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Homework Help: [Physical Science] Force

  1. Jan 25, 2005 #1
    I was just hoping someone could check the work I did on this two part problem? :)

    18a. A car has a mass of 1200 kg. What's the weight of the car?

    Answer: 1200 kg*9.8 m/s^2 = 11760 N.

    b. Disregarding friction, what force must the car motor apply to accelerate the car along a level highway at the rate of 4.0 m/s^2?

    Answer: 11760 N/4.0 m/s^2 = 2940 kg.

    Thanks so much in advance!
    Last edited: Jan 25, 2005
  2. jcsd
  3. Jan 25, 2005 #2

    Doc Al

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    No. You need to use F = ma. (Note that force must have the units of force, which is N. Mass (measured in kg) is not a force.)
  4. Jan 25, 2005 #3
    Oh, okay! :) Thank you for helping me.

    So is it 11760 N*4.0 m/s^2 = 47040? I don't understand what the unit is, though - I thought I'd already used N in the problem, so it had to be mass. Does that not matter?
  5. Jan 25, 2005 #4

    Doc Al

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    No! This problem has nothing to do with the weight of the car.

    Use F = ma, which means:
    Force needed to accelerate car = (mass of car) x (acceleration).

    The answer will be in N, the unit of force.
  6. Jan 25, 2005 #5

    1200 kg (mass of car)*4.0 m/s^2 (acceleration) = 4800 N?

  7. Jan 25, 2005 #6


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    Yes, that's right.
  8. Jan 25, 2005 #7
    Wonderful! Thanks, both of you.
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