• Support PF! Buy your school textbooks, materials and every day products Here!

Problem of photon torpedo

  • Thread starter Delzac
  • Start date
389
0
1. Homework Statement
A star-wars laser satellite, whose mass is 5000 kg, is orbiting Earth at a speed of 10.0 km/s. It launches a photon torpedo at an enemy missile which is directly in front of it. The photon torpedo contains [tex]10^3^3[/tex] photons, each having a wavelength of 200nm. What is the speed of the weapons satellite just after the launch?


2. Homework Equations
Conservation Of Momentum(?)(COM)
[tex]K_m_a_x = hf - \varphi[/tex]
[tex]KE = \gamma mc^2 - mc^2[/tex]
[tex]p = E/c[/tex]
[tex]v = f\lambda[/tex]
3. The Attempt at a Solution

Since we have [tex]\lambda = 200nm[/tex] we can then obtain f = 1.5*10^15 Hz.

However, from here on, i am lost. How do we use the number of photons and equate it in to [tex]p = E/c[/tex] and finally COM. And then there is also a problem of E which i am unable to find since i don't have h, [tex]K_m_a_x = hf - \varphi[/tex].

Any help will be appreciated. Thanks
 

Answers and Replies

3
0
This feels like the typical momentum problem involving light, but using sci-fi scale numbers. In these cases, there are a few things to take into consideration:
-conservation of momentum
-conservation of energy

As for the relevant equations, there may have been some confusion on what [tex]K_m_a_x = hf - \varphi[/tex], the photoelectric work function, is used for.
There is also another equation that can be used to find the energy of the photon, not involving [tex]p[/tex]. And h is Plank's constant. Take another look at the possibilities. =)
 
389
0
Well i have calculated i out heres what i have obtain :

[tex]p = \frac{h}{\lambda} * 10^3^3[/tex] Since no. of photon is [tex]10^3^3[/tex]

Then using Conservation of momentum, i get :

[tex]mv = p_p_h_o_t_o_n[/tex]
Which works v out to about 662.6 m/s
The speed of satellite is therefore 10,000 - 662.6 = 9337.4 m/s

Can anyone confirm my result?
 
Meir Achuz
Science Advisor
Homework Helper
Gold Member
2,169
63
What you do seems correct, but the answer should be to 2 sf, so
\delta v=660 and v_sat=9,300.
 
389
0
Thanks for the help
 

Related Threads for: Problem of photon torpedo

Replies
1
Views
11K
  • Last Post
Replies
6
Views
4K
  • Last Post
Replies
1
Views
858
Replies
1
Views
4K
  • Last Post
Replies
3
Views
4K
  • Last Post
Replies
8
Views
4K
  • Last Post
Replies
5
Views
4K
  • Last Post
Replies
1
Views
2K
Top