# Homework Help: Subway train problem

1. Sep 1, 2008

### mossfan563

1. The problem statement, all variables and given/known data
(a) If the maximum acceleration that is tolerable for passengers in a subway train is 1.54 m/s2 and subway stations are located 816 m apart, what is the maximum speed a subway train can attain between stations? (b) What is the travel time between stations? (c) If a subway train stops for 20.0 s at each station, what is the maximum average speed of the train, from one start-up to the next?

2. Relevant equations
velocity & time:
v = v_0 + a t

displacement & time:
x = x_0 + v_0 t + (1/2) a t^2

velocity & displacement:
v^2 = v_0^2 + 2 a \\Delta x

3. The attempt at a solution
I tried the displacement and time equation for part A and it didn't work. I had no idea how to do the other two parts.
How do you start this problem?

2. Sep 1, 2008

### tiny-tim

Hi mossfan563!

It's difficult to say why it didn't work if you don't show us what you did!

I'll guess for now … did you remember to use only half the distance?

3. Sep 1, 2008

### mossfan563

Well I simply plugged in the distance between stations as the equation's displacement and the acceleration is given. I have to use half the distance?

4. Sep 1, 2008

### tiny-tim

erm … depends on whether you want people to be able to get on and off without nets!

5. Sep 1, 2008

### mossfan563

Seriously, how do i start this problem?

6. Sep 1, 2008

### tiny-tim

You need to accelerate it for 408m at 1.54 m/s2, and then decelerate it for the other 408m.

7. Sep 1, 2008

### mossfan563

So the maximum speed would be the point between accelration and deceleration. So 408 would be the correct displacement instead of 816.
Speed would be 35.449 m/s.
Does that sound right?

And for part B I would use the number I just got in the formula:
V = V_0 + at?

8. Sep 1, 2008

### mossfan563

Ok I got part B wrong. How do you do part B?

9. Sep 1, 2008

### tiny-tim

Why do you make us guess?

Well … I'm going to guess … that you forgot to double the distance?

10. Sep 1, 2008

### mossfan563

Well I'm sorry if I'm making you guess. Trying to multitask. I know you use 816 as the displacement for this part. Acceleration is still 1.54. I'm just confused as to what formula to start with.

Maybe this formula?:
$$x = x_0 + v_0 t + (1/2) a t^2$$

11. Sep 1, 2008

### mossfan563

Can anyone point me towards the right formula?