# B Weight of pulley with mass attached to it.

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1. Sep 12, 2016

### donaldparida

Suppose there is a pulley(of negligible mass) and a string(of negligible mass) passes through the groove and two equal masses are attached to each end of the string. Now, if the whole arrangement is attached on a spring balance what will be the reading of the spring balance. Will it be zero?

2. Sep 12, 2016

### jbriggs444

This spring balance is supporting the pulley and the pulley is supporting the string supporting the masses? What do you think?

3. Sep 12, 2016

### BvU

If it is zero, you can as well take away the connection attaching it to the spring balance, right ?

4. Sep 12, 2016

### donaldparida

I think that the reading will be zero since the weights are equal and will thus neutralize each other but then it can also be argued that the spring balance measures the weight of the object attached to it and so it will show the sum of the two weights(since the pulley and the string have negligible mass).

5. Sep 12, 2016

### jbriggs444

You may want to draw some free body diagrams. What is the net force on the pulley due to the two masses hanging from it?

6. Sep 12, 2016

### BvU

Suppose you stand on a scale and it reads 98 kg (mine does). Someone hands you two full shopping bags of 6 kg each. One in each hand or both in one hand: would it make a difference ? What would the scale read ?

7. Sep 12, 2016

### donaldparida

BvU said:
@BvU what do you mean by this?

8. Sep 12, 2016

### BvU

If the spring balance shows zero, it does not exert any force and can therefore be taken away without influencing the remainder: the pulley would remain in place !?!?!

9. Sep 12, 2016

### donaldparida

So in conclusion the weight shown by the spring balance will not be zero but the sum of the weights suspended at the two ends of the string passing through the pulley.Right?

10. Sep 12, 2016

### BvU

Any other possibilities ?
Convinced ?

Now the same question but with two different masses, $m$ and $3m$ ?

11. Sep 12, 2016

@BvU