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Homework Help: HW Help

  1. Sep 25, 2006 #1
    HW Help!!!

    I have no idea how to answer these questions!!!:confused:

    1) Newton's second law indicates that when a net force acts on an object, it must accelerate. Does this mean that when two or more forces are applied to an object simultaneously, it must accleerate? Explain.

    2) A father and his seven-year-old daughter are facing each other on ice skates. With thei hands, they push off against one another. (a) Compare the magnitudes of the pushing forces that the experience. (b) Which one, if either, experiences the larger acceleration? Acocount for your answers.

    3) According to Newton's third law, when you push on an object, the object pushes back on you with an oppositely directed force of equal magnitude. If the object is a massive crate resting on the floor, it will probably not move. Some people think that the reason the crate does not move is that the two opposite directed pushing forces cancel. Explain why this logic is faulty and why the crate does not move.

    4) A 10-kg suitcase is placed on a scale that is in an elevator. Is the elevator accelerating up or down when the scale reads (a) 75 N and (b) 120 N? Justify your answers.
    Last edited: Sep 25, 2006
  2. jcsd
  3. Sep 25, 2006 #2


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    Staff Emeritus
    Science Advisor

    Please write your thoughts regarding the answers to these questions.

    But consider:

    1. Forces are vectors. Two opposing vectors of equal magnitude will produce a null result. Otherwise, the sum of two or more vectors has a magnitude and direction.

    2. Equal forces - different masses.

    3. What's the significance of friction? Static. Dynamic.

    4. If one has stood in an elevator which acclerates down, one feels light. If the elevator accelerates upward, one feels heavier. Gravity is always pulling matter (mass) down.
  4. Sep 25, 2006 #3
  5. Sep 26, 2006 #4


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    Homework Helper

    1. Think of a ball lying on the floor. It is experiencing at least two forces - its weight downwards and the upwards supporting force of the floor. When someone kicks it into the air it also experiences two forces - its weight downwards and the force from the boot of the player.

    2. This is a demonstration of Newton's third law. The daughter pushes her father just as hard as he pushes her, the action-reaction forces are the same size, but in opposite directions. One cannot push harder than the other, no matter who is the strongest! Their acceleration will differ though since their mass is not the same. The daughter will experience a much higher acceleration due to her smaller mass.

    A friendly warning though. If you do not think about these concepts and try to understand them you will find that physics will become much more unpleasant and difficult to understand. In which case we will hear much more from you in the future about problems of this nature.
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