# Compare torque from force applied at different points

1. Apr 5, 2007

### future_vet

1. The problem statement, all variables and given/known data
Two equal forces are applied to a door. The first force is applied at the midpoint of the door; the second force is applied at the doorknob. Both forces are applied perpendicular to the door. Which force exerts the greater torque?

A) The first force at the midpoint of the door.

B) The second force at the doorknob.

C) Both forces exert equal non-zero torques on the door.

D) Both forces exert zero torque on the door.

3. The attempt at a solution
I think it's B. Would this be correct?

Thanks!

2. Apr 5, 2007

### Hootenanny

Staff Emeritus
Indeed, you are correct. The torque is given by the cross product of the force vector and the position vector of the applied force. Since the force is applied orthogonal to the door this simplifies to T = F*r.

3. Apr 5, 2007

### denverdoc

or as archimedes once said, give me a place to stand and a long enough lever, ....

4. Apr 5, 2007

### future_vet

Thank you! :)