1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding time with force, mass and velocity

  1. Nov 22, 2012 #1
    1. The problem statement, all variables and given/known data

    Small rockets are used to make tiny adjustments in the speeds of satellites. One such rocket has a thrust of 35 N. If it is fired to change the velocity of a 71700 kg spacecraft by 66 cm/s, how long should it be fired?

    2. Relevant equations

    p = mv
    Impulse = FΔt

    3. The attempt at a solution

    Well I think that you might have to find momentum and then plug that into another equation. I'm just not sure what equation that would be since the impulse = FΔt doesn't use momentum.
     
  2. jcsd
  3. Nov 22, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Look up the 'impulse-momentum' theorem.

    Start here: The Impulse-Momentum Change Theorem
     
  4. Nov 22, 2012 #3
    So that would make it:

    F Δt = Δ p

    So that makes that

    Δt = Δp/ F
    Δt = 4732200 - 0 / 35
    Δt = 135205.7

    But now how do I get rid of the delta?
     
  5. Nov 22, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Right.

    Careful with units. Convert the speed to standard units of m/s.

    Δt is what you are solving for. (You don't need to get rid of the delta; Δt is the time that the thrust must be applied.)
     
  6. Nov 22, 2012 #5
    So is it in cm/s now because that's what it says in the problem? And then do I have to change it to minutes because that's what it asks for next to the answer blank. (Sorry forgot to put that in.)
     
  7. Nov 22, 2012 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Yes. You need to convert that to m/s.
    Exactly. The equation will give you the time in standard units of seconds. If you need the answer in minutes, you'll have to convert units.
     
  8. Nov 22, 2012 #7
    Awesome! Thanks :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Finding time with force, mass and velocity
Loading...