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Force of acceleration in car?

  1. Jan 9, 2007 #1
    1. The problem statement, all variables and given/known data
    Newton's Second Law of motion states that there must be a net force for a body to have an acceleration and that the acceleration is in the direction of the net force. When you push down on the accelerator of a car it accelerates forward. What force is exerted on the car to accelerate it?

    2. Relevant equations

    3. The attempt at a solution
    The accelerator triggers a device in the car which then triggers something that accelerates the car by exerting a force to something. Does the gear wheel spin faster? What I don't understand is the internal workings of the car. This question may not ask it directly but for a complete answer it would be good to incoprate it in or at least mention aspects of the car.
  2. jcsd
  3. Jan 9, 2007 #2

    Doc Al

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    You don't need to know the inner workings of a car to answer this question. Hint: An external force is required to accelerate an object.
  4. Jan 9, 2007 #3


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    I'm no car mechanic, so the internal workings of a car are beyond me. But as far as what the net force is that accelerates car, does a car accelerate forward , with your foot on the pedal and the car in gear, if it were on a very icy slippery road, especially with an uphill slope? Why or why not?
  5. Jan 9, 2007 #4
    So you are saying the (external) force of my foot accelerates the car. What about the 'literal' force that is accelerating the car? i.e. what makes the gear wheels turn faster?

    Out of interest, I like to know the chain reaction i.e. foot on accelerator => ... ... =>observed acceleration of car
    Last edited: Jan 9, 2007
  6. Jan 9, 2007 #5
    Think about what PhantomJay said about a slippery road. No, the force of your foot does not accelerate the car--that's silly. Think external! something outside the car literally pushes it.
  7. Jan 9, 2007 #6


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    When you press the accelerator, you increase the amount of fuel injected into the engine's cylinders. The burning of the fuel creates gaseous pressure, and the pressure pushes on the pistons. The pistons then push on the crankshaft, resulting in a torque that causes it to spin faster. The crankshaft is ultimately connected to the wheels, and the wheels experiences a torque which causes them to spin faster. The friction of the tires against the pavement resists the torque, and the car experiences a force, equal and opposite that of the friction, that propels it forward.

    - Warren
  8. Jan 9, 2007 #7
    just to clarify what he said, friction is the force that I was trying to get at. Keep in mind that earth has a huge mass--if it did not, a car would go forward and earth would go backwards at the same time.
  9. Jan 9, 2007 #8

    Doc Al

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    The force of your foot is not an external force--you (and your foot) are part of the system being accelerated. As has been explained, the external force that accelerates the car is the friction force of the road on the tires--take away that friction and you can step on the gas all you like and get nowhere. (Of course, how to arrange things to create that external friction force is the trick!)

    Warren explained that perfectly.
  10. Jan 9, 2007 #9
    Ahh, friction is the external force, that makes sense.

    thanks chroot for the explanation of the inner workings of the car. It's amazing how well designed the modern car is. There is an almost instaneous acceleration of the car after pushing down the accelerator.
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