# Accleration of 3 masses

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1. Dec 21, 2016

1. The problem statement, all variables and given/known data
3 blocks A,B and C are attached between them by massless and frictionless strings , that passes through frictionless , massless pullies , the blocks A and B have the same weight 25 N and the same coefficient of kinetic friction 0.35 , the bloc C goes down with a constant speed .

Using Newton's 2nd law we get : $$a_{C} = 0$$ because the speed is constant ,

what I can't figure out is why in the correction they said that : $$a_{B} = 0$$ and $$a_{A} = 0$$
why both of block A and B have ALSO a zero acceleration , how do we know that ?

Last edited: Dec 21, 2016
2. Dec 21, 2016

### Replusz

They are attached by a string to C, meaning that they cannot get further from C. But because they would slide to the other side, they cannot get closer to C. Thus, they will be equally far from C. This means that they dont accelerate.

3. Dec 21, 2016

why being equally far from C means that they don't accelerate , they coud be equally far but their speed is increasing so accelerating .

4. Dec 21, 2016

### Replusz

But you know that c isnt acclerating. So it goes with a constant velocity (v) at any time. But B and A have to have the same velocity at any time, because as you said:
So A and B have to have v at any time. But v is constant => no acceleration

5. Dec 21, 2016

does it means that A and B have the same velocity as C ?

6. Dec 21, 2016

### Replusz

yes. do you understand why?

7. Dec 21, 2016

truthly no , because there is also friction that acts on A and B , so how could they have the same velocity ?

8. Dec 21, 2016

### Replusz

okay. so:
if you have a train, with coaches connected to each other with "ropes", all coaches have the same velocity, right?

9. Dec 21, 2016

### Replusz

A and B cannot be slower because the rope is inextensible, right?

10. Dec 21, 2016

yes ,sure .

11. Dec 21, 2016

### Replusz

okay, lets do this in private convreastion. i sent you messages

12. Dec 28, 2016

### Staff: Mentor

Be aware that Physics Forum rules don't allow homework related technical discussions to be moved to private messages. The purpose of the forums is to allow others to benefit from and contribute to the discussion.