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Explain Why we get hut by kicking a brick

  1. Jan 10, 2010 #1
    Explain Why we get hurt by kicking a brick

    1. The problem statement, all variables and given/known dataYou can easily move a brick lying on a floor by gently pushing it by your foot. But
    you hurt your foot when you try to kick the brick. Explain why? (in 60 to 80 words)



    2. Relevant equationsTheory Question



    3. The attempt at a solution Explanation: When we gently push the brick with foot, The Force we apply on the brick by pushing and friction by floor makes it move. But when we kick the brick lying on the floor, From Newtons third law of motion: Every action has equal and opposite reaction. the brick applies the same opposite force that we apply on the brick by kicking and we get hurt.

    is it right?
     
    Last edited: Jan 10, 2010
  2. jcsd
  3. Jan 10, 2010 #2

    HallsofIvy

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    No, that's not right. Newton's third law of motion applies in both cases. Even when slowly moving the brick, the brick applies the same force to your foot.

    The difference is the amount of the force. If you move a brick slowly, you are applying a very slight acceleration to get it moving, then almost no acceleration so, since F= ma, very slight force, then almost no force. When you kick a brick quickly, you apply a lot of acceleration so a large force- which, by Newton's third law, the brick also applies to your foot.

    By the way, "friction on the floor" does NOT make the brick move. It slows the brick- friction always acts against motion. The friction is the reason that, once the brick is moving, you must apply "almost" no force. If there were no friction, once the brick started moving, it would continue to move with no additional force.
     
  4. Jan 10, 2010 #3
    so in ur words, can u answer it in 60-80 words the above question
     
  5. Jan 10, 2010 #4
    so is this answer correct,

    Answer: Here Newton's third law of motion applies on both cases. when we push the brick lying on the floor with our foot. The Brick applies the same opposite force to our foot. here the difference is amount of force. when we push the brick we apply less acceleration to move it. its almost no acceleration. But when we kick the brick quickly we apply a lot of acceleration on it so the force increases, by newton's third law "every action has an equal and opposite reaction", the brick also applies the same force on our foot and we get hurt.

    ur post helped me a lot and cleared my confusion in physics, hope u'll help me in future
     
    Last edited: Jan 10, 2010
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