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1. Nov 11, 2015

### ajaysabarish

in string pulley systems(like atwood machine),the string applies a force on pulley of force 2T downwards,but why is it so?

why should it apply that force?
explain in detail

2. Nov 11, 2015

### Dr. Courtney

Think about net force. The force of the string on the pulley is -T on the left side and another -T on the right side. The total contribution of the string to the net force is -2T.

3. Nov 11, 2015

### ajaysabarish

thank you very much sir for replying,but please explain why should string apply that force T on the pulley on each side.please explain this part in detail sire.

4. Nov 11, 2015

### Dr. Courtney

Draw the free body diagram.

5. Nov 11, 2015

### ajaysabarish

sir,unable to do that.i understand that the clamp connecting ceiling and pulley exerts some force but i couldn't understand the direction of force and why should that force be equal to 2T.sir,i would be really grateful to you if you can please attach it here,i am struggling with this doubt for the past 2 days,thinking of nothing else.

6. Nov 11, 2015

### Chandra Prayaga

OK. So, let us go at it step by step. Call the two masses hanging from the ends of the string as m1 and m2. Concentrate on one mass, say m1.

1. There are two forces on m1. The weight, downwards, and the tension in the string, upwards. Agreed?
2. Since the string is pulling m1 up with the tension T, m1 is pulling down on the string with the same force T. Agreed?
3. The tension in the string is the same throughout. (This assumes that the string is massless, and it does not stretch). So the string is pulling up on m2 on the other side, with the same force T. Agreed?
4. So m2 is also pulling down on the string with the force T. Agreed?
5. So there are two forces on the string, each equal to T, and pointing downwards. Agreed?

Let me know if these steps are clear, and if you can continue on your own from here

7. Nov 11, 2015

### ajaysabarish

thank you very much sir,for replying i understand all those steps,please proceed on like this.