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Forces, find net with velocity and distance

  1. Oct 2, 2013 #1
    1. The problem statement, all variables and given/known data

    Driver brakes 1420kg car moving at 64.8km/h [W]. Car slows down and comes to stop after 729m [w]

    find net force


    2. Relevant equations

    a=v/t f=ma

    3. The attempt at a solution

    t=40.5 a= 0.44

    f=1420(0.44) f= 624.8

    My answer is 624.8N

    TB Answer: 316N


    what am i doing wrong?
     
    Last edited: Oct 2, 2013
  2. jcsd
  3. Oct 2, 2013 #2

    NascentOxygen

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    Staff: Mentor

    Hi yogoes123. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Your first mistake is probably in how you worked out that 40.5 secs, as you haven't shown the equation you used.
     
    Last edited by a moderator: May 6, 2017
  4. Oct 2, 2013 #3
    I used v=d/t swapped it around and solved for t=d/v and that gave me 40.5s, is that wrong?
     
  5. Oct 2, 2013 #4

    NascentOxygen

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    Staff: Mentor

    That equation relates time to the distance travelled at a constant speed.
     
  6. Oct 2, 2013 #5
    So it would be correct, no? I cant figure out what i did wrong.
     
  7. Oct 2, 2013 #6

    NascentOxygen

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    You [somehow] worked out a figure of 624.8N
    then you decided to halve it, and call that your answer?

    How to explain that halving?
     
  8. Oct 2, 2013 #7
    The TB answer is 316, my answer is 624.8, sorry I didnt make that clear.
     
  9. Oct 2, 2013 #8

    NascentOxygen

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    As I emphasised "at a constant speed". At no time here is the vehicle travelling at a fixed speed, the vehicle is constantly slowing.

    To begin over....

    you need equations relating distance, initial velocity, final velocity (= 0 here) etc.

    So, write down some suitable candidates.
     
  10. Oct 2, 2013 #9
    I thought it starts to slow down after the 729m. Are you saying that is it slowing down within 729m?
     
  11. Oct 2, 2013 #10

    NascentOxygen

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    At the 729m mark, it is motionless. :smile:

    Where speed is constant it has acceleration = 0 (for straight line travel).
     
  12. Oct 2, 2013 #11
    Ok I think I got this, I used Vf^2=Vi^2+2ad

    Then I did a= 18^2/2/729 and i got 0.2222222 then I used F=ma and got 316, thanks so much for clearing that up.
     
  13. Oct 2, 2013 #12

    NascentOxygen

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    Staff: Mentor

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