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Dog crash

  1. Sep 18, 2012 #1
    I am involved in a dispute related to the forces that a dog's head is exposed to when he, of its own power, runs into a stationary object.
    Because of the dog's structure (this is a small dog) the head is getting pinched between the stationary object and his own back part of the body (which we believe is directly behind the head).
    Let's say the dog's weight is 10 kg and the head hold 20% of the mass and the rest is behind the head. At the collision it would be one force against the forehead from the object and another force toward the back of the head because of the kinetic energy from the 8 kg's that is behind the head. At the same time the parts are linked together.
    If I want to calculate the force on the front and back of the head How should I do? Calculate as if it are two separate bodies that collide. First the head with a mass of 2 kg that collied with the solid object and than the back part of the body with a mass of 8 kg that collied with the now stationary head.
  2. jcsd
  3. Sep 18, 2012 #2


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    Staff: Mentor

    I doubt that this calculation will give any meaningful result, but well...

    The forces involved in the head->wall crash depend on the properties of the head, especially the time during which the head is decelerated. I would expect that this part is the most dangerous one for the dog.

    What about the other part?
    If the body has mass m, length L and velocity v, the total transferred momentum to stop it is m*v and the maximal time is L/v, giving a minimal average force of mv^2/L. I would expect that the real force is larger.
    You can use the same formula as approximation for the head, with stopping distance L' << L.
  4. Sep 18, 2012 #3


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    The question of "An object of mass m hits a surface with speed v and stops. What is the force the surface exerts on the object" recurs here over and over again. But it is impossible to answer because that is not enough information. In order to find the (average) force exerted you would also need to know the time required for the object to come to a stop or the distance it moved (perhaps "crumpled" would be better) as it stopped.
  5. Sep 18, 2012 #4
    So how much did the dog crumple?

    Side question, this mental picture makes me smirk inside. Am I a bad person?:tongue:
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