Question by Dallas Fan: PHYSICS PROBLEM HELP NEEDED ASAP?
I got this problem and I asked the teacher for help so we solved it together and now i have a question.
THis is the problem and my solution is under it .
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. For a 1275-kg spaceship starting from rest and attached to a square sail 24 m wide, how fast could the ship be moving after 1 hour?
(3.84×10^21)(24^2) = 2.21184 x 10 ^ 24 — > number of protons string the sail in 1 sec
P = mv
v = p/m
[( 2 ) ( 1.30×10^-27 ) ( 2.211x 10 ^ 24 ) ]/ 1275 = 4.5087 x 10^ -6
1 hour = ( 4.5087 x 10^-6 ) ( 3600 s ) = 0.01623 m/s
My question is what Why do you have to multiply the the momentum/ mass by 2 .
Best answer:
Answer by kuiperbelt2003
the factor of 2 arises because the force is a result of the change in momentum of the photons, remember that momentum is a vector, so if the direction away from the sun is positive, the direction back toward the sun is negative…and when you find momentum change you get:
change in momentum = final momentum – initial momentum = -p-p=-2p where p is the magnitude of the momentum
hope this makes sense
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