Why do both Block A and B have a zero acceleration in this scenario?

[mamadou]

1. Homework Statement
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 ?

 

[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 don't accelerate.

 

[mamadou]

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

 

[Replusz]

they could be equally far but their speed is increasing so accelerating

But you know that c isn't 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:

being equally far from C.

So A and B have to have v at any time. But v is constant => no acceleration

 

[mamadou]

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

 

[Replusz]

yes. do you understand why?

 

[mamadou]

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

 

[Replusz]

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

 

[Replusz]

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

 

[mamadou]

yes ,sure .

 

[Replusz]

okay, let's do this in private convreastion. i sent you messages

 

[gneill]

okay, let's do this in private convreastion. i sent you messages

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