# Homework Help: Time period of oscillation

1. Apr 16, 2014

### Tanya Sharma

1. The problem statement, all variables and given/known data

A small plate of mass m is suspended by a light string of length l .A monochromatic light beam starts falling on it and is completely absorbed. If the energy falling on the plate per unit time is W, then find the time period of oscillation?

2. Relevant equations

3. The attempt at a solution

I feel the light falling on the disk exerts some force on the disk .If it is a constant force then the equilibrium position is shifted.

But I am unable to relate the energy falling on the plate with the force it exerts.

I would be grateful if someone could help me with the problem.

#### Attached Files:

• ###### shm.GIF
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2. Apr 17, 2014

### Saitama

Suppose $n$ photons strike the plate per second. Can you work out the change in momentum of these photons and the force acting on the plate?

Do you know of a relation between energy and number of photons?

You should be able to work out the force acting due to photons with the two equations you get.

3. Apr 17, 2014

### ehild

If you have found the average force the light exerts, find the equilibrium position of the plate in terms of W and mg, then the net force on the plate when it is displaced from that equilibrium position.

ehild

4. Apr 17, 2014

### Tanya Sharma

Thank you Pranav...

Thanks ehild...