# Thread: axis of pulley?

1. ## axis of pulley?

I am stuck on a simple question that I belive could be a trick.
it says -

A 50kg body hangs at rest from a rope wrapped around a pully 15cm in diameter. what is the torque about the axis?

the formula I have used for a similar question is

Torque = Force x distance

Force= Mass x Acceleration
seeing as there is no acceleartion in this problem do i assume that Force is equal to 50kg ie 50Newtons or should i have to mulitiply by gravity (9.81) and assume this is the accelartion?

would the distance be 15cm or .15m and where is the axis of a pulley?? please help

2. ## Free Body Diagram

The torque about the axis should be the force, T, of the Tension in the rope multiplied by the radius, r, of the pulley.

If you do a free body diagram of the 50kg body you have the force, T, pulling up on the body and the weight of the body which is either 50kg*9.81m/s^2 or 50N they are equivalent.

T = mg
T = 50N

The radius, r, of the pulley is either .15m or 15cm they are also equivalent.

So the torque, J, is equal to 50N * .15m or,
J = 7.5 Nm