There's two force problems I couldn't get on my study guide.(adsbygoogle = window.adsbygoogle || []).push({});

1. Two blocks (A and B) are in contact on a horizontal frictionless surface. A 36-N constant force is applied to A as shown. The magnitude of the force of A on B is:

There is a picture. Block B is on the right of Block A. 36N is applied to block A. Block A = 4.0kg and block B = 20kg.

A. 1.5 N

B. 6.0 N

C. 29 N

D. 30 N

E. 36 N

What I did is:

F = (m_{A}+ m_{B})a

a = 36 / (4.0 + 20)

a = 3/2 m/s^{2}

FAB = 4.0(3/2)

= 6.0 N

The correct answer is D. 30N. I can see where it comes from. I thought "the magnitude of the force of A on B" means I should multiply m_{A}and a to get the force, but to get 30 N they multiplied m_{B}and a. Why?

2. Two blocks with masses m and M are pushed along a horizontal frictionless surface by a horizontal applied force F as shown. The magnitude of the force of either of these blocks on the other is:

In this picture. M and m are in contact. m is on the right of M. Force F is applied to M.

A. mF/(m + M)

B. mF/M

C. mF/(M - m)

D. MF/(m + M)

E. MF/m

The correct answer is A. I don't even know how to start this problem. I tried to solve for a and substitute but that failed. Any ideas?

These came from a study guide I was given in class. I thought I knew this topic well but these problems really scare me. Can someone please explain these to me? I have a test coming up pretty soon. I read this section in the book, but sometimes I can't apply what I read to every problem.

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# Homework Help: 2 simple force problems

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