A 300 mega-watt microwave beam (traveling at 3×108m/s) bounces off of a network of carbon fibres floating in space (a 'star-wisp'). The force on the network is
(A) 3×10-8N
(B) 6×10-8N
(C) 1N.
(D) 2N


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A 300 mega-watt microwave beam (traveling at 3×108m/s) bounces off of a network of carbon fibres floating in space (a 'star-wisp'). The force on the network is
(A) 3×10-8N

--- No. Use the formula for the momentum of photons of energy E.

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A 300 mega-watt microwave beam (traveling at 3×108m/s) bounces off of a network of carbon fibres floating in space (a 'star-wisp'). The force on the network is
(B) 6×10-8N --- No.

Use the formula for the momentum of photons of energy E.

Try again.
Questions --- What this question is about
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A 300 mega-watt microwave beam (traveling at 3×108m/s) bounces off of a network of carbon fibres floating in space (a 'star-wisp'). The force on the network is
(C) 1N. --- No.

Remember that the waves bounce.
What is their momentum change?

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A 300 mega-watt microwave beam (traveling at 3×108m/s) bounces off of a network of carbon fibres floating in space (a 'star-wisp'). The force on the network is
(D) 2N --- Yes.

Force = momentum change in 1 sec.
= 2(E/c)/(1s) = 2×(3×108J/3×108m/s)/(1s)

Check the other answers.
Questions --- What this question is about
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