Calculating Potential Energy Changes in a Roller Coaster System

[Tmtamrak]

A 800 kg roller-coaster car is initially at the top of a rise, at point A. It then moves 135 ft, at an angle of 40.0° below the horizontal, to a lower point B.
(a) Choose the car at point B to be the zero configuration for gravitational potential energy of the roller coaster-Earth system. Find the potential energy of the system when the car is at points A and B, and the change in potential energy as the coaster moves.



GPE = mgy cos theta


This is my attempt to solve, however I keep getting that my answer is wrong.
(800kg)(9.8)(41.15m)x cos 40 =247,138.19J

Can someone please let me know what is incorrect in my process?

 

[sylas]

A 800 kg roller-coaster car is initially at the top of a rise, at point A. It then moves 135 ft, at an angle of 40.0° below the horizontal, to a lower point B.
(a) Choose the car at point B to be the zero configuration for gravitational potential energy of the roller coaster-Earth system. Find the potential energy of the system when the car is at points A and B, and the change in potential energy as the coaster moves.



GPE = mgy cos theta


This is my attempt to solve, however I keep getting that my answer is wrong.
(800kg)(9.8)(41.15m)x cos 40 =247,138.19J

Can someone please let me know what is incorrect in my process?

Suppose that the angle is zero degrees below the horizontal. What would be the height change?

Now put zero into your formula instead of 40.

Why is it different? Just becos.

 

[Tmtamrak]

The height changes by 9.6m...

If I do (800kg)(9.8)(41.15m) = 322,616J
.. but that is not the correct answer either.

 

[sylas]

The height changes by 9.6m...

If I do (800kg)(9.8)(41.15m) = 322,616J
.. but that is not the correct answer either.

0 degrees from horizontal is horizontal...