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I have a chair like this(shown in figure). I was kneeling on the part where we sit. I just gave a sudden push to the part where we rest the back, without moving my legs. I found that the chair is moving little in the direction I applied the force. I couldn't find any reason why this chair would move forward because I am inside this system and no external force is coming from outside. When I apply the force on it , equal amount of force I am applying there on the seat in the opposite direction, so there should not be any movement.

I tried this experiment again right now. Now holding on the handles of the chair. Did the same thing. Again found the chair moving.

Where I am wrong and how does the chair move?


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Cool, I am just sitting on an almost identical one. ;-) google.cz/… –  Luboš Motl Jan 10 '13 at 18:53
@LubošMotl: Did you try above experiment? –  Inquisitive Jan 10 '13 at 18:54
Sure, it's just momentum conservation, isn't it? Or, a better law: the conservation of the horizontal location of the center-of-mass of the whole system chair+you. So when your body moves in one direction, the chair moves in the opposite direction. However, all these things are modified by friction - which also differs depending on the weight on each wheel, and so on. –  Luboš Motl Jan 10 '13 at 19:04
I am sitting on exactly the same kind of a chair... –  Dimensio1n0 Jul 6 '13 at 16:13

2 Answers 2

up vote 3 down vote accepted

In free space, this would be impossible, but there's friction, so if you impulse the chair forward, by moving yourself a little back (by conservation of momentum the chair must move forward), and then you get back to you initial position, the chair will move back again, but there's friction and energy is being lost during the process so the distances will be smaller and the chair, at the end, will be a little bit moved forward.

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Well , brother, lets assume that you are on a frictionless surface along with your chair. Now , lets consider the chair as a system. Now when you applied the force , you are a different system , hence your force is an external one. This external force moves the chair ,all according to newton's laws. Now your question about being inside a system..... as a whole if you think yourself and the chair as a system you would have moved due to the reaction force from the chair, which I guess you didn't because frictionless surfaces are hypothetical. If you were on a friction-less surface both you and the chair would have moved due to mutual action and reaction but you center of mass would not have moved. I guess you are thinking internal force force as something from inside its body. Well, any force which from something not lying on your system is an external force, like in case of your chair, your body (as it is a different system) is another system exerting the external force, however the force that the molecules of the chair exert on each other are internal forces, they can move individually due to their forces.. but as a whole their movement is in such w]a way that the center of mass of the chair does not move,... The motion of the center of mass of the chair is what is required for the translational motion of the chair to be percieved.

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