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Homework Help: Momentum and Collision with a seat belt

  1. Feb 26, 2008 #1
    Need some help, got a 50 question review packet of questions to finish and am having trouble with these 2. please and thank you:

    1. A friend claims that as long as he has his seat belt on, he can hold on to a 12.0-kg child in a 60.0 mi/h head-on collision with a brick wall in which the car passenger compartment comes to a stop in 0.0500 s. Show that the violent force during the collision will tear the child from his arms.

    2. A proton, moving with a velocity of vii, collides elastically with another proton that is initially at rest. If the two protons have equal speeds after the collision, find (a) the speed of each proton after the collision in terms of vi and (b) the direction of the velocity vectors after the collision.
  2. jcsd
  3. Feb 27, 2008 #2
    i would really appreciate some help guys.
  4. Feb 27, 2008 #3
    ok so for question 1
    what you want to figure out is the force exerted by the 12kg as it decelerates from 60->0 in.05 seconds

    there is a formula that says
    which means your final velocity is equal to your inital velocity plus acceleration multiplied by time
    so knowing that your final velocity is zero and time is .05 you can work out "a"

    be carefull because 60mph is about 100kmph which about 1700 m/minute
    which is about 30 odd m/s [prob a bit less than that]

    anyway point is you now have "a"
    you also know the mass of the child which i'll call "m"

    now you calculate the force required to produce this acceleration [or deceleration] for this mass.

    i'm guessing it would be like trying to lift a half ton. anyway your neck would probably break.
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