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Homework Help: Confused by Forces!

  1. Nov 30, 2005 #1
    Hey All,
    This is my first post and I'm glad I joined. It looks like a lot of good advice is given around here. Anyway, on to my problem:

    If only an external force can change the velocity of a body, how can the internal force of the brakes bring a car to rest.

    This question totally stumps me. I'm not really sure what I am supposed to be doing at all. I am not sure exactly what forces are being taken into account, and I'm not sure what my answer is supposed to be. I'm pretty lost. I'm not necessarily looking for an answer, as much as I am looking for a little bit of explanation or clarification.
  2. jcsd
  3. Nov 30, 2005 #2


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    First of all, can you figure out what force causes the car to slow down?

    If the road was very slippery and you hit the brakes, will the car slow down faster or slower when compared to a normal road?
    Last edited: Nov 30, 2005
  4. Nov 30, 2005 #3


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    I like that question. It raises a couple of other questions:

    Internal to what? Are the brakes internal to the object they're acting on?

    Once the brakes act on the tires, is the resulting interaction internal or external?

    Edit: By the way, a better definition of internal and external is what the object interacts with, not its physical location. For example, electromagnetic torquing coils in a satellite might physically be located inside the satellite body, but they interact with the external magnetic field surrounding the Earth, making them an external actuator vs. an internal actuator.
    Last edited: Nov 30, 2005
  5. Nov 30, 2005 #4
    well the car has many forces acting upon it, in all actuallity it is not the car that stops itself, it is the friction on the road. think of it like this, if the car was in space and breaked would it stop? no because the friction on the road stops it, and by external force they mean one thats not in the scope of the problem, such as a gust of wind, or a tennis ball that hits it, ect....
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