# Homework Help: Simple f=ma problem?

1. Feb 20, 2007

### derajfast

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

three blocks (m1=2kg, m2=3kg, m3= 4kg) are in contact with each other on a frictionless, horizontal surface as shown below. a horizontal force f=18n is applied to m1. blocks look like this --force-->(m1)(m2)(m3)

find the acceleration of the blocks

find the magnitudes of the net forces on each block

find the magnitudes of the contact forces between the blocks

2. Relevant equations

f=ma
f=mg

3. The attempt at a solution

for the acceleration of the blocks part, is it f=ma?
so for block one is it 18=2a
block 2 is it 18=3a
and block 3 18=4?

i dont know for the next 2 parts

thanks

2. Feb 20, 2007

### denverdoc

Can you visualize this problem at all? Imagine an air hockey table with 3 blocks of different sizes ranging from smallest to largest. And you push against the smallest. What happens? Would the three blocks separate? Your eqns would suggest that this is the case. If it were so, maybe I could arrange ever increasing weights on a string, like a clothes line and knock on one, and the 50'th flies off at 2000mph. I like to add comments like these as I think trying to get your minds eye around these situations can be very helpful in focusing on a soln.

Now you have posted a couple of eqns, one of which is helpful, the other has no bearing because in this case friction has been preempted. So weight f=mg doesn't matter. You're missing a crucial eqn, any idea about which one? Hint: go back to the beginning of your text chapter and look at all three of newtons laws.

3. Feb 21, 2007

### sidrox

To solve the above you can either draw the free body diagrams of each body and then mess with it for a long tym, or simple consider the three bodies as one for the first part of the question.
Thus, the mass of the new combined block is 10 kg.
Now find the accln.
After this, you need to work out the contact forces, say N1 and N2 by writing out individual force equations for each block starting with the 2 kg block. Try it and tell me what your answers are....

Last edited: Feb 21, 2007