First slide

Momentum and Radiation pressure

Question

Light with energy flux of 24 Wm^–2 is incident on a well polished disc of radius 3.5 cm for one hour. The momentum transferred to the disc is

Moderate

A

1.1 $μ$ kg ms^-1
B

2.2 $μ$ kg ms^-1
C

3.3 $μ$ kg ms^-1
D

4.4 $μ$ kg ms^-1

Solution

Total energy incident
U = flux × Area × time
$\begin{array}{rcl} = & 24 x π {(3.5 x 10^{- 2})}^{2} x 60 x 60 \\ = & 332.34 \\ p & = & \frac{2 U}{C} \\ = & \frac{2 x 332.34}{3 x 10^{8}} \\ = & 2.2 x 10^{- 6} kg {ms}^{- 1} \\ = & 2.2 μ kg {ms}^{- 1} \end{array}$

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