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Homework Help: Applying Newton's Laws to general questions.

  1. Oct 2, 2005 #1
    Can any explain the following concepts to me? I am clueless. And these are suppose to be general concepts...

    -Why does a child's wagon seem to fall backaward when you give the wagon a sharp push?

    -If you walk along a log floating on a lake, why does the log move in the opposite direction?

    -If a stone hangs by a fine thread from the ceiling, and a section of the same thread dangles from the bottom of the stone, and someone give a sharp pull on the danging thread, where is the thread likely to break: Below or above the stone? What if the person gives a slow and steady pull?

    Thanks to anyone that can help me gain a better understanding,
  2. jcsd
  3. Oct 2, 2005 #2


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    Are you sure this was the way it was given? What does it mean for a wagon to seem to fall? Is it possible that this was referring to a child sitting in the wagon?

    What can you say about the center of mass of both you and the log?

    Now this is a hard question. I assume we are to take the stone to be almost at the maximum that the thread will support. Think about what happens to the upper thread if you pull slowly- give the stone time to start moving downward and stretch the upper string- as opposed to if you pull quickly and the stone doesn't have time to move (so that the upper string doesn't even "know" the stone is being uplled!).

  4. Oct 2, 2005 #3
    Yes, I understand it now that you pointed that out. It does say the child falls. And the reason for that is because the child continues it's state of motion, maintaining his/her velocity as the velocity of the wagon increases?

    I still don't understand why the log is going in a reverse direction? I don't know what to say about the center mass...

    As for the stone. I thought if you pulled out it quickly it would break above the stone, where it is tied onto the ceiling because of the weight of stone? And if you pull losely it will break the other way because you exert more fore by gradually pulling on it instead of quickly???
  5. Oct 2, 2005 #4


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    The log: Well, think about momentum.
  6. Oct 2, 2005 #5


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    for the log question I would say consider the log and the person as one complete system. Consider the net force on this combined system and how this relates to the center of mass of this system. If there is no net force, what does this say about the motion of the center of mass?
  7. Oct 2, 2005 #6
    If there is no net force than there is no motion? Can someone please just explain the concept to me? This isn't homework assignments. There are things we went over in class, which I still do not understand.
  8. Oct 2, 2005 #7
    We have a test this week. If anyone knows any of the concepts please explain them as easy as possible. I get confused easily.
  9. Oct 2, 2005 #8
    Not really. You can move in one direction and the log can move in the other while the center of gravity between you remains in the same place. I'm not really convinced that that's the explanation though. I would have to ask because it's not to clear, are you on the shore or wading through the water? If you're on the shore, I imagine it's just a matter of perspective. When you walk forward, stationary objects will appear to move backwards in your peripheral vision. On a very calm lake, you won't have much to judge the distance by and I think this will give you a sense it's moving backwards.

    These questions seem a little metaphysical for a physics class (if that's what you're taking.) I wouldn't say that's a bad thing though. In some of my higher level courses I could go through an entire semester and never know what the heck we were doing. I could solve the equations and get the right answer at least 50% of the time :eek: but I just never really knew what possible application they had.
  10. Oct 3, 2005 #9
    Doesn't the log just move backward because the foot pushes back on the log, which causes it to push back on the water, and the water doesn't supply enough resistive force to keep the log in equilibrium?

    Is this a question of the stone having time to move, or an issue of reaction forces? It seems to me if you pull slowly, then the tension in the lower thread will be less, since F=ma. where the m is the mass of the stone. If you pull faster, then the tension in the lower string will be greater, since the stone is subjected to a higher acceleration. The reaction force on the string will be much greater, so it will break before it is able to move the stone very far, and probably the stone will look like it hasn't moved at all. But I still think you have to think of the stone and upper thread moving in both cases.
  11. Oct 3, 2005 #10
    Thanks! That really helped out! :)
  12. Oct 3, 2005 #11
    And child falls backward because of the change in velocity, which has nothing to do with his/her state of motion?
  13. Oct 3, 2005 #12
    And child falls backward because of the change in velocity, which has nothing to do with his/her state of motion?
  14. Oct 3, 2005 #13
    It is an inertia problem, so it has everything to do with the childs state of motion. It's similar to what happens to your head when your car suddenly moves forward. The car moves forward, but your head stays in the same place, so to speak. So it moves backward, relative to the car. A body at rest, and all that.
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