# Basic problem and confusion

by JayJohn85
Tags: basic, confusion
 P: 44 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 20-10=10 ???????? 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 10 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
Engineering
Thanks
P: 6,041
 Quote by JayJohn85 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
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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