1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: How do i find the final velocity of an object?

  1. Jan 11, 2017 #1
    1. The problem statement, all variables and given/known data

    the car took 0.35 sec to stop
    it weighs 975kg
    the force on the car during the crash was 25,000
    3. The attempt at a solution
    i think i may have found impulse but dont know how to go from there
    Last edited: Jan 11, 2017
  2. jcsd
  3. Jan 11, 2017 #2


    User Avatar
    Gold Member

    The car stopped at the end, which means finally it wasn't moving!
  4. Jan 11, 2017 #3
    oopsies i meant before it hit the tree, small typo :O
  5. Jan 11, 2017 #4
    Well knowing that the car will come to a stop, this means you know it's final velocity ##v=0##
    How should you construct your flow of equations that links force to a change in velocity ##\delta v##?
  6. Jan 11, 2017 #5
    i did: time x force =+ J change in momentum which is 8750 so does that mean the initial velocity is 8750?
  7. Jan 12, 2017 #6
    Well, that is wrong because 8750 is just the change in momentum not the velocity.
    What is momentum defined as?
  8. Jan 12, 2017 #7
    8750 divided by the wight 975 would be 8.9 or so but i think thats incorrect for some reason :/
  9. Jan 12, 2017 #8
    Well how I interpreted the question is taking 2 rigid objects without any elasticity in its collision.
    So if the 25kN is constantly felt by the car throughout the entire period it's decelerating then its deceleration is simply ##F/m## which amounts to 25.6 ##m/s^2##
    Then just using simple kinematics the initial velocity is just ##v=at## which will give the 9.0m/s

    But if you're saying it's wrong then I suspect that the force given may be the total force experienced by the car throughout the course of its crash which is equivalent to its impulse. Then that'll be different.

    Yet again I might be wrong, I tend to misunderstand questions so pardon me if I did so.
  10. Jan 12, 2017 #9
    this is high school physics, not too complex, i think this is correct as most of the class is freshmen,its pretty easy we only use 2 equations as well. Thank You very much though!
  11. Jan 12, 2017 #10
    Oh I see if that's the case I hope you understood how the solution works! Cheerio
  12. Jan 12, 2017 #11
    well i figured it out after i took a second look at my notes when you pointed out the definition of momentum, thanks for being so helpful :D.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted