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Homework Help: Car Force question

  1. May 23, 2015 #1


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    A car whose mass is 1000 kg is travelling at a constant speed of 10ms-1. Neglecting any friction, how much force will the engine have to supply to keep going the same speed?

    F = ma

    Since it is travelling at a constant speed then acceleration = 0 ms-2 hence the answer is 0.
    F = m x a
    F = 1000 x 0
    F = 0

    My question is:
    Firstly am I right?

    And secondly why does everything have to be accelerating? If the car was travelling at a constant speed of 10ms-1 and there was friction how would you calculate the force needed?
  2. jcsd
  3. May 23, 2015 #2
    If the car is traveling in a straight line without going up hill or anything like that, then yes, the force is zero. If there was friction, the force would have to be equal to force friction, so that the ΣF=0.
  4. May 23, 2015 #3


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    Ok thanks
  5. May 24, 2015 #4


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    Science Advisor
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    To be clear, friction between tyres and road only slows a car down when the car is braking. In the real world, a car travelling at constant speed on the flat needs power to overcome drag, axle friction and rolling resistance. Rolling resistance is the force required to deform the tyres as they rotate.
  6. May 24, 2015 #5
    It"s right if you are in space no friction .
  7. May 24, 2015 #6
    Uh, there is friction in space..
  8. May 24, 2015 #7
    There's just a lot less air resistance or drag force
  9. May 25, 2015 #8
    If there is friction, and you want to keep the car travelling at constant speed, the force going forward would have to be equal to the retarding force of the friction (to balance it out). But yes, your answer is correct :)
  10. May 25, 2015 #9


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    Thanks everyone
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