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Newton's Second Law - Net Force and Constant Acceleration

  1. Feb 24, 2015 #1
    1. A 92 kg water skier floating in a lake is pulled from rest to a speed of 12 meters/second in a distance of 25 meters. What is the net force exerted on the skier, with the assumption that his acceleration is constant?


    2. Force = Mass x Acceleration


    3. As far as I know, from using the equation from #2, I did 92kg x 12m/s, and got 1.104k Newtons.
     
  2. jcsd
  3. Feb 24, 2015 #2

    BvU

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    Hello guy, welcome to PF :smile:

    As far as I know is not far enough: if you compare the dimensions you'll see that you don't get Newtons (kgm/s2) this way, but kgm/s -- which is the dimension of momentum.

    So you'll need some more equations to find something with the dimension of time to get this in order. How long does it take to cover 25 m with constant acceleration and then have a speed of 12 m/s ?
     
  4. Feb 24, 2015 #3
    Hi!

    If I want to solve a problem in Dynamics, the first thing I do is to write down Newton's Second Law. You stated it right: F = ma
    The two things we do not know here are the force, which is the solution of the exercise, and the acceleration which we have to express in another way, because we have two other data we didn't use up: the change of speed and the distance. Try to think about the definition of the acceleration (change of velocity over unit of time). We need an equation that links the distance and the time in function of the acceleration.
     
  5. Feb 24, 2015 #4
    I got approximately 2.08 seconds.
     
  6. Feb 25, 2015 #5

    BvU

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    2.08333 s is a result. What is the calculation that gave you that result ? Because I get something else...

    Suppose you have found this t. Then how do you calculate the net force exerted on the poor skier ? (I mean an equation with variable names, not a numerical result)
     
  7. Feb 25, 2015 #6
    In answer to the first question, I took the distance (25m) and divided it by the speed (12m/s); as for the second question: Fnet = F1 + F2.
     
  8. Feb 25, 2015 #7

    BvU

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    The speed isn't 12 m/s ! It clearly says the skier is accelerated with a constant acceleration from rest to 12 m/s.
    RE the equation: FNET is supposed to be the answer for the exercise, but what are F1 and F2 ?
     
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