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Mist786
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#1
Report Thread starter 9 years ago
#1
I keep getting the wrong answer for this question

A train, travelling on a straight track, is slowing down with constant deceleration 0.6ms-2. The train passes one signal with speed 72km h-1 and a second signal 25s later. Find a) the speed, in km h-1, of the train as it passes the second signal, b)the distance between the signals.


For a) I used v=u+at but I got the wrong answer

Any help?
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matt2k8
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#2
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I suspect you didn't convert the units - the decleration is in m/s, the speed is in km/h.
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Mist786
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#3
Report Thread starter 9 years ago
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(Original post by matt2k8)
I suspect you didn't convert the units - the decleration is in m/s, the speed is in km/h.
Stupid question but how do you convert the units?
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hassan_s
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#4
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s=?
u=72*1000=72000/3600=20
v=?
a=-0.6
t=25

a)v=u+at
v=20+(-0.6*25)
v=20-15
v=5ms-1

b)s=((u+v)/2)t
s=((20+5)/2)25
s=12.5*25
s=312.5m
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Mist786
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#5
Report Thread starter 9 years ago
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(Original post by hassan_s)
s=?
u=72*1000=72000/3600=20
v=?
a=-0.6
t=25

a)v=u+at
v=20+(-0.6*25)
v=20-15
v=5ms-1


b)s=((u+v)/2)t
s=((20+5)/2)25
s=12.5*25
s=312.5m
The answer for is a) is 18 according to the answers at the back of the book
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hassan_s
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(Original post by Mist786)
The answer for is a) is 18 according to the answers at the back of the book
Really? It does work out at 5.

It is because, to convert 72kmh to m/s, you multiply it by 1000 (72000) to get m/h, then divide by 3600 to get m/s. This is 20. Then, you multiply a (-0.6) by t(25), which gives the answer 15. Then, according to v=u+at, you just do 20-15 to get 5.
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Mist786
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#7
Report Thread starter 9 years ago
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(Original post by hassan_s)
Really? It does work out at 5.

It is because, to convert 72kmh to m/s, you multiply it by 1000 (72000) to get m/h, then divide by 3600 to get m/s. This is 20. Then, you multiply a (-0.6) by t(25), which gives the answer 15. Then, according to v=u+at, you just do 20-15 to get 5.
Its okay

I emailed my tutor, I think it was a mis-print

anyways thanks
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mathsisverycool
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#8
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hello, only 9 years later do i stumble upon the question i was stuck on, in this chat. 5ms-1 is 18kmh-1, thats why the answer is 18. lol its not a misprint glad to see that they are updating our questions at alevel maths aha
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