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Homework Help: Final velocity, three forces, time, mass and velocity given

  1. Nov 23, 2011 #1
    1. The problem statement, all variables and given/known data
    A small robotic spacecraft (m= 145kg) is initially moving in the +x-direction at .150 m/s. Then, its x-axis thruster exerts the following force on the spacecraft:
    Fx(t) = -5.00 N during 0s<t<1.5s
    -9.00 N during 1.5s< t< 3.25s
    -6.00 N during 3.25s <t< 4.00s
    When the thruster stops firing at t=4.00s, what is the spacecraft's final velocity?

    2. Relevant equations

    v= mt2

    3. The attempt at a solution

    well...don't you have to find the displacement/"?
  2. jcsd
  3. Nov 23, 2011 #2
    Well, you can find the deceleration of eachtime interval with Newton's second law, then go from there.
  4. Nov 23, 2011 #3


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    Science Advisor

    What??? where did you get this? It doesn't even have the right units: v is m/sec and mt^2 (which I guess you mean) is in kg sec^2. I think you meant F= ma. And then v= v0+ at.

    No, you don't. It is not asked for and you don't need it to find the velocity, Use F= ma to find a and then use v= v0+ at to find t at the end of each time interval. v0 is the speed at the beginning of each time interval.
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