# What is the reading of the following spring balance?

I am not a physics expert, but I think you have a trick question here.

I think the answer is c) 10 kg

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Look at the diagram and the paragraphs that follow "The Purpose of Pulleys".  That's a similar situation to what you have on this table only doubled.

Presuming that the pulleys and ropes are massless, frictionless, and do not stretch, you know that the force applied to each rope of the pulley is equal on both sides and that the sum of these two forces is equal to the force that it will apply to your spring balance.

You know that each side of the rope on each pulley is exerting a force of (5kg X g).

Most spring balances that read in kilograms actually assume that you are operating at earth surface gravity as kg is a unit of mass, not force.  Newtons would be more accurate, but the world works the way that it does, not the way I think it should ...

So, call it 5kg and ignore the acceleration part of the F=ma equation for now.

Each side of the rope is pulling 5 kg.  There are two pulleys, so there should be 5kg + 5kg pulling to the left and 5kg + 5 kg pulling from the right.

Equal and opposite forces balance one another, so really this would be no different if one side of the spring balance were attached to an immovable object rather than a weight and pulley.

Thus, my answer would be 10 kg showing on the spring balance.
thanked the writer.