# Finding the coefficient of friction of a block on a table

#### 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

#### Tom Mattson

Staff Emeritus
Gold Member
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.
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?

I feel like I need to know the acceleration.
You DO know the acceleration! In your very own words:

...it descends at a constant speed...
and

I have gotten this hint (mastering physics problem):

In this problem, blocks A and B are in dynamic equilibrium; their velocities are constant.

#### 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)

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