TheStoryteller
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I have this problem:

An athelete takes part in a 400m race. He gains a speed of 4.5 m/s over the first 30m. It asks to cakculate the time to reach the speed, assuming uniform acceleration which I managed and got 13.3 seconds. Then it asks: he then continues to run at this speed. Calculate the time he takes to run the first 100m.

I can't figure it out at all. Is it very simple/am I missing something?

The answer is 28.6 seconds.
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TSR Jessica
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Sorry you've not had any responses about this. Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get responses.

I'm going to quote in Puddles the Monkey now so she can move your thread to the right place if it's needed. :yy:

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randlemcmurphy
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(Original post by TheStoryteller)
I have this problem:

An athelete takes part in a 400m race. He gains a speed of 4.5 m/s over the first 30m. It asks to cakculate the time to reach the speed, assuming uniform acceleration which I managed and got 13.3 seconds. Then it asks: he then continues to run at this speed. Calculate the time he takes to run the first 100m.

I can't figure it out at all. Is it very simple/am I missing something?

The answer is 28.6 seconds.
He has another 70m to travel until he reaches 100m, this is at a constant speed of 4.5 m/s? Can you work out the time this takes? If so, for the 100m the total time is 13.3+(ans from above).
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Dieselblue
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(Original post by randlemcmurphy)
He has another 70m to travel until he reaches 100m, this is at a constant speed of 4.5 m/s? Can you work out the time this takes? If so, for the 100m the total time is 13.3+(ans from above).
Have you been using SUVAT equations?

S- 70m U- 0m/s V- 4.5m/s A- ? T- x

S= ((u+v)/2)t
70 = ((0+4.5)/2)t

70/2.25 = t
It gets 31.1

If the 2.25 were 2.5 it would be correct although I am not sure were the extra 0.5 would come from.
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randlemcmurphy
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(Original post by Dieselblue)
Have you been using SUVAT equations?

S- 70m U- 0m/s V- 4.5m/s A- ? T- x

S= ((u+v)/2)t
70 = ((0+4.5)/2)t

70/2.25 = t
It gets 31.1

If the 2.25 were 2.5 it would be correct although I am not sure were the extra 0.5 would come from.
I don't think there is a need to use SUVAT as the OP states the runner runs at this speed (4.5 m/s), so I assume OP means a constant speed, so I think you can just use distance=speed/time.

EXTRA: You can use SUVAT, but remember u would be 4.5 m/s? Can you see why?
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jonnypdot
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why would you put initial speed, U=0 when you are trying to find the time from 30m to 100m? Doesnt that assume that the runner waited at 30m then finished the race
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randlemcmurphy
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(Original post by jonnypdot)
why would you put initial speed, U=0 when you are trying to find the time from 30m to 100m? Doesnt that assume that the runner waited at 30m then finished the race
You are right, 'u' should be 4.5 m/s, as I said in my previous post.
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