Question

Light with an energy flux of 18 W//cm^2 falls on a non-reflecting surface at normal incidence. If the surface has an area of 20 cm^2, find the average force exerted on the surface during a 30 minute time span.

Answer 1

The average force exerted on the surface during a 30 minute time span is

Explanation:

Given :

Energy flux $I = 18$ $\frac{W}{cm^{2} }$

Area $A = 20$ $cm^{2}$

Average time $\Delta t = 30$ min = 1800 sec

From the force equation,

 $F = \frac{\Delta P}{\Delta t}$

Where $\Delta P =$ change in momentum,

Momentum is given by $\Delta P = \frac{E}{c}$

∴ $F = \frac{E}{c \Delta t}$

From power formula $P = \frac{E}{\Delta t}$

  $P = I A$

  $P = 360$ W

  $F = \frac{P}{c}$

  $F = \frac{360}{3 \times 10^{8} }$                     ( ∵ $c = 3 \times 10^{8} \frac{m}{s}$ )

  $F = 1.2 \times 10^{-6}$ N

Therefore, the average force exerted on surface is $1.2 \times 10^{-6}$ N