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

1. Aug 28, 2008

### imac

1. The problem statement, all variables and given/known data
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

3. The attempt at a solution
I have a) and b) correct but I can't figure out c).

Thanks.

2. Aug 28, 2008

### granpa

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

reverse it. imagine the train is accelerating at 0.22 m/s/s

3. Aug 28, 2008

### LowlyPion

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

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. Aug 28, 2008

### imac

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

I know this equation but I get the wrong answer: d=(vf^2-vi^2)/2a

5. Aug 29, 2008

### LowlyPion

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

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))