Question

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

Answer 1

Given :

Energy flux = 18 ω/cm²

Surface Area = 20 cm²

Time period (T) = 30 min

To find :

The average force exerted on the surface = ?

Solution :

• Pressure = Force / Area = F/A

       P = (dp/dt) / A

• Momentum (P) = E/c

        P = (1/Ac) × dE/dt

• P =  Flux / c

        P = 18×10⁴ / 3×10⁸

        P = 6×10⁻⁴ N/m²

The average force exerted on the surface is 6×10⁻⁴ N/m²