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A theoretical question about forces

  1. Jul 16, 2015 #1

    Imagine the following scenario: a person is lying in bed, straight on the back. A second person passes the bed and accidently hits the edge with it's knee. The created force results in a fast head movement of the person on the bed, towards the direction where the impact was.

    Is it correct that the force would have been the same high, when the knee hit the head directly?
  2. jcsd
  3. Jul 17, 2015 #2


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    Staff: Mentor

    I'm assuming there's a headboard, and in such a case it is the headboard that moves towards the person's head. This exerts a force on the head opposite of the direction of impact.
  4. Jul 17, 2015 #3
    No, it' not correct. Supposing the bed was like a clean table with some kind of oil to eliminate the friction between the body on it and the bed the force on the head is going to be close to zero or zero (idealy).
  5. Jul 17, 2015 #4


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    No. Consider the trick where you pull a table cloth from under a plate. The plate does not accelerate as quickly as the cloth. The forces could be very different.
  6. Jul 17, 2015 #5
    F=ma, in the two cases the mass is different, the force is the same, so you would get a different acceleration.
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