Max Subway Speed & Time b/w Stations, Avg Max Speed w/Stops

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Homework Help Overview

The discussion revolves around a physics problem involving the maximum speed of a subway train between stations, given specific acceleration and distance parameters. Participants are exploring the implications of acceleration and deceleration on travel time and average speed, while also considering the effects of stops at stations.

Discussion Character

  • Exploratory, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants are attempting to apply kinematic equations to find maximum speed and travel time. There are questions about the correct use of displacement in calculations, particularly whether to use the full distance between stations or half of it during acceleration and deceleration phases.

Discussion Status

The discussion is ongoing, with participants providing hints and suggestions about the approach to take. Some have offered guidance on how to interpret the problem, while others express confusion about the formulas and calculations needed for different parts of the question.

Contextual Notes

There is a mention of the need to consider the effects of acceleration and deceleration over half the distance, as well as the impact of stops at stations on average speed. Participants are also navigating the challenge of multitasking while discussing the problem.

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Homework Statement


(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?

Homework 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

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?
 
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mossfan563 said:
I tried the displacement and time equation for part A and it didn't work.

Hi mossfan563! :smile:

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

I'll guess for now :rolleyes: … did you remember to use only half the distance? :smile:
 
tiny-tim said:
Hi mossfan563! :smile:

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

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

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?
 
mossfan563 said:
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?

erm … depends on whether you want people to be able to get on and off without nets! :biggrin:
 
Seriously, how do i start this problem?
 
mossfan563 said:
Seriously, how do i start this problem?
You need to accelerate it for 408m at 1.54 m/s2, and then decelerate it for the other 408m. :smile:
 
tiny-tim said:
You need to accelerate it for 408m at 1.54 m/s2, and then decelerate it for the other 408m. :smile:

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?
 
Ok I got part B wrong. How do you do part B?
 
mossfan563 said:
Ok I got part B wrong. How do you do part B?

Why do you make us guess? :confused:

Well … I'm going to guess :rolleyes: … that you forgot to double the distance? :smile:
 
  • #10
tiny-tim said:
Why do you make us guess? :confused:

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

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
Can anyone point me towards the right formula?
 

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