Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, when,

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Homework Statement


Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, when the engineer spots a cow on the track. The train brakes to a halt in 2.3 min, stopping in front of the cow.

a) What is the magnitude of the train's (constant) acceleration?
_______m/s^2 <-----(0.22m/s^2) Correct

b) What is the direction of the acceleration?
- eastward
- westward <-----Correct
- southward
- northward

c) How far was the train from the cow when the engineer first applied the brakes?
______km

The Attempt at a Solution


I have a) and b) correct but I can't figure out c).

Thanks.
 

Answers and Replies

  • #2
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reverse it. imagine the train is accelerating at 0.22 m/s/s
 
  • #3
LowlyPion
Homework Helper
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Homework Statement


Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, when the engineer spots a cow on the track. The train brakes to a halt in 2.3 min, stopping in front of the cow.

a) What is the magnitude of the train's (constant) acceleration?
_______m/s^2 <-----(0.22m/s^2) Correct

b) What is the direction of the acceleration?
- eastward
- westward <-----Correct
- southward
- northward

c) How far was the train from the cow when the engineer first applied the brakes?
______km

The Attempt at a Solution


I have a) and b) correct but I can't figure out c).

Thanks.

First what equation do you know that relates distance acceleration and time?

(Btw, I think the 20th Century Limited stopped running in 1967. Today it's the Lake Shore Limited that runs the route.)
 
  • #4
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First what equation do you know that relates distance acceleration and time?

(Btw, I think the 20th Century Limited stopped running in 1967. Today it's the Lake Shore Limited that runs the route.)

I know this equation but I get the wrong answer: d=(vf^2-vi^2)/2a
 
  • #5
LowlyPion
Homework Helper
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I know this equation but I get the wrong answer: d=(vf^2-vi^2)/2a

I would have simply used the x = 1/2 a t2 relationship.

x = 1/2 * .22544 * (138 sec)2

The other equation works as well

x = (31.111)2/(2*(.22544))
 
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