In my school days, I had been taught that we cannot move a vehicle by applying any force from inside. That is we cannot push the vehicle from inside. But when I was sitting in a scrolling chair, I was able to move it without touching anything other than the chair. Can anyone explain the reason for this?
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$\begingroup$ Does this answer your question? Internal forces in an isolated system $\endgroup$– BioPhysicistCommented Feb 15, 2020 at 13:05
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You are absolutely right about the fact that an internal force cannot cause movement to the system. But your problem does involve an external force - friction. If you were present on the same chair which was present on a frictionless surface the above event will not take place, through conservation of momentum we have learnt that the center of mass of a system will not undergo any change in position without the application of an external force; on a frictionless surface when you lean forward to cause motion the chair would move backwards so that the net movement of the center of mass is zero, But in the above situation friction prevents the chair from doing so and thus you move forward.
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$\begingroup$ I would add the difference between static and dynamic/kinetic/sliding friction. You change your center of mass by moving slowly, such that static friction keeps the chair in place. Then you move quickly in the other direction such that static friction is overcome and the chair moves backwards underneath you (subject to the weaker dynamic friction), similarly to your frictionless case. $\endgroup$– BBeastCommented Feb 15, 2020 at 11:54