on the seesaw to be zero? Jack’s dog spot jumps on his end of the seesaw. Spot has a mass of 10 kg and sits on a point 3m from the axis of rotation. What is the net torque on the seesaw? If the seesaw has an angular acceleration of 1rad/s^2 after the dog jumps on, what is the moment of inertia of the seesaw system including the kids and dogs?

Answer 1

(a) Given mass of Tina mT = 25kg, the mass of Jack mJ = 35kg. Tina is at a distance rT = 1.8m from the axis of rotation anf let Jack be at a distance rJ. For the net torque to be zero, clockwise torque should be equal to anti-clockwise torque. Therefore,

So to make the net torque zero, Jack must sit at 1.28m from the axis of rotation on the other side.

(b) Given the mass of Spot mS = 10kg and sits at a distance rS = 3m from axis of rotation. Now the net torque on the seesaw is,

So when the dog sits on the seesaw, the net torque on the seesaw is 29.8Nm

(c) Given the angular acceleration of the seesaw = 1 rads^2. If I is the moment of inertia of the system, the torque is given by,

So the moment of inertia of the system including the kids and dog is 29.8kgm^2.