Physics HW help? Normal Force between two objects?

1. Feb 3, 2010

VoNi

Physics HW help!? Normal Force between two objects?

Hi, Ive had trouble trying to gigure out this problem! I would really appreciate the help!

The Problem:

Three blocks rest on a frictionless, horizontal table (see figure below), with m1 = 7 kg and m3 = 12 kg. A horizontal force F = 103 N is applied to block 1, and the acceleration of all three blocks is found to be 3.4 m/s2.

A. Find m2
B. What is the normal force between blocks 2 and 3?
I coouldnt post the figure(picture) but I can explain it.. The three blocks are all lined up against each other. So block 1(m1) is first, then block two(m2) is right next to block1, then block 3(m3) is against block 2.. they are all lined up on top of the table.. side by side..
Relevant equation:

I found the mass of block 2 by doing F=ma. or M= F/A
7+12+m2= 103/3.4
19+m2= 30.29
m2= 11.29

So M2 is 11.29

My only problem is that I don't know how to find the normal force between blocks 2 and 3

Attempt at a Solution:

The way i tried to find the solution was figuring out what force is acting on block three so that it can accelerate 3.4 m/s^2.
I did:
F=m*a
F=12*3.4
F= 40.8

Now, is 40.8 the normal force between block two and three or is that just an irrelevant number?

Thank you so much for the help!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 3, 2010

PhanthomJay

Re: Physics HW help!? Normal Force between two objects?

very good.
Yes, but don't forget, F is actually F_net (in Newtons, don't forget the units)
It is very relevant. If F_net = 40.8 N, what forces act on that block in the x direction? Draw a free body diagram of block 3. Is F_net and and F_normal the same??

3. Feb 3, 2010

VoNi

Re: Physics HW help!? Normal Force between two objects?

Thank you SO much! that was very helpful! and the normal force, is 40.8 N. Thanks for taking the time to help me out! =)

4. Feb 3, 2010

PhanthomJay

Re: Physics HW help!? Normal Force between two objects?

You are welcome. Note that the normal force should probably (I think) be rounded off to 41 N (it's a significant figure thing), but your answer is essentially correct. Wecome to PF!