# A small doubt on pulley :)

#### smalldoubt

Imagine an ideal pulley with equal masses M hanging on it on each side. So I raise the level of one mass with my hand, so right now , when i am holding one mass, they are at different levels. Now I let go off the mass.
Would they move and try to rebalance to come to same height again ??
My theory say NO .. because both are pulled down my equal Mg force so none would move.
But now if we come to practical case ... Why would a pulley or a weighing balance try to rebalance itself in real life?
What are the forces that make the body at the higher position to come down even though the mass is same on both sides ?
I hope I am clear

#### Unknown008

PHF Hall of Honor
I would suggest that there is some potential energy stored in the pulley as it gets turned in one direction, which is slowly released when the system is left on its own.

The weighing balance I once did for my class never went back to equilibrium once it was touched though :x

#### smalldoubt

But i have seen instances where the weighing balance would come back to same level.

When either side of weighing balance is not loaded , people generally just tap one side down.. that side would go down then come come back up ... oscillate a few times and then settle at equilibrium. Thats why I was confused.
In such case what kinda forces shall i consider ?
May be in ur case you had very little air resistance or friction. Dont know exactly.

And if you say weighing balance also has some potential energy somehow... Ok agreed may b somehow it has.. But still Energy doesnt make a body move ... net force does .. So what forces would be these including friction etc..

#### Unknown008

PHF Hall of Honor
Energy is the ability to do work

W = Fd

If energy was stored, it can be released and act as a force on an object to make it move. It'll be simply kinetic energy. Maybe to a lesser degree, sound and heat, but those would be caused by the opposing frictional force.

#### smalldoubt

I agree that Energy conversion can move a body ... eg a chemical bomb

But then another doubt comes that our original pulley system also had one mass with higher PE .. So that could have moved because of that but it didnt... cause the net force was equal.

This way Energy and external forces would compete with each other to make a body move rather than the different forces acting on it. Like a box kept on a table doesnt move though it has PE cause the net force Mg = N is balanced.
So i am a little confused if Energy can move a body ..or a force ... newton law supports external Force theory. hope you can give me a clear example to resolve this confusion.

And watever the case be .. these energies are stored in ideal case also ... so that system should also try to equalize.

Hope I am not troubling you

#### Unknown008

PHF Hall of Honor
Hmm, well, I think of the pulley like a little spring, which coils when it rotates a bit, and uncoils to regain its initial position.

Like on a rubber table, if you push that box down, and let it go, the box (and table) would move up and down until they are back to their original position.

If this isn't that, then I am really clueless

And not at all! Sometimes thinking a bit out of the box is good, and I love to learn new things, if what I initially thought is proved to be wrong

#### smalldoubt

I think pulley and elastic things(spring, rubber etc) are completely different wrt physics and motion.

A pulley doesnt coil up when it is moved ! u can think it as a chain and sprocket pulley .. like we hav in our bicycles. It will just move without slipping and without coiling . So there wud b no potential energy (not considering mass of chain or rope).

If it was like this then our 2 equal masses ideal system wud also try to rebalance though it might take time. But I think it would not.

And i would try to think how to prove that forces make things move and not energy stored in them. The chemical bomb etc can give me a hard time though
Cause when i pose the ideal pulley question to people they generally say "as the masses hav different PE so they would try to balance it out"
I think that is not correct.

#### Unknown008

PHF Hall of Honor
I believe that everything has some elastic potential, even if the spring constant is not as big as in springs.

Otherwise, no, I don't think that the argument of gravitational potential energy can be used. It really comes down to F = GMm/r^2, and using this, the higher mass would be experiencing less gravitational pull, and hence, the mass below would fall down, pulling the other one up.