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Forces of a person on a sled.

  1. Jan 2, 2004 #1
    I'm doing a project on isaac newton for global studies and was trying to find graphics on the internet demonstrating his laws of motion. I came across a website which had a diagram of an object on a totally level surface with a force of n being applied from both the top and bottom and no other forces shown being applied. Then they asked the reader whether or not it was possible for the object to be moving, and I thought of course not, I checked to see what the website said was the answer and they said that an object with equal forces being applied to it from above and below could be in motion. To demonstrate this they showed a picture of a person on a sled who had perviously gone down a hill and was travelling along a straight surface. The website elaborated that the only forces affecting the rider were gravity pulling him towards the earth and the snow pushing back against gravity.

    This instantly seemed wrong to me. Firstly I thought that inertia is a force and must be somehow illustrated. Also, there would be force exerted on him by the air he's displacing, pulling him in all sorts of directions and slowing him. Then there's the drag that the sled would experience due to the snow it's riding on.

    But perhaps they just chose a poor example to demonstrate their point.

    Essentially, my question is; if you have an object on a flat surface and the only forces being exerted on it are the downward force of gravity and the upward force of the surface it's resting on pushing against gravity, could it be moving?

    If an object is moving due to inertia, does it still count as a force?
    Last edited: Jan 2, 2004
  2. jcsd
  3. Jan 2, 2004 #2

    Doc Al

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    Staff: Mentor

    Inertia is not a force. Consider it as a property of mass.

    You are correct that a more realistic situation would consider air resistance and friction. But these forces would tend to slow down the sled, not keep it moving. (They are assuming that these forces are small and can be ignored. A reasonable assumption for the exercise.)

    The example is meant to illustrate that a net force is not required for motion to occur. This is Newton's first law.
    Of course it can.
    No. An object can be said to have inertia, but that isn't a force. (A force requires an agent: something to exert the force.) The point of Newton's laws is that force is not needed to maintain motion. The natural inertia of mass keeps things moving (or not moving) until a force compels it to change its motion (speed up, slow down, or turn).
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