# How long does it take for the train to stop moving?

## Homework Statement

a train is traveling down a straight track at 20m/s when the engineer applies the brakes resulting in an acceleration of -1.0m/s^2 as long as the train is in motion. how far does the train move durin g a 40 second time interval starting at the instant the brakes are applied?

## The Attempt at a Solution

I'm new to physics and have no idea what to do. i keep getting zero when i plug it into the formula. also...i know that the train eventually stops before 40 seconds and doesn't travel anywhere...but how do i incorporate that into the equation?

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Doc Al
Mentor
How long does it take for the train to stop moving?

i think it takes 20 seconds...but i'm not so sure.

Doc Al
Mentor
That's correct. If you're not sure, review the definition of acceleration.

okay...so if it takes 20 seconds for the train to stop moving...doesn't that mean it travels for a total of 210 meters?

Doc Al
Mentor
How did you deduce that? (Always show your work.)

i'm using common sense for the problem but i don't know what equation to use to get the answer of 200 meters. i just did 20+19+18+...+1+0=210 meters.

Doc Al
Mentor
i'm using common sense for the problem but i don't know what equation to use to get the answer of 200 meters. i just did 20+19+18+...+1+0=210 meters.
Commonsense is OK if you can justify it. What do those numbers mean? 20 = ?; 19 = ?...

If you are thinking that the train travels 20 m in the first second, etc., that's not quite correct. Hint: What's the average speed during the first second?

I suspect you should be learning some kinematic relationships.

thanks so much...i figured it out...and finally understand what i'm doing with all my other hw problems!