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A laser device of power 3 mW and mass 50 g emits photons of wavelength 500 nm while floating in gravity-free space. It fires continuously for exactly one hour. What is the approximate distance the device travels due to recoil during this time?

1. 1.3 mm
2. 2.1 micro-m
3. 1.3 micro-m
4. 2.1 mm

Correct answer: 1.3 mm

Solution

The laser imparts a recoil force F = P/c on the device (by Newton's third law). With constant force on mass m in free space, the acceleration a = P/(m*c) is constant, and the device travels distance s = (1/2)*a*t² in time t = 1 hour.

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