# Homework Help: Newtons' Laws Problem

1. Oct 17, 2012

### whoareyou

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

2. Relevant equations

Newtons' Laws
Kinematics Equations

3. The attempt at a solution

I don't know what I'm doing wrong, but I'm not getting the correct answer of 17cm ...

2. Oct 17, 2012

### ehild

A will not stop. It comes to rest on the surface of B, that is they move together with the same velocity.

ehild

3. Oct 17, 2012

### whoareyou

Does that imply that the time is wrong? Do I set up 2 equations, one for block A and block B and determine when their speeds will be the same?

4. Oct 17, 2012

### ehild

Yes, the time is wrong. The force of friction decelerates A and accelerate B till their velocity becomes equal.

ehild

5. Oct 17, 2012

### whoareyou

so...

3.5 - 5.88T = 4.704T
T = 0.331

d = (1/2)(4.704)(0.331)^2 = 0.257m --> still the wrong answer :(

6. Oct 17, 2012

### ehild

You use wrong value for the acceleration of B. It should be the force of friction divided by 1 kg.

ehild

7. Oct 17, 2012

### whoareyou

23.52N (the reaction force from the friction of block A) = ma

Now I assumed that it was the total mass of the system, since block A is on top of it and its moving with it. Was that assumption wrong?

23.52N = (m1 + m2)a
a = 23.52 / (m1+m2) = 23.52 / (4 +1 ) = 4.704m/s^2

8. Oct 17, 2012

### whoareyou

Oh, okay I get the right answer now. But how come you don't use the total mass when finding the acceleration for B? It's directly on top of it ...

9. Oct 17, 2012

### ehild

It is wrong. Zero external force acts on the whole system. The blocks interact, the force of friction is an internal force. A acts B and B acts A through the friction. So block B moves according to the force exerted on it by block A: F= 23.52 N.

ehild

10. Oct 17, 2012

### whoareyou

OH ! okay, makes sense. Thanks :)