Basic ques but getting confused

Feb 2013
8
0
Just a simple Pulley question to start with -
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 because if you see the torque equation both masses havesame torque pulling it down.
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 :)
 

ChipB

PHF Helper
Jun 2010
2,361
289
Morristown, NJ USA
You are correct that the two masses will remain at their initial un-equal levels, if that's where they start from and no other forces are applied. They do not come to equilibrium. This is why when using a balancing scale you have to adjust the two sides to be level, then let go and see if they stay level. If you start with them uneven, and the masses are identical, they will stay uneven.
 
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Feb 2013
8
0
Thanks ChipB for the reply
I have checked it myself by putting equal mass on a weighing balance used in shops and if i lower one of them , it'll rise up immediately and will become even after few oscillations.
Did i mess up there ? Should i be out more ?
 

ChipB

PHF Helper
Jun 2010
2,361
289
Morristown, NJ USA
Can you post a photo of the balancing scale that you are using? Does it look like the attached? It's important that the pans on each side are free to rotate relative to the cross-arm. Notice how the pic shpows the balance tilted.

 
Feb 2013
8
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I saw that in a regular pan balance at shops.
Another thing is that when the weight is balanced on both sides , and we gently tap on one side pushing it down , The beam balance would do SHM , so there is a restoring force in the system and that force I am searching.
For reference check this video or you can find other videos on youtube showing beam balance calibration.
https://www.youtube.com/watch?v=UY-jDO58vSs
 
Feb 2013
8
0
Hi ChipB,

I think I got the answer. The balance should not move to a horizontal position ideally but it does because it is designed in such a way .
http://physics.stackexchange.com/questions/13474/why-does-the-weighing-balance-restore-when-tilted-and-released
Thanks a lot! :)
For many years I have quizzed people over the pulley version of this ques and almost all failed to answer but then i myself got confused when i thought about the pan balance version of it. Still I continued to ask both version of it (after someone gets the pulley version right, just to messup his mind ;) ) as a tea time discussion topic :p
Finally peace has been restored :)
 
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ChipB

PHF Helper
Jun 2010
2,361
289
Morristown, NJ USA
The balance in the video has a pointer that hangs down and is tilted when the beam is tilted. Its weight causes a restoring torque to the mechanism. Also, as explained in the other link, it seems the pivot point for this balance is located above the cross beam, so the center of mass of the beam is below the pivot, and hence when the beam is tiled there is a restoring torque (like a pendulum).