Velocity and acceleration of a train

[honey1060]

A train travels due south at 60 m/s. It reverses its direction and travels due north at 60 m/s. What is the change in velocity of the train? Whats the average acceleration if it took 2 sec. to reverse.

 

[dmoravec]

Welcome to the forum...the rules state you need to show some work for us to help you. If the train went from 60m/s south to a stop what would the change in velocity be?

 

[honey1060]

60 m/s and since it goes back north would the velocity not change?

 

[dmoravec]

well it would change. it would change another 60 m/s. making the total 120 m/s. the overall speed did not change, but velicity has a direction and since the direction changed completely oposite the total velocity change would be 120m/s in the north direction. Think of an axis that represents velicity either north or south (north being positive south being negative). Originally you are at -60 (since your velocity is southbound). When you are done you are at 60 (since your velocity is northbound). Therefore the change in velocity is 60-(-60) = 120 m/s.

 

[honey1060]

ok now i got it. So if it took 2 sec. to reverse the average velocity would be -60-60/2 and since acceration is always positive it would be 60 m/s squared?

 

[dmoravec]

the answer is correct, but you can have negative acceleration. it just depends on how you define your axis. Negavie acceleration can be thought of as slowing down and then going in reverse, but again, that's assuming your axis was defined as positive in the direction you were going in. I would report it as the 60 m/s^2