Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Physics momentum problem

  1. Aug 14, 2010 #1
    1. The problem statement, all variables and given/known data

    Small rockets are used to make small adjustments in the speed of satellites. One such rocket has a thrust of 35 N. If it is fired to change the velocity of a 72 000 kg spacecraft by 63 cm/s, how long should it be fired?


    2. Relevant equations

    Momentum = Mass x Velocity


    3. The attempt at a solution

    After reading the instructions, I'm still having difficulty on this particular problem.

    There is a formala called pbefore = pafter

    Where momentum in one object transfers to another when hit. But I just don't know where to begin.

    Thanks.
     
  2. jcsd
  3. Aug 14, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Hint: Consider the impulse delivered by the force. (Look it up!)
     
  4. Aug 14, 2010 #3
    Thanks but I don't know what that has to do with my problem. Can you post an explanation of the formula or the solution?
     
  5. Aug 14, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    What's the definition of impulse and what does it have to do with momentum? Look it up!
     
  6. Aug 14, 2010 #5
    How am I supposed to calculate impulse with the information given?
     
  7. Aug 15, 2010 #6

    Doc Al

    User Avatar

    Staff: Mentor

    What's the definition of impulse?
     
  8. Aug 15, 2010 #7
    Iv'e gotten this solution:


    F = (mass x velocity) / time
    35N = 72,000 x 0.63 / t
    35N = 45,360 kg m/s / t
    35t = 45,360kg m/s
    T = 1296 seconds

    Is it right?
     
  9. Aug 15, 2010 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Yes, very good.

    Impulse = FΔt

    And impulse equals the change in momentum, Δ(mv).

    So:
    FΔt = Δ(mv) = mΔv
    Δt = mΔv/F
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook