1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Simple Kinematics problem

  1. May 12, 2009 #1
    1. The problem statement, all variables and given/known data

    A small steel ball, dropped from rest, takes 0.5s to fall through a vertical distance of 1.25m.
    Determine the actual speed of the ball when it has fallen through a vertical distance of 1.25m, assuming that acceleration is constant.

    2. Relevant equations


    3. The attempt at a solution

    Long ago I was helped by a friend to get the result
    ( 1.25m x 2 ) / 0.5s = 5ms-1

    but I've forgotten how did he come to that conclusion, and the reason behind (1.25mx2), help is much appreciated .
  2. jcsd
  3. May 12, 2009 #2


    User Avatar
    Homework Helper

    The formula [itex]s=vt[/itex] is only valid for constant speed. Does an object fall with constant speed?

    Your friend used the following, correct equations:

    1) [itex]s=\frac{1}{2}at^2+v_0t+x_0[/itex]
    2) [itex]v=at+v_0[/itex]

    These are valid for constant acceleration. You will need to combine both of them to get the answer.
  4. May 12, 2009 #3
    s = t*(u+v)/2
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook