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!(adsbygoogle = window.adsbygoogle || []).push({});

1. A passenger train, which is travelling at 80 m s[itex]^{-1}[/itex] is 1500 m behind a goods train which is travelling at 30 m s[itex]^{-1}[/itex] 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 + [itex]\frac{1}{2}[/itex]at[itex]^{2}[/itex]

v = u + at

s = ([itex]\frac{u + v}{2}[/itex])t

v[itex]^{2}[/itex] = u[itex]^{2}[/itex] + 2as

3. I worked out that at current speeds, the trains would collide after 30 seconds, using a velocity-time graph, letting the distance travelled by the passenger train = distance travelled 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[itex]^{-1}[/itex]). Then I used v = u + at for the passenger train, and had 30 = 80 + a(30), where a comes out at - [itex]\frac{5}{3}[/itex] m s[itex]^{-2}[/itex]. I've compared this answer to the correct one.

Thanks,

project_ILE

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Linear Motion - Minimum Retardation to Avoid Crash?

**Physics Forums | Science Articles, Homework Help, Discussion**