# Homework Help: Newton's 3rd Law, pulley, only given friction coefficients

1. Sep 1, 2014

### aatgomez

1. The problem statement, all variables and given/known data
An object consists of 3 connected masses as seen in the figure. None of the surfaces have friction except the surface on m1 which has the coefficients μs=0.4 y μk=0.3.
What is the acceleration of each one of the masses?
It looks like the following image but m1 is the table and m2 is equal to m3 or the block thats hanging from the pulley of the table.

2. Relevant equations
∑Fx = m1a → τ - fk = m1a
∑Fy = 0 → n- m1g = 0
T= μkm1g+ m1a
∑Fy = m2a → m2g - T =m2a
m2g - (μkm1g+m1a) = m2a
a = m2 - μkm1/m1 + m2

3. The attempt at a solution
I got up to the finally equation but I can't figure out how to solve the equation without mass. I've attached a picture just in case the problem needs some clearing up. Thanks!!

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2. Sep 2, 2014

### Astronuc

Staff Emeritus
Please separate the work from the relevant equations.

The attached image is a bit confusing since it has two masses, m2, but one mentions a mass m3.

The falling mass is 'pulled' by gravity, so there must be 'g' in the final solution. Gravity is 'forcing' the system.

The small mass is pulled by the falling mass through tension in the cord or string.

Opposing the motion of the horizontally moving smaller mass is the friction with the table. If the table is free to slide, then it too will experience acceleration due to the frictional force sliding on top of it.

The accelerations will be related to g by some coefficient which will be a complicated function of the masses.

3. Sep 2, 2014

### Staff: Mentor

Is it safe to say that your first image is not relevant to this problem?

Are there any constraints on the problem? Are things starting from rest? Is the hanging mass falling? Is the system being accelerated?

Please revise your description of the problem to be clear about which surfaces have friction.

4. Sep 2, 2014

### Astronuc

Staff Emeritus
Particularly, one's needs to clarify "None of the surfaces have friction except the surface on m1 which has the coefficients μs=0.4 y μk=0.3." As shown in the attached diagram, m1 has two surfaces - between m1 and the ground, and between m1 and the mass on top. Friction on either of those surfaces, or both, will significantly affect the answer.