# Homework Help: Newton Third Law problem. Help pleaseee

1. Oct 6, 2009

### irNewton

Newton Third Law problem. Help pleaseee!!

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

The 1.0 kg block is tied to the wall with a rope. It sits on top of the 2.0 kg block. The lower block is pulled to the right with a tension force of 20 N. The coefficient of kinetic friction at both the lower and upper surfaces of the 2.0 kg block is uk = 0.370.

What is the acceleration of the 2.0 kg block?

2. Relevant equations

Manipulations of F=ma.

3. The attempt at a solution
I have given this two tries.

First try,

Fx=Fa2-Fk2-Ft+Fk1=ma
20N-(0.370)(3.0kg)(9.81m/s^2)-3.63N+(0.370)(1.0kg)(9.81)=3a

and I get it wrong..

2. Oct 6, 2009

### semc

Re: Newton Third Law problem. Help pleaseee!!

How did you get 3.63N? Why did you add Fk1? I believe the summation of forces is 2a

3. Oct 6, 2009

### irNewton

Re: Newton Third Law problem. Help pleaseee!!

Hmm but isn't the force being applied to the whole system which is 3kg?

anddd, Fk1 is the kinetic friction acting between block 1 and block 2 and 3.63 is the tension in the rope for block 1.

I was guessing all of the forces on block 1 and 2 should be taking into account...
I guess not?

4. Oct 6, 2009

### semc

Re: Newton Third Law problem. Help pleaseee!!

Ops i misread the question didn't notice the 1kg mass is tied to the wall no wonder i don't understand why static coefficient is not given. If we say the force applied is F, is this force F in your free body diagram for the smaller mass? Maybe you can try looking at just the 2kg mass and draw the free body diagram and resolve the forces. Does it give you the correct answer?

Last edited: Oct 6, 2009
5. Oct 6, 2009

### irNewton

Re: Newton Third Law problem. Help pleaseee!!

I already tried adding up the sum of all the forces only on block 2 and got a wrong answer :(

6. Oct 6, 2009

### semc

Re: Newton Third Law problem. Help pleaseee!!

Mind writing out how you do it?

7. Oct 6, 2009

### irNewton

Re: Newton Third Law problem. Help pleaseee!!

20N-0.37*2*9.8=2a
6.4 m/s = a

this is wrong :(

8. Oct 6, 2009

### semc

Re: Newton Third Law problem. Help pleaseee!!

0.37*2*9.8 is the frictional force between the smaller mass and the bigger one how about the friction between the bigger mass and the floor?