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

[blader324]

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?

Homework Equations

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?

 

[Doc Al]

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

 

[blader324]

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

 

[Doc Al]

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

 

[blader324]

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]

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

 

[blader324]

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]

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.

 

[blader324]

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

 

[Feldoh]

These equations might help you out, they're for problems involving motion, including problems that involve constant acceleration:

http://en.wikipedia.org/wiki/Equations_of_motion#Linear_equations_of_motion