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Homework Help: Just some conceptual stuff about car crashes

  1. Nov 13, 2007 #1
    1. The problem statement, all variables and given/known data

    During a crash test, dummy x12 has a mass of 80 kg and rides in a 1120-kg car. Dummy y17 has a mass of 80 kg and rides in a 7120-kg truck. The vehicles approach each other with velocities of 30 m/s (about 67 miles per hour) in opposite directions (car heading east). The vehicles collide and their bumpers lock together after the collision. The collision takes 0.015 s to complete.

    -You should find that while the car and truck undergo equal and opposite changes in momentum and are subject to equal and opposite forces, this is not true for the dummies. Explain why this is so.

    2. Relevant equations

    [tex]\Delta[/tex]P = Ft
    P = mv

    3. The attempt at a solution

    I did out all the momentum and force work for the cars, but I'm just not sure conceptually how to explain this. It makes logical sense dealing with inelastic collisions, and we constantly see this happen in real life where a passenger is a car is severely injured when hit by a truck/bus/larger object. Any help? I dont think I should be proving this with math, but even so, I'm not sure how. Any ideas?
  2. jcsd
  3. Nov 14, 2007 #2


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    The forces that the dummies will experience depends on the decelerations of the vehicles during the collision. That is they only experience the force when they hit the inside front of the vehicle - which transfers the force to the dummies via the difference in relative motion between the two (dummy and vehicle). These decelerations will differ due to the different masses of the vehicles since both vehicles experiences the same magnitude of force at all times during the collision.
  4. Nov 14, 2007 #3
    i figured it out: its because during the collision between the car+truck, its inelastic so momentum is conserved and thus forces is conserved as well (impulse = Ft); however, the two dummies never actually collide with each other and thus do not conserve momentum. Instead the dummies collide with the dashboard/windshield/ground/steering-wheel/soda bottle in front seat.
  5. Nov 15, 2007 #4


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    The forces between the two vehicles are also "generated" due to the difference in their relative motions.
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