Calculating Net Torque: An Example Problem with Formula and Image

[dsptl]

http://img225.imageshack.us/img225/6275/25534482ej9.jpg
http://g.imageshack.us/img225/25534482ej9.jpg/1/

attempt:

Tnet = F x r = (40N + 30N) x .03m

but something is wrong here...i don't know wht

 

[Hootenanny]

You're on the right lines, but you need to be careful here.

What is the definition of F and r?

 

[dsptl]

F is the force and r is the radius

 

[Hootenanny]

F is the force

Good

r is the radius

Not quite.

 

[dsptl]

Good

Not quite.

then?

 

[Hootenanny]

then?

It is the position vector directed from the centre of rotation to the line of action of the force. Another important point to realize is that you need to take the cross product of F and r, not simply multiply them.