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Newton's second law

  1. Feb 19, 2006 #1
    Hi,
    I have a question on newton's second law.

    I have a box in which an egg rests inside. Egg is surrounded by some sort of padding. A force is applied to the box. (Force has an x and -y component.

    Are my conclutions right?

    1) Force act on the box, not on the egg. Applying newton's second law to the whole system one can get the system's accelertation,

    F = (m1 + m2 ) a

    2) Net force on the egg = zero. Egg does not move.

    3) What does the padding do? Is it to avoid the egg moving at the beginning of the application of the force? At the beginning, due to newton's third law, egg will move against the applied force and go hit the back of the box. So if I gradually increase the applied force can I avoid the padding?

    (4) Force applied in the negative y direction would make it difficult to move the box.

    I get confused with my first statement. Technically egg moves with respect to a person out side the box. So the egg shoud have accelerated which means egg shoud be experiencing a force?? How to clear my confusion?


    Please pour in your knowledge on this egg problem. I know egg box system is a very common school problem in USA.

    Thank You.


    Gamma.
     
  2. jcsd
  3. Feb 19, 2006 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Okay, you are treating the box, egg, and padding as a single system. I would have preferred you told us what m1, m2 mean (there are actually three masses- box, egg, padding).

    relative to what? The egg does not move relative to the box but since the "box,egg,padding" have acceleration a (relative to the boxes original position), so does the egg.

    The padding spreads the force over the entire surface of the egg. It's a mistake to think of it as having to do with time. "Gradually increasing the applied force", just means you start with low force which, hopefully, would be low enough not to crack the egg even without padding spreading the force. Of course if you continued increasing the force, eventually it would become enough to crack the egg.

    If your box is sitting on a surface with non-zero friction coefficient, then, yes, friction is proportional to the normal force.

    Yes, the egg is accelerated so the egg is experiencing a force. You push against the box, the box pushes against the padding, the padding pushes against the egg. As I said before, the padding spreads the force over the surface of the egg so there is no force at any one point on the egg enough to crack it.

     
  4. Feb 19, 2006 #3
    Thank you,

    Yes, I should have included the weight of the padding.

    After your suggestions, my new conclutions are:


    (1) F = (m1 + m2 + m3) a

    (2) Egg experiences the applied force.

    If the applied force is F = Fx + Fy (x and y componetns of the applied force)

    Net force on the egg in horizontal direction is Fx.
    Net force in the vertical direction is N + Fy - mg

    However, these forces are uniformly distributed over the surface of the egg so that the egg would not crack.

    (3) If I apply newton's second law to the egg only,

    Fx = m(egg) a where a is given by F = (m1+m2+m3) a

    N + Fy - mg = 0

    Please let me know if these conclutions are correct. Thank you in advance.


    Gamma.
     
  5. Feb 19, 2006 #4
    So what you mean is that the applied force act on the egg too. Please look at the following.

    Newton's second law applied to the whole system horizontally,

    Fx = (m1 +m2+m3) a ---------------(1)

    (m1 mass of the egg, m2 mass of the padding, m3 mass of the box)

    If we apply newton's second law to only the egg,

    Fx = m1 a
    -----------------------(2)

    (m1=mass of the egg)

    If we look at (1) and (2) they both can't be true. We know egg's acceleration is a. This means, force on the egg can not be Fx???
    This is the confusion I have.



    Please explain some one. Thanks,

    Gamma.
     
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