# How is the tension equal in both sides of the string

1. Dec 11, 2013

### Miraj Kayastha

How is the tension equal in both sides of the string, if the string is over a pulley with two different masses attached to its sides?

2. Dec 11, 2013

### A.T.

The smaller mass accelerates up, which requires more force than the just the weight. On the other side its the opposite.

3. Dec 11, 2013

### Miraj Kayastha

Can you explain what you mean by "On the other side its the opposite."

4. Dec 11, 2013

### A.T.

The greater mass accelerates down, which requires less force in the rope than the weight.

5. Dec 11, 2013

### sophiecentaur

The same force acts on both masses but they are not in equilibrium so there is acceleration. If the pulley is massless and frictionless, there can be no extra force on one side or the other of the string length - so the forces must be equal. The force on the pulley will be less than the sum of the weights because the CM of the two masses is accelerating downwards.

6. Dec 12, 2013

### Miraj Kayastha

I did not get you, can you explain in a different way plz

7. Dec 12, 2013

### sophiecentaur

How's this?
If there were two rockets, pulling at either end of a string then would not the tension be the same in either direction and all along the ideal string? (That's the basis for all the Mechanics we ever use.)
Merely putting a pulley in the way and re-directing the forces, there is nothing to change that. The force on the pulley will be twice the tension because the pulley is not accelerating and must be in equilibrium.

8. Dec 13, 2015

### sumanprakasam

i want to know whether it is an assumption or not

9. Dec 13, 2015

### DrGreg

The assumption is: