# Contact Forces Question(s) part 2

1. Oct 6, 2008

### Inertialforce

1. The problem statement, all variables and given/known data
Three blocks are placed onto a frictionless horizontal surface. These blocks are side by side and are in contact with each other. Let m1=2m2=5m3. Answer the following exercises.
(Part 1)
a)If a force is applied to m1, then show that

i)Fa = 17/2m3a

ii)F12 = 7/2m3a where F12 is the contact force of mass 1 on mass 2

iii)F23=m3a

Question: Consider the same three blocks as in the question above. If these blocks rest upon a rough floor (μ=0.11), show that if a force is applied to m1, then:
(Part 2)
i)Fa = 17/2 m3(a+μg)

ii)F12 = 7/2 m3(a+μg)

iii)F23 = m3(a+μg)

2. Relevant equations
Σ Fx = ma

3. The attempt at a solution
All in all both questions (part 1 and part 2) are more or less the same except this question (part 2) includes the force of friction while the other (part 1) does not. I have already solved all the questions for part 1, and where I am having difficulty is, is in getting rid of the mass (m) in "μmg" for this question so that I could get the values as seen above.

2. Oct 6, 2008

### Inertialforce

Never mind I figured out where I went wrong and now know what to do for this question so a reply is not needed. But thank you to those who were trying to help by trying to explain the question to me.

3. Oct 6, 2008

### Rake-MC

It would be good to see your attempt at the questions, then maybe I could understand what you are saying a little better. From what I have read you are finding it difficult cancelling the masses in all of part 2.

F = ma
Therefore you have ma2 = 17/2 m3 (a+μg)
Divide by m.