# Homework Help: Atwood's machine

1. Dec 15, 2016

### KevinFan

1. The problem statement, all variables and given/known data
http://[url%3Dhttps://postimg.org/image/pov8gzkr5/][ATTACH=full]200173[/ATTACH]

[ATTACH=full]200174[/ATTACH]
The question is shown above in the image.
[B]2. Relevant equations[/B]
F=ma

[B]3. The attempt at a solution[/B]
I personally think the answer should be option B. The movable pulley requires 1/2 of the force compare with the fixed pulley, therefore the a1 is 2 times a2. Can anyone confirm my logics please?

Last edited by a moderator: May 8, 2017
2. Dec 15, 2016

### ehild

Last edited by a moderator: May 8, 2017
3. Dec 15, 2016

### KevinFan

Could you elaborate a bit?

4. Dec 15, 2016

### ehild

Imagine that m1 raises by 1 cm. How much does the piece of rope keeping the left pulley get shorter? The length of the whole rope does not change. So the right piece gets longer. What is the diplacement of m2?

5. Dec 15, 2016

### KevinFan

I think the displacement of m2 would be 1cm as well

6. Dec 15, 2016

### ehild

Why? The right piece of string should be longer by the same as the left piece gets shorter. But the left piece consists of two pieces, and both of them gets shorter by 1 cm.

7. Dec 15, 2016

### KevinFan

oh... so the displacement of m2 should be 2 cm and a1=1cm/t^2, a2=2cm/t^2. abs(a1)=abs(1/2(a2))
Option C is the correct answer

8. Dec 15, 2016

### ehild

Yes

9. Dec 15, 2016

### KevinFan

Thank you very much for your explanation