# Inclines and friction

1. Dec 5, 2013

### pumpernickel

The figure below shows a block of mass m resting on a 20° slope. The block has coefficients of friction µs = 0.74 and µk = 0.52 with the surface. It is connected via a massless string over a massless, frictionless pulley to a hanging block of mass 2.0 kg.

(a) What is the minimum mass m that will stick and not slip?
(b) If this minimum mass is nudged ever so slightly, it will start being pulled up the incline. What acceleration will it have?

fs= μs*N
F=M*A

I got part A correct.
m = 19.6 ÷ (9.8 * sin 20 + 0.74 * 9.8 * cos 20) = 1.93

but part B is not, I tried to use the same equation but change the static friction to kinetic

and got 2.4 as the mass

then F= MA
F = 2.4 (A)

I made a FBD and came up with

WME-fk - wsin$\theta$=a(M1+M2)

Is this on the right track?

thanks

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2. Dec 5, 2013

### Staff: Mentor

But the question states that the mass remains at the value 1.93