Calculate the z component of the torque relative to position

[magma_saber]

Homework Statement

http://img3.imageshack.us/img3/5681/32832.jpg

All seven forces have a magnitude |F| = 32 N. w = 10 m, h = 16 m, and d = 15 m. Calculate the z component of the torque relative to position A.

Homework Equations

T = r*F

The Attempt at a Solution

Not sure how to start this.
F1 = F*h*w = 32*16*10?
F2 = F*h = 32*16?

 

[LowlyPion]

T = r X F

So you want to figure the force acting at a perpendicular to the point at interest.

In this case your r X F1 will equal |F1|*h, where h is the distance at which it is acting ⊥ to a line through A.

 

[magma_saber]

T = r X F

So you want to figure the force acting at a perpendicular to the point at interest.

In this case your r X F1 will equal |F1|*h, where h is the distance at which it is acting ⊥ to a line through A.

i tried that, its wrong. it says the z component. would that make a difference?

 

[LowlyPion]

i tried that, its wrong. it says the z component. would that make a difference?

The resultant of r X F is a z component by the right hand rule for taking a cross product.

Of course you still need to sum up all the other torques from the other forces.

 

[magma_saber]

it asks just for the torque for each force that is applied, i.e. F1, F2, F3, ect. After that, it asks for the net torque. for F1 i tried F*h and it was wrong. Would it be F*h*w, since the tail starts at w?

 

[LowlyPion]

it asks just for the torque for each force that is applied, i.e. F1, F2, F3, ect. After that, it asks for the net torque. for F1 i tried F*h and it was wrong. Would it be F*h*w, since the tail starts at w?

Sorry I thought you were supposed to be determining the sum.

The |T| is F1*h. But because it is the cross product of y X x, that yields a clockwise torque that is -z

So for F1 you have the z component is -|F1*h|z

Counterclockwise torques are positive. Clockwise are negative.
http://en.wikipedia.org/wiki/Right-hand_rule