# Three Blocks

1. Sep 25, 2010

### farrah003

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

Blocks of mass 5, 10, and 25 kg are lined up from left to right in that order on a frictionless surface so each block is touching the next one. A rightward-pointing force of magnitude 14 N is applied to the left-most block.

a) What is the magnitude of the force that the middle block exerts on the rightmost one?
Fright, middle = 8.749999995
b) What is the magnitude of the force that the leftmost block exerts on the middle one?
Fmiddle, left = ?
c) Suppose now that the left-right order of the blocks is reversed. Now find the magnitude of the force that the leftmost block exerts on the middle one?
F'middle, left = ?

i figured out the first part i just need help to do the last two
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 25, 2010

### MHrtz

I don't think (a) is right. Here's what I would do.

F = ma

The force pushing on the blocks is equal to their total mass times their acceleration but we have to solve for acceleration.

14 = 30a

a = 7/15

Use F = ma for every question and don't forget Newton's 3rd law

3. Sep 25, 2010

### farrah003

part a is correct because the homework is on a software and it accepted my answer

4. Sep 25, 2010

### MHrtz

how did you get a

5. Sep 25, 2010

### MHrtz

Nevermind. I screwed up. The total mass of the blocks is equal to 40 not 30. So a is right.