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Homework Help: Constant Power

  1. Nov 24, 2008 #1
    A constant direction force delivers a constant power of 45.0 W to an 8.00-kg mass which is
    initially moving in the direction of the force. At t = 0 s, the force is 5.00 N.
    Calculate the speed of the mass after 4.00 s.

    I've tried to do several different things, however I believe you need to integrate. My intregration skills are really weak, so please don't assume I'll know what you're doing if you skip alot of steps.

  2. jcsd
  3. Nov 24, 2008 #2


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    Welcome to PF.

    You're in luck. You won't need to integrate.

    But you will need to understand what power is.

    Like what units is power expressed in?
  4. Nov 24, 2008 #3
    Kg m^2/s^3

    Few more points would be helpful.

  5. Nov 24, 2008 #4


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    OK and how is power related to Force?

    Power = Force times ...?
  6. Nov 24, 2008 #5
    velocity.. however unfortunately the answer is not 9 m/s. The other information is needed to solve this.. as the force isn't the same throughout.. (it's 5.00 N only at t = 0 s)
  7. Nov 24, 2008 #6


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    You are jumping ahead a bit.

    The 9 m/s is the speed at t=0 however.

    Now think about what is a watt? A Joule/sec perhaps?

    And how many seconds? So how many Joules were added to the energy of the mass? ...

    And how many joules of energy did the mass have at t=0? And added together as Kinetic energy how fast does that mean the mass is going?
  8. Nov 25, 2008 #7
    Ahh thanks alot man.. appreciate it.
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