Finding the coefficient of friction of a block on a table

1. Feb 18, 2009

ssj2pizza

1. The problem statement, all variables and given/known data
Block A is on a table and is connected by a pulley to block B.
Block A has weight Wa and block B has weight Wb. Once block B is set into downward motion, it descends at a constant speed. Assume that the mass and friction of the pulley are negligible.

Calculate the coefficient of kinetic friction between block A and the table top.
Express your answer in terms of some or all of the variables Wa , Wb, and g (the acceleration due to gravity).

2. Relevant equations
fk=$$\mu$$kn

I have gotten this hint (mastering physics problem):

In this problem, blocks A and B are in dynamic equilibrium; their velocities are constant. This means that the net force on each,F net , is equal to zero.

The tension is constant throughout the rope. By setting the sum of the forces acting on both blocks A and B equal to zero, you should be able to obtain two different expressions for the tension in the rope. Set these equal to each other and solve for $$\mu$$.

3. The attempt at a solution
I really don't know how to go about starting this. I know the equation above but don't know what to do with it. I feel like I need to know the acceleration. How would I find the tension?

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 18, 2009

Tom Mattson

Staff Emeritus
You don't know what to do with it? Haven't you ever seen a free body diagram? Don't you think it would be a good idea to draw one?

You DO know the acceleration! In your very own words:

and

3. Feb 18, 2009

ssj2pizza

Ok so their accelerations were equal to zero and their net forces were also equal to zero. Thanks about the free boday diagram (duh). Well i figured out the answer. It ended up being the weight of block b divided by the weight of block a. I have tried looking in my book and looking through my notes but there were no examples for finding the coefficient of friction. (frustrating)