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Mechanical Properties Of Fluids

Question
CBSEENPH11019241

A particle is in equilibrium at O. When it is displaced from O, it experiences a force directly proportional to displacement and directed towards O. Explain the motion of particle.

Solution
Consider a particle of mass m placed at O.
Let it be displaced from O by OB = r.
When the particle is released, it will experience a force F = – kr and starts accelerating towards O.
          
Let at any instant, the particle be at x from O.
Let the acceleration of the particle be a and velocity be v. 
straight a space equals space minus kx over straight m space

and space straight v space equals space square root of straight k over straight m left parenthesis straight r squared minus straight x squared right parenthesis end root space
When space particle space reaches space straight O comma space

Force comma space straight f space equals space minus kx space equals space minus space straight k space left parenthesis 0 right parenthesis space equals 0 space

Velocity comma space straight v space equals space square root of straight k over straight m left parenthesis straight r squared minus 0 squared right parenthesis end root space equals space square root of straight k over straight m end root space straight r space
That is, the force acting on the particle is zero but velocity is maximum.
Because of inertia, it will not stop at O, but continue to move in the same direction, i.e. right towards A. 
As the particle escapes from O towards right, it experiences the force in the left direction and hence retards.
Ultimately the particle will stop at A.
Restoring force acts at point A and the particle starts accelerating towards left. 
When the particle reaches O, force on it will be 0 and velocity will be maximum. 
Due to inertia, it will again overshoot O, and due to resorting force it retards and comes to rest at B. Thus, the particle oscillates between B and A.