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# Mechanics help! Watch

1. can someone tell me why the time doesn't cancel as I couldn't get it to work and it only works when the t is squared?

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2. (Original post by zachariou1)
can someone tell me why the time doesn't cancel as I couldn't get it to work and it only works when the t is squared?

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To put it simple you cnt do tht bcuz s and -ut are seperate at the top so u cnt just divide the top by t only if the top was sut could u do tht dyu understand?
3. You only need to find the acceleration: therefore -
S=2500km = 2500000m
u= 0
a = ?
t = 30m = 1800s

and so then rearrange S=ut+1/2*at ² to give a=s/(1/2*t ²) - note that the 'ut' can be got rid of as U=0
and so the just put in the numbers to give: 2500000/(1/2*1800²) = 1.543209876ms-2 which is roughly equal to 2
Hope this helped!
4. (Original post by Revisionbug)
To put it simple you cnt do tht bcuz s and -ut are seperate at the top so u cnt just divide the top by t only if the top was sut could u do tht dyu understand?
You have divided one half of the fraction by t but not the other half, you must do both sides otherwise it doesn't stay equal!
Think of the fraction (11 - 4) / 4
You can split that into 11/4 - 4/4 or 2.75 - 1 = 1.75
If however, you cancel the 4 from just the RH numerator you would get:
11 - 4/4 = 10 which is incorrect.

[s - ut] / t² can be rewritten as s/t² - ut/t²
However, you have split it into s - u/t which is something different so it won't work.
You must divide every term of the fraction when you're cancelling something out.
5. Thanks guys, stupid me didn't realise u was 0 should really have listed out what each term is. Thanks a lot!

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