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: Car's velocity after being hit by a force

  1. Nov 14, 2007 #1
    1. The problem statement, all variables and given/known data

    A 2500 kg car traveling to the north is slowed down uniformly from an initial velocity of 25 m/s by a 6240N braking force acting opposite the car's motion. What is the car's velocity after 2.52 s?


    2. Relevant equations

    F=m(Vf-Vi)/t

    3. The attempt at a solution

    This is how I used the equation

    6240N=2500kg(Vf-25m/s)/(2.52s)

    Solving for Vf I got 31.29m/s

    The problem is that would mean the car's velocity is increasing, which it should be decreasing I think...
     
  2. jcsd
  3. Nov 14, 2007 #2
    I think its because you forgot to make your acceleration negative.
     
  4. Nov 14, 2007 #3
    I think it would have been better to use two formulas instead:

    F = ma
    a = (vf - vi)/t
     
  5. Nov 14, 2007 #4
    Yay thank you, I got the right answer once I did that :)
     
  6. Nov 14, 2007 #5
    Ooo one more question

    to find the distance the car traveled in that problem I'm using the equation

    d=1/2(Vf+Vi)t

    d=1/2(-18.71m/s+25m/s)(2.52s)

    Do I make the initial velocity of 25 m/s negative there? and the 18.71 should still be negative correct?
     
  7. Nov 14, 2007 #6
    Actually I realize they should both be negative velocities.

    So last part of the question asks how long would it take for the car to come to a complete stop? Does that basically mean when does final velocity equal zero?
     
  8. Nov 15, 2007 #7
    Yes it does.
     
  9. Nov 15, 2007 #8

    Shooting Star

    User Avatar
    Homework Helper

    Why are the velos negative? You should put accn as -ve. If you make the velos -ve, then accn should be +ve.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook