Linear Motion - Minimum Retardation to Avoid Crash?

[project_ILE]

First post! I would be grateful if anyone could give me any advice on this particular type of problem (i.e min retardation to avoid a crash). I'm not necessarily looking for the answer to this specific question, I would rather if someone could point me in the right direction as to how to go about these questions. Thanks!

1. A passenger train, which is traveling at 80 m s^{-1} is 1500 m behind a goods train which is traveling at 30 m s^{-1} in the same direction on the same track. At what rate must the passenger train decelerate to avoid a crash? (Ignore the lengths of the trains)



2. s = ut + \frac{1}{2}at^{2}
v = u + at
s = (\frac{u + v}{2})t
v^{2} = u^{2} + 2as



3. I worked out that at current speeds, the trains would collide after 30 seconds, using a velocity-time graph, letting the distance traveled by the passenger train = distance traveled by the goods train + 1500. (30T + 1500 = 80V). From there I summized that for the trains to never crash, the passenger train should decelerate to at least the same speed as the goods train ( 30 m s^{-1}). Then I used v = u + at for the passenger train, and had 30 = 80 + a(30), where a comes out at - \frac{5}{3} m s^{-2}. I've compared this answer to the correct one.

Thanks,
project_ILE

 

[Spinnor]

You wrote,

"I've compared this answer to the correct one."


%^) So did you get the right answer? If you did fine, if not then someone might care to find the error.

 

[project_ILE]

You wrote,

"I've compared this answer to the correct one."


%^) So did you get the right answer? If you did fine, if not then someone might care to find the error.

Sorry, I thought I had finished my post! This question is from an exam paper, I have the numeric answer, but not the method. My own answer (above) is not correct.

 

[Spinnor]

The answer might help potential helpers. What was it 8^)