Basic problem and confusion

  1. Sorry I can't get latex working so bear with me as I am new to it. I start A-level physics in a week and during my own research I have come across something that confuses me.

    Acceleration= Final velocity - Initial velocity divided by time.

    What is confusing me is when your deriving other equations from this you get

    at=v - u
    Multiply both sides by t gives at=v-u. This confuses me cause I would get something like this

    at=vt - ut

    So I substituted some values in to see if at=v - u works.

    Acceleration= 10 m/sec^2
    Time= 2 seconds
    Final velocity= 20 m/s
    Initial velocity= 10 m/s

    10*2= 20
    ???????? 20 does not equal 10 so how does at=v - u work?

    Doing it my way I got at= vt - ut
    20*2= 40
    10*2= 20
    40-20= 20
    at=vt - ut

    Then I thought about cancelling the time term for some reason and I got
    20-10= 10
    a= v - u

    Probably being really stupid here and need to go over my rudimentary algebra. Ignore this I got it acceleration is actually 5 I didnt use the equation at the start right
    Last edited: Nov 23, 2013
  2. jcsd
  3. AlephZero

    AlephZero 7,248
    Science Advisor
    Homework Helper

    Those numbers are inconsistent. with each other Either the acceleration should be 5 m/sec^2, or the final velocity should be 30 m/s, or something else is wrong.

    The equations you stated is correct (for the average acceleration, or if the acceleration is constant):
    a = (v - u)/t

    Multiplying by t you get
    at = v - u
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