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April 2nd, 2013, 01:47 AM   #1
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Mechanics basic question confusion.

I am have trouble trying grasp this question. What I am saying, I can sort of see it working in my head, but can't seem to put it into algebra.

Question:2. An underground train accelerates uniformly from rest at station A to a velocity of 24 m s-1.
It maintains this speed for 84 seconds, until it decelerates uniformly to rest at station B.
The total journey time is 116 seconds and the magnitudes of the acceleration and deceleration are equal.

(a) Find the time it takes the train to accelerate from rest to 24 m s-1.

The mark schem say I reall can't see this. Here is how I view the problem; in algbra.

clearly this is wrong, but I can't seem to put this problem in terms of t. I know that not what the question asks but; for peace of mind I would like to know where I am going wrong.

Big thanks in advance.
taylor_1989_2012 is offline  
 
April 2nd, 2013, 06:07 AM   #2
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Re: Mechanics basic question confusion.

You are along the right tracks, I'm not sure where the value of 32 came from though.

As you have put the train accelerates for t seconds and then travels at a constant velocity for 84 seconds and then decelerates for t seconds. As we know the total journey time is 116 seconds we can write:



or



Rearranging this gives:



As you have seen in the mark scheme.
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April 4th, 2013, 05:50 AM   #3
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Re: Mechanics basic question confusion.

Thanks for that, don't worrie about the 32. I think what happen is I miss read the question, and did not look at the fact that as the so consequently didn't think that there times were the same.
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