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Homework Help: Weather Rocket Problem

  1. Mar 29, 2008 #1
    1. The problem statement, all variables and given/known data
    A small rocket to gather weather data is launched straight up. Several seconds into the flight, its velocity is 120 m/s and it is accelerating at 18 m/s^2. At this instant, the rocket's mass is 48 kg and it is losing mass at the rate of .5 kg/s as it burns fuel. What is the net force on the rocket? Hint: Newton's second law was presented in a new form in this chapter

    2. Relevant equations
    p=mv F=dmv/dt F=dp/dt

    3. The attempt at a solution

    I tried to come up with a formula for momentum with respect to time and then differentiate it to find force, but it's not making sense to me.

    I tried to say that at this instant dm/dt= -.5 and dv/dt = 18 and then I tried to multiply those but that clearly is not correct reasoning. I'm pretty sure I'm not looking at this problem right. Any hints?
    Last edited: Mar 29, 2008
  2. jcsd
  3. Mar 29, 2008 #2


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    Welcome to PF!

    Hi bcjochim07 ! Welcome to PF! :smile:

    A hint? :rolleyes: … yes! …

    Product rule: d(mv)/dt = mdv/dt + vdm/dt. :smile:
  4. Mar 29, 2008 #3
    Oh yes, I see. Thank you very much!
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