# Homework Help: Help with newton's second and third laws

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1. Nov 4, 2014

### hollymason

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

Two blocks are pushed to the right so that they move together with increasing speed. Block B remains at the height shown. Ignore friction between the ground and block A but not between block A and block B. The mass of block A is 10kg and the mass of block B is 2kg. Let system S represent the system consisting of both blocks A and B. (use g=10m/s^2)

A-for block a, b, and system s: draw free-body diagrams, identify any Newton's third law force pairs, and write out the algebraic form of Newton's second law.

B-using only the forces in your free-body diagram for system S, calculate the magnitude of the force exerted on system C by the ground (N_sg).

C-using only the forces in your free-body diagrams for block A and block B, calculate the magnitude of the force exerted on block A by the ground (N_ag).

D- How should the value of N_sg compare to N_ag?

2. Relevant equations
Force = mass X acceleration

3. The attempt at a solution

This is my attempt so far. I'm stuck on C because I feel like I would use the same equation F=m X a but mass would be 10kg instead of 12 because it's asking about the force exerted on A. But the blocks move together so would I account for the weight of B as well? I've attached an image so you can see my attempt at parts A and B as well.

http://oi58.tinypic.com/2z4gw11.jpg

2. Nov 4, 2014

### Staff: Mentor

OK.

The weight of B only acts directly on B, not A. Of course, the weight of B affects the force that B exerts on A. Set up two equations, one for A and one for B.

3. Nov 4, 2014

### hollymason

Sorry if this is a stupid question, but do you mean write different force equations for A and B?
So for A it would be F=10kg X 10m/s2=100N
and for B it would be F=2kg X 10m/s2 = 20N

4. Nov 4, 2014

### Staff: Mentor

I mean apply Newton's 2nd law to each block separately. ($\Sigma F = ma$.) You already have the forces identified, and even the equations written. (To answer part C, all you need are the vertical forces.)