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Car pulls a trailer of weight

  1. May 4, 2012 #1
    EDIT: I JUST REALISED YOU CAN'T DO 1 WORD THREAD TITLES, SORRY ! WON'T DO IT AGAIN!

    A car pulls a trailer of weight 2500 N with a force of 20 N for a distance of 8 km along
    a horizontal road.

    How much work is done by the car in pulling the trailer?
    A 160 J
    B 20 000 J
    C 160 000 J
    D 20 000 000 J

    I know work done = force x displacement

    so it should be 20 x 8000 = 160,000 J (so the answer is C)

    But what I am confused about is surely the weight of the trailer will affect how much work the car does in pulling it? I mean for instance if the trailer was 5 N, shouldn't the work done be MUCH less .. BTW can you please try explain in simple terms since I am not that advanced at Physics as you guys.

    Cheers :)
     
  2. jcsd
  3. May 4, 2012 #2

    tiny-tim

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    hi influx! :smile:
    work done = change in energy

    if you applied the same force for the same distance, the two trailers' increased kinetic energy would be the same

    but the heavier trailer's speed would have increased less :wink:
     
  4. May 4, 2012 #3
    Hi :)

    Oh! Never knew that! When you say change in energy, what exactly do you mean? And does the speed increase less because kinetic energy = 1/2(mv2) , so as the value for 'm' increases, the value of 'v' decreases?

    Cheers
     
  5. May 4, 2012 #4

    tiny-tim

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    if the speed goes from u to v, then the change in energy (on a horizontal road) is 1/2mv2 - 1/2mu2

    if that is the same for two masses, the one with larger m must have smaller (v2 - u2)
     
  6. May 4, 2012 #5
    Who says that the 20N force is increasing the KE of the trailer, and not just overcoming air resistance?
     
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