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Impact Force

  1. Nov 23, 2013 #1
    A moving car crashes with a tree. Will the impact force on the car by the tree equal to the impact force on the tree by the car?

    Does newton's third law apply here?
  2. jcsd
  3. Nov 23, 2013 #2

    Newton's third law of motion is naturally applied to collisions between two objects. In a collision between two objects, both objects experience forces that are equal in magnitude and opposite in direction. Such forces often cause one object to speed up (gain momentum) and the other object to slow down (lose momentum). According to Newton's third law, the forces on the two objects are equal in magnitude. While the forces are equal in magnitude and opposite in direction, the accelerations of the objects are not necessarily equal in magnitude. In accord with Newton's second law of motion, the acceleration of an object is dependent upon both force and mass. Thus, if the colliding objects have unequal mass, they will have unequal accelerations as a result of the contact force that results during the collision.
  4. Nov 26, 2013 #3


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    And many people gets confused with Newtons third law ,saying "why not the forces cancel out?"
    Ans:The forces act on two different objects.i.e.In case of a ball bouncing of a wall,the wall applies a force on the ball and the ball applies a force on the wall,all same in magnitude.
  5. Nov 27, 2013 #4
    The question deals with real objects; i. e., a car and a tree. Some of the kinetic energy of the car will be turned into friction by the deformation of both the car and the tree. The resulting heat energy and thermal radiation will be non-directional--especially if the accident results in a fire. Hence, total momentum will not be conserved. Also, without knowing more about the characteristics of both the car and the tree, it would be extremely difficult to estimate how much force was exerted by one upon the other.

    The same caveat applies to the ball and wall example. I think that you will find the real balls rebound from real walls with diminished momentum. A dropped ball will not keep bouncing forever. Similarly, sound waves disappear into the random and non-directional motion of the atmosphere's constituent molecules. Entropy will have its toll.
    Last edited: Nov 27, 2013
  6. Nov 27, 2013 #5


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    N3 must always apply but the consequences are not necessarily intuitive. A small object hitting a massive object will transfer an amount of momentum which is equal to the momentum it loses (tree plus Earth will move a tiny amount) but the speed of recoil of the tree will be very low and the Kinetic Energy gained (half m v squared) is even more vanishingly small, due to the square factor. 'All' the energy of the collision will be dissipated in the car (crumpling and graunching).
    In an elastic collision, the small object will rebound with virtually the same speed as it approached, not transferring any appreciable KE, despite having experienced the same contact forces.
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