A piece of metal is pressed against the rim of a 1.6kg, 19-cm-diameter grinding wheel that is turning at 2400 rev/min. The metal has a coeffiecient of friction of .85 with respect to the wheel. When the motor is cut off, with how much force must you press to stop the wheel in 20 sec.? Not sure I did this right, here's what I did: 2400 rev/min = [tex] 80 \pi[/tex] rad/sec. = [tex] \omega_0 [/tex]. t=20, and [tex] \omega_f = \omega_0 + \alpha t [/tex]. After plugging everything in (w_f=0) I got [tex] \alpha = -40 \pi [/tex] rad/sec^2. Since [tex] a = r \alpha [/tex], I got a = [tex] -3.4 \pi [/tex] m/sec^2. Then somehow from there I jumped to an answer of [tex] 4.624 \pi [/tex] N. I don't think that's right.