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Homework Help: Momentum and Collision help needed (2 questions)

  1. Mar 23, 2016 #1
    1. The problem statement, all variables and given/known data
    A car braking will decelerate at .8g. If the car's mass is 1200kg, how much horizontal force does each tire exert on the road? What is the coefficient of friction between the tire and the road?

    A 2kg steel ball rolling at 4m/s strikes a stationary 5kg steel ball elastically. What is the resultant motion of each ball? How and why would the collision change if it were elastic?
    2. Relevant equations

    P1+P2=P3+P4 ??????
    KE=1/2mv^2 ??????

    3. The attempt at a solution
    I am having a very hard time even understanding where to start here. I started off by multiplying (1200kg)(.8g)(-9.8m/s). I am unfamiliar with the measurement being in g's. How would I factor that into the question? I then divided the number by 4 to reference the tires in the work.

    The work i have for the second question is basically me attempting to do the momentum equation in the relevant equations. There is two velocities you have to solve for though and I am unsure on how to progress in this problem. I am fairly sure that I started the question wrong and I should be using a different equation. Some guidance would be appreciated!
    Last edited by a moderator: Mar 23, 2016
  2. jcsd
  3. Mar 23, 2016 #2


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    You did so already, just with two mistakes. Just replace g by its value (and correct units) and you are done.
    Your textbook probably has a formula for both velocities. Otherwise, think which quantities are conserved, and use those.
  4. Mar 23, 2016 #3
    for question #1, what do u mean by done. Am i done with the problem or am i done with referencing g's in the question? If not, what is the next step?

    for question #2, could you elaborate? My knowledge on collisions is iffy at best. What formulas are you talking of for velocity?
  5. Mar 24, 2016 #4


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    You have multiplied the mass by two accelerations, 0.8g and 9.8m/s2. Does that seem right?
    What else is conserved in an elastic collision?
    By the way, the question did not mention any friction in this question, so presumably you should ignore it.
  6. Mar 24, 2016 #5


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    Staff: Mentor

    F=m*a. You know m and a, I don't see the problem.
    Divide it by 4 and you are done.
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