Thread: Physics Quiz
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Unread 19-11-2011, 10:41
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Re: Physics Quiz

The sum of the moments around the pivot must be zero. The forward friction on the wheel and the sideways friction on the wheel are the only two forces with a component perpendicular to the rod. So

0 = forward friction * length * cos(theta) - sideways friction * length * sin(theta)

the forward friction would be F= tau/r, so solving for the sideways friction gets:

0 = tau/r *length * cos(theta) - sideways friction * length * sin(theta)

sideways friction = tau/r *cos(theta)/sin(theta)
sideways friction = tau/r * cotan(theta)

The total magnitude of the total friction force is just
F = sqrt ((tau/r)^2 + (tau/r * cotan(theta))^2)
F = tau/r * sqrt(1+cotan(theta)^2) = tau/r * (1/sin(theta))^2 = tau/r * cosec(theta)^2

Edit: should be F= tau/r * cosec (theta) (forgot to take square root)

The angle from the horizontal is
arctan((tau/r)/(tau/r * cotan(theta))) = arctan (theta) =theta

This makes sense because the friction force has to act exactly opposite to the tension force, or the wheel won't be in static equilibrium.

Last edited by John : 19-11-2011 at 11:01.