# College Physics

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Although they don’t have mass, photons—traveling at the speed of light—have momentum. Space travel experts have thought of capitalizing on this fact by constructing solar sails—large sheets of material that would work by reflecting photons. Since the momentum of the photon would be reversed, an impulse would be exerted on it by the solar sail, and—by Newton’s Third Law—an impulse would also be exerted on the sail, providing a force. In space near the Earth, about 3.84·1021 photons are incident per square meter per second. On average, the momentum of each photon is 1.30·10–27 kg m/s. Assume that we have a 1291 kg spaceship starting from rest attached to a square sail 27.9 m wide.
a) How fast could the ship be moving after 45 days? b) How many months would it take the ship to attain a speed of 7.75 km/s, roughly the speed of the space shuttle in orbit?

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