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: Calculating deceleration

  1. Jan 12, 2009 #1
    Sorry about this, must seem a trivial question, but anyways here goes.

    A car is travelling at 13ms^1. It is allowed to slow down natrually without appyling the brakes. The distance travelled in coming to rest is 640m.

    Show that the average deceleration of the car is about 0.1ms^2.

    -------------------

    Ok so first of since time aint metioned i used.

    v^2 = u^2 + 2as

    giving me

    v = 0
    u =13
    a = ?
    s = 640

    ----------------------------------
    v^2 = 13^2 + 2(a)640.

    v^2= 13^2 +2(640)
    ___________
    a

    a * v^2 = 13^2 +2(640)

    a = 13^2 +2(640)
    __________
    v^2

    Giving me a very strange answer, no were remotely close to 0.1, ive obviously screwed up the re-arranging or used the wrong formula.

    Would like a nudge in the right direction please.
     
  2. jcsd
  3. Jan 12, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    Welcome to PF.

    Your equation is ok. But solving it looks not right.

    V² = V² + 2*a*x

    13² = 2 * a * 640

    a = 169/1280
     
  4. Jan 12, 2009 #3
    Cheers, knew i did something wrong, thanks.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook