# Roller coaster and Potential Energy

## Homework Statement

A 1000 kg roller coaser train is intially at the top of a rise, at point A. It then moves 135 ft, at an angle of 40 degrees below the horizontal, to a lower point B.
Choose the train at point B to be the zero configuration for gravitational potential energy. Find the potential energy of the roller coaster-Earth system at points A and B, and the change in potential energy as the coaster moves.

Ug=mgy

## The Attempt at a Solution

Y at Point A=86.8 m
Ug= 850640 Kj

This isn't the right answer, so where am I going wrong?

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Doc Al
Mentor
Check your units. Kj = 1000 Joules.

That still does not give me the right answer even if I divide by 1000.

CompuChip
Homework Helper
Actually, if you'll allow me to nitpick a bit:
The unit is Joules (J), the prefix k(ilo) is written in lower case; therefore kilojoule is abbreviated kJ instead of Kj.

My bad! Either way, I don't get the right answer though :)

Doc Al
Mentor
My bad! Either way, I don't get the right answer though :)
When you input your answer, do you also input your units? (No need to divide by 1000. The kJ was a mistake on your part.)

135 ft is not 86 m. More like 45m. Try to convert properly.
Good Luck

Doc Al
Mentor
135 ft is not 86 m. More like 45m. Try to convert properly.
He wasn't converting, he was finding the vertical component. (He should have converted first.)

Good catch though: The distance was given in feet, not meters. D'oh! :uhh:

Then this is the mistake (besides the units).
You don't need to do this (find component). 135 ft is the height, if I read the problem right. The angle doesn't matter.

Doc Al
Mentor
You don't need to do this (find component). 135 ft is the height, if I read the problem right. The angle doesn't matter.
That's not the way I read it.
It then moves 135 ft, at an angle of 40 degrees below the horizontal, to a lower point B.
That sounds like it moved 135 ft at an angle of 40 degrees below the horizontal.

We'll find out soon enough when the OP corrects the feet vs. meters error. That's not the way I read it.

That sounds like it moved 135 ft at an angle of 40 degrees below the horizontal.

We'll find out soon enough when the OP corrects the feet vs. meters error. Yes, you are right. I did not focus enough...