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Newton's Third Law of Motion

  1. Oct 9, 2013 #1
    "For every action there is an equal and opposite reaction." I'm having a difficult time wrapping my head around this. I'm having a hard time applying it to certain situations. For example, a hammer hitting a nail, but more presently a problem in my homework.

    A building is being torn down. The wrecking ball smashes through a wall. Does the ball put a larger force on the wall than the wall puts on the wrecking ball? Explain your answer.

    So I understand that as the wrecking ball swings there is
    Tension force
    Weight
    and Drag (as the ball swings)
    that act on it.

    For the wall there would be
    Weight
    Normal Force
    Static Friction

    I guess my big problem, is that if a wrecking ball smashes through a wall and they exert the same amount of force on each other, how would the ball go through the wall? Is it simply because the wrecking balls surface area is smaller?
     
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  3. Oct 9, 2013 #2

    phinds

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    Yes, that would be weird wouldn't it? What makes you think the total forces are equal? What would the ball do if the wall were not there?
     
  4. Oct 9, 2013 #3
    Edit: Never mind :)
     
  5. Oct 9, 2013 #4

    arildno

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    Remember that action and reaction forces act on DIFFERENT objects.
     
  6. Oct 9, 2013 #5
    I guess the "Equal and opposite reaction" part. That means to me that the wall would exert the same amount of force back onto the wrecking ball.

    I read somewhere in my search for knowledge that there is better way of wording Newton's Third law, but they couldn't remember what it was.
     
  7. Oct 9, 2013 #6
    Well....

    A more complete statement of Newton's third law would be something like "For every action force, there exists a reaction force that is equal in magnitude and opposite in direction and that acts on a different body." Or in other words, if body A pushes on body B then body B will push back with an equal and opposite force on body A.

    This is easy to understand for systems in equilibrium. A common example is to think about a person standing on the floor. The weight of the person pushes down on the floor and reaction force from the floor pushes back on the person.

    In your examples (hitting a nail ect.) when the objects are in contact there are the same action - reaction force pairs working. If we hit a nail with a hammer then the force on the hammer will change its momentum by the same amount as the momentum of the nail is changed. This is why we use hammers that have a mass that is much larger than the mass of the nail.

    You could start off by thinking about how the motion of the hammer changes during the collision. Also, what happens to the motion of the nail?
     
  8. Oct 9, 2013 #7
    The force of the hammer is enough to drive the nail into the wood to some degree but at some point the hammer would be driven back. (Unless you have the accuracy and confidence to hit the nail home in one swing.)

    Is this what you're getting at?
     
  9. Oct 9, 2013 #8

    PhanthomJay

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    Newton's 3rd law refers to force pairs (so called action-reaction forces) that are equal in magnitude but opposite to each other, but which act on different objects. The ball hits the wall and applies a normal contact force to it, say to the right, and thus the wall applies an equal normal contact force to the ball, to the left. Newton 3 applies to that specific force pair, not to rope tension, weight, friction forces, etc., each of which has its own set of force pairs (like the rope tension force acts on the ball with a force T, and thus the ball acts on the rope with a force T in the other opposite direction).

    In your example, the ball hits the wall with a force F, and thus the wall hits the ball with an opposite force F. The wall breaks but not the ball because the wall is not as resistant to the impact forces as the ball. If the ball was made of glass instead of say steel, I bet the ball would break instead of the wall.

    edit: 6 responses before I finished typing mine!
     
    Last edited: Oct 9, 2013
  10. Oct 9, 2013 #9
    The idea that I was working towards was that the hammer will slow down slightly and the nail will start moving a lot more quickly. The force on the nail is the same size as the force on the hammer but the effect is different because the two objects have different properties.

    Your question asked about why it is that the demolition ball doesn't just stop when it hits the wall. It's true that the force on the ball and the wall are the same size, just like in the hammer and nail example. But the effect of a force of that size on the wall is different to the effect of the same size force on the ball.
     
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