Calculate the net torque about the axle of the wheel

confusedbyphysics

This problem is due on my webassign homework in 3 hours and I can't get it right!!

Calculate the net torque about the axle of the wheel shown in Fig. 8-39. Assume that a friction torque of 0.43 mN opposes the motion and that F = 26.

The picture is here: http://www.webassign.net/giancoli/8-39alt.gif

What I did:

The 35 N force will move the wheel clockwise so the torque will be -

28 N force will move it counterclockwise so torque will be +

26 N force will move it clockwise so force will be -

They forces are all perpendicular to the radius so its just T = r X F

35 N force T = 35 N X .12 m = -4.2 (moves cw)

28 N force T = 28 X .24 m = 6.72 (moves ccw)

26 N force T = 26 X .24 m = -6.24 (moves cw)

So then including the friction torque which opposes the motion of .43 I put

6.72 - 4.2 - 6.24 - .43 = -4.15 is the net torque and because its - it movies in the clockwise direction

Web assign says -4.15 is WRONG! What am I doing wrong, please help!

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lightgrav

Homework Helper
You haven't said yet which direction (cw , ccw) the disk is rotating...
if it was already rotating ccw, the friction torque would be cw.
have you tried that?

whozum

It says 0.43 mN, which isnt the same as 0.43N

confusedbyphysics

Ohhhh...so if the motion is going clockwise than the friction will be counter clockwise so it would be +.43 instead of -.43?

6.72 - 4.2 - 6.24 + .43 = -3.29

is that right?? ( I only have one guess left on my webassign and I dont want to screw it up, lol)

lightgrav

Homework Helper
Well, I don't know the whole scenario of the problem, but ...
the 3 forces in the diagram MIGHT be slowing the spin of the disk.
There should be some indication of the spin, in the problem text.

I expect your .43 mN means meter x Newton (not milliNewton)
since it is a torque - unless it is 43 milliN applied at the .12m hub!

Seems right.

confusedbyphysics

Ok I just guessed and -3.29 is right. That was stupid of me not to catch that the friction is positive. Thanks for the help!

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