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Very simple but I don't understand how

  1. Feb 15, 2004 #1
    This is not a problem but more of a question, and I just don't understand why this is so...
    There is a book placed on top of a car and the car drives off. What are the two horizontal forces acting on the car?

    Now, I understand that acceleration is one of the forces but, how can garvity be considered a horizontal force??? I thought if it pulls things down it is a vertical force????
    I know that I'm in college but I think nature forgot to provide me with common sense. please help. Thank you.
  2. jcsd
  3. Feb 15, 2004 #2
    Gravity is not a horizontal force, so I do not think that is correct.
    I believe the other horizontal force on the car would be friction. That is, the friction due to the book.

    I figure that the book is going to slide off the back of the car. Really, it is not sliding backward, rather the car is sliding out form under it. No matter how small compared to the acceleration force of the car, the friction is still present.

    What does everyone else think?
  4. Feb 15, 2004 #3
    neglecting weak interactions between small objects, gravity should not act horizontally, it acts only on the y-axis.

    friction is most of what makes a car go anyways...try driving on sheer ice and you'l find out why ;)
  5. Feb 16, 2004 #4


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    But friction is typically a constant (coefficient of friction) times the normal force. In the case of the book sitting on top of the car, the normal force is the book's weight: the gravitational force acting on the book. That may be what is meant by "considering gravity as a horizontal force".

    That would be a force acting on the book in the same direction as the motion of the car. The point is that that force probably would not be as great as the force acting on the car- the book does not accelerate as fast as the car and so "is left behind"- it slides of the back.

    (I once laid a pair of gloves on the back "seat" of a motorcycle and then forgot about them. When I stopped 15 miles later, one glove was still there!)
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