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: Kinemetics Question - theoretical subsitution

  1. Oct 21, 2011 #1
    1. The problem statement, all variables and given/known data

    This is for personal study and as im on study leave its a bit hard to contact a teacher.

    This is the question; (the circled answer was the teachers answer)


    I'm unsure as to why you would individually square the times before subtracting as the actual time is (t2-t1) substituted into time on the equation, squaring before will result in a larger value (small value for acceleration though). It is possible the teacher merely got it wrong (has been known to happen).

    2. Relevant equations

    s=ut + (1/2)at^2

    3. The attempt at a solution

    Last edited: Oct 21, 2011
  2. jcsd
  3. Oct 22, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Your equation,  h = 0(t2-t1) + (1/2)a(t2-t1)2  is in error.

    The velocity, u1, at time, t1 is not zero. This velocity is give by:
    u1 = u0 + a(t1-t0) = 0 + a t1 = a t1 .​

    ∴ h = u1(t2-t1) + (1/2)a(t2-t1)2
    = (a t1)(t2-t1) + (1/2)a(t2-t1)2

    If you multiply this all out, combine like terms and then factor it, you will find that your teacher did indeed give the correct answer.

    BTW: There is an easier way to come up with his answer. I thought it better to use what you started with.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook