# Homework Help: Newton's second law and tension

1. Sep 19, 2011

### warrior2014

Newton's second law and tension!!!!

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

A helicopter is lifting two crates simultaneously. One crate with a mass of 139 kg is attached to the helicopter by cable A. The second crate with a mass of 88 kg is hanging below the first crate and attached to the first crate by cable B. As the helicopter accelerates upward at a rate of 1.2 m/s2, what is the tension in each of the two cables (in kN)?

2. Relevant equations

Fnet=ma

3. The attempt at a solution

I drew a free body diagram and for the first crate (139 kg) I had tension pointing upwards and gravity pointing downwards. So I did the following calculations:

-mg + Ta= ma
Ta= m (a + g)
= 139 (1.2 +9.8)
= 1529 N
= 1.529 kN

For the second crate (88 kg), I had tension of the second crate pointing upwards, tension of the first crate pointing downwards and gravity pointing downwards. So I did the following calculations:

-mg- 1529 + Tb= ma
Tb= ma + mg + 1529
= m (a+g) + 1529
= 88 (1.2 + 9.8) + 1529
= 2497 N
= 2.497 kN

Both answers were wrong so I'm not so sure what I'm supposed to be doing for this question. Any help will be appreciated!!!

2. Sep 19, 2011

### cepheid

Staff Emeritus
Re: Newton's second law and tension!!!!

Are you sure that those are all of the forces acting on crate A? After all, it has a rope (that is in tension) attached to the bottom of it as well...

3. Sep 19, 2011

### Dick

Re: Newton's second law and tension!!!!

The first crate is the one with two tensions acting on it (Cable A and Cable B). The second crate is only attached to Cable B. Do the second crate first!

4. Sep 19, 2011

### warrior2014

Re: Newton's second law and tension!!!!

So the calculations I did for crate A should actually be for crate B? and the calculations I did for crate B should be for crate A?

5. Sep 19, 2011

### Dick

Re: Newton's second law and tension!!!!

Sort of. Your logic was swapped. But don't just reverse the numbers. They will change when you redo it.

6. Sep 19, 2011

### warrior2014

Re: Newton's second law and tension!!!!

If I'm not only supposed to reverse the numbers then how exactly am I supposed to solve this problem??

7. Sep 19, 2011

### cepheid

Staff Emeritus
Re: Newton's second law and tension!!!!

Exactly as you were intending to solve it. Draw a free body diagram for crate B. Solve for the forces on it. Draw a free body diagram for crate A. Do the same.

What, specifically, is the source of your confusion?

8. Sep 19, 2011

### warrior2014

Re: Newton's second law and tension!!!!

I understand how to go about doing the question now. thank you for all of your help :)