Photons and mometum

  • Thread starter thatguy101
  • Start date
  • #1
14
0

Homework Statement


In space near earth, about 3.84*10^21 photons are incident per square meter. On average, the momentum of a photon is 1.3*10^-27. Assume we have a 1205 kg spaceship, and a square sail that is 26.3m wide.
How fast could the ship be travelling after 21 days?


Homework Equations


F*Δt=m*v


The Attempt at a Solution


I started out by finding the force the photons would have on 1 m^2 and then multplied that by 26.3 to get the force on the sail
[itex] (1.30*10^-27)(*3.84*10^+21)*(26.3^2) [/itex]
and got .00345 N
Since F=ma, I divided that by 1205 to get 2.865*10^-6
I then took 21 days and changed it to 1814400 s. So i multiplied 2.865*10^-6 by1814400 to get 5.19914 m/s. Did i go wrong somewhere cause the computer says im wrong.
 

Answers and Replies

  • #2
TSny
Homework Helper
Gold Member
13,068
3,393
Hello.

Is any information given about whether or not the photons are reflected back by the sail?
 
  • #3
Andrew Mason
Science Advisor
Homework Helper
7,662
385

Homework Statement


In space near earth, about 3.84*10^21 photons are incident per square meter. On average, the momentum of a photon is 1.3*10^-27. Assume we have a 1205 kg spaceship, and a square sail that is 26.3m wide.
How fast could the ship be travelling after 21 days?


Homework Equations


F*Δt=m*v


The Attempt at a Solution


I started out by finding the force the photons would have on 1 m^2 and then multplied that by 26.3 to get the force on the sail
[itex] (1.30*10^-27)(*3.84*10^+21)*(26.3^2) [/itex]
Your method is right. It may be a matter of significant figures. Try to work out the solution using symbols and plug in numbers at the end. It makes it easier to follow:

Fsail = dpship/dt= ndpph/dt = σAdpph/dt where σ is the number of photons per unit area

mΔv = FΔt = ΔtσAdpph/dt

AM
 
Last edited:

Related Threads on Photons and mometum

  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
3
Views
7K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
3
Views
1K
Replies
5
Views
17K
  • Last Post
Replies
6
Views
7K
  • Last Post
Replies
4
Views
3K
Replies
14
Views
1K
Top