Homework Help: Acceleration with friction

1. Nov 9, 2013

MathewsMD

A bureau rests on a rough horizontal surface (μs=0.50, μk=0.40). A constant horizontal force, just sufficient to start the the bureau in motion, is then applied. The acceleration of the bureau is _____?

Attempt:

Fapp = ma = mgμs if there is no acceleration. But since there is movement, it must be the smallest value of a possible. We also know Fapp is just slightly greater than fs, right?

FNET = ma = Fapp - fs
ma = Fapp - mgμs

Well, I'm stuck here. I don't know what the value of a would be, I would just guess it's the smallest possible value. But how would that be figured out from what is give? Any help would be amazing! :)

2. Nov 9, 2013

CWatters

The coefficient of static friction is higher then the coefficient of dynamic friction.

The force required to start it moving is that required to overcome static friction = mgμs and that's the constant horizontal force that continues to be applied for the rest of the problem.

Once moving the net forces acting on the bureau is therefore the applied force mgμs minus the force due to kenetic friction mgμk

Then apply Newton.

3. Nov 9, 2013

tiny-tim

Hi MathewsMD!

You find Fapp as the minimum F that gives you movement, using µs.

Then you use that same Fapp and µk to find the acceleration.