Solving Torque and Force Homework Problems

[jaredmt]

Homework Statement

A 50kg grindstone is a solid disk .26 in radius. You press an ax down on the rim with a normal force of 160N. The coefficient of kinetic friction between the blade and the stone is .6, and there is a constant friction torque of 6.5 NM between the axle of the stone and its bearings. (a) How much force must be applied tangentially at the end of a crank handle .5 m long to bring the stone from rest to 120 rev/min in 9 seconds? (b) After the grindstone attains an angular speed of 120 rev/min, what tangential force at the end of the handle is needed to maintain a constant angular speed of 120 rev/min? (c) How much time does it take the grindstone to come from 120 rev/min to rest if it is acted on by the axle friction alone?

Homework Equations

for solid disk: I = (1/2)MR^2

The Attempt at a Solution

120 rev/min = 12.56 rad/s
i found the acceleration would have to be 1.4 rad/s^2 and it would travel 56.7 radians
I = 1.69

i tried this:

t - 6.5Nm(from torqu friction) - torque friction from ax = I(Arad)

but I am not entirely sure how to get torque friction from ax. is it just Uk(Normal)(r) ?

and after i get total torque required I am not sure how to convert it into tangential force with the handle.

 

[weatherhead]

think about the defining equation for torque...

 

[jaredmt]

ok torque is FD so i guess Uk(normal)(2pie)(r) i'll try that

 

[jaredmt]

ok nevermind i found all the answers. I am just tired and Physics homework takes sooooo long lol + i have 2 more assignments to do after this in phyiscs :(

 

[weatherhead]

first work out what tangential force the normal force will produce... you are given the coefficient of friction. You then know what this tangential force is and how far it is from the centre, so you can work out the torque