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Homework Help: Simple, but confusing question about Newton's laws

  1. Nov 1, 2006 #1
    Ok, if you need to determine a tension in a string that is being pulled by a human that is on a sled (the sled is accelerating outwards). Is the normal force the two weights combined??

    I have Friction force + (- Tension)= ma

    But for the friction force I don't know whether or not I need to find the human's normal force or the sled or both.

    The human isn't moving, the sled is pulling away ---> but he is holding onto a string attatched to a wall

    l------O The O is the human and he is standing on the sled.

  2. jcsd
  3. Nov 1, 2006 #2
    It depends what you're considering to be your system. If it's just the human, then the normal force is applied by the sled on the human. If the system is the sled and the human, then the normal force is the force applied by the earth on the sled. Remember that Newton's laws don't apply when the relative body is itself accelerating.
  4. Nov 1, 2006 #3
    So which one do I use? Do I add their weights together to get the normal force? Only the human is holding onto the string, but he is not moving. The sled is accelerating from below him, he is standing on the sled and resisting the motion because he is holding the string. =/
  5. Nov 1, 2006 #4


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    For stacked objects in general there are multiple normal forces. Draw a free body diagram for each object in the stack. If there is no vertical acceleration, each object must have zero net force in the vertical direction. As you move lower in the stack, the normal force magnitudes will increase.
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