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Quick Equations of Motion questions (2). Rxn time; deceleration

  1. Sep 4, 2007 #1
    Quick "Equations of Motion" questions (2). Rxn time; deceleration

    1. A meteorite strikes on October 9, 1992, a 27-pound meteorite struck a car in Peekskill, NY, leaving a dent 22 cm deep in the trunk. If the meteorite struck the car with a speed of 130 m/s, what was the magnitude of its deceleration, assuming it to be constant?

    27# meteorite
    22cm dent in trucnk
    130 m/s



    2. Relevant equations

    v^2=v0^2+2a(x-x0)=v0^2+2a(change in)x

    v=-22
    v0=130m/s
    a=? (to be solved for)
    x=?
    x0=?

    I'm pretty sure this is not the equation since distance is not given.



    3. The attempt at a solution

    I'm lost without the correct equation... :(



    1. You are driving through town at 16 m/s when suddenly a car backs out of a driveway in front of you. you apply brakes and begin decelerating at 3.2 m/s^2.
    c. If the car backing out was initially 55 m in front of you, what is the maximum reaction time you can have before hitting the brakes and still avoid hitting the car?


    v0=16m/s
    a=-3.2 m/s^2
    (change in)x=55 m
    v=0




    2. Relevant equations

    [(change in)x=v^2-v0^2]/[2a]

    v=v0+at=0



    3. The attempt at a solution


    0=16m/s+(-3.2m/s^2)t

    t=5s?

    answer given is 0.945s, but i want to learn how to get there.

    Thanks for looking!
    Steve

    If I could just get an equation, that would help greatly!

    Thanks again!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 4, 2007 #2

    hage567

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    There is no v = -22. :confused: The 22 cm refers to the depth of the dent (i.e. the distance the meteorite travels), so you know the change in x. You also know what the final velocity is. The equation you have will work fine.
     
  4. Sep 5, 2007 #3

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    The time you have calculated is the time it takes your car to stop, once you apply the brakes. The problem asks for your max reaction time before you apply the brakes, to avoid collision. You are assumed moving at constant speed during that reaction period. The distance you travel during the reaction period, plus the distance you travel during the braking period, must add to 55m. (I got t = .938s).
     
  5. Sep 5, 2007 #4
    okay, for question 1:

    0^2=(130m/s)^2+2a(0-22)
    -16900=-44a
    a=384.90m/s^2

    and for question 2:
    (change in)x=[0^2-(16m/s)^2]/[2(-3.2)m/s^2]=40m

    55m-40m=15m

    [15m]/[16m/s]=0.9375s=~0.94s

    thanks for the kick in the right direction guys! great website!
     
  6. Sep 5, 2007 #5

    hage567

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    Be careful of your units. The distance is given 22 cm but your velocity is in m/s. You must be consistent. Also, it's x-x0. Your acceleration should come out negative since the meteorite comes to a stop.
     
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