First slide

Momentum and Radiation pressure

Question

Light with an energy flux of 20 W/cm² falls on a non-reflecting surface at normal incidence. If the surface has an area of 30 cm² the total momentum delivered (for complete absorption) during 30 minutes is

Moderate

A

36 × 10^–5 kg m/s
B

36 × 10^–4 kg m/s
C

108 × 10⁴ kg m/s
D

1.08 × 10⁷ kg m/s

Solution

The total energy falling on the surface is
$U = IAt = (\frac{20 W}{{cm}^{2}}) x (30 {cm}^{2}) x (30 x 60) \sec$
= 20 x 30 x 1800 joule
Total momentum delivered (for complete absorption)
$P = \frac{U}{C} = \frac{20 x 30 x 1800}{3 x 10^{8}}$
= 36 x 10^-4 kg m / s

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