Gravitational Potential Energy of a swing

[bearhug]

A 395 N child is in a swing that is attached to ropes 1.8 m long. Find the gravitational potential energy of the child+Earth system relative to the child's lowest position when the ropes make a 30° angle with the vertical.

What I did with this problem is I drew a diagram and then figured that y would be 1.8cos30

U=mgy
U=395(1.8cos30)= 6.16e2 J

Is there anything that I did wrong with this problem it's confusing me with the angle. I feel like there's more to do than this but I'm not sure what.

Thanks for any help

 

[Noein]

The 'y' in the formula is the vertical displacement measured from the lowest point on the swing.

 

[bearhug]

That doesn't really help.

 

[Noein]

Draw the swing with the child just sitting there. Then draw the swing when it's pulled back 30o. The 'y' is how high the child has moved vertically.

 

[Rozenwyn]

The gravitational potential you calculated just now is at a height of [tex]1.8\cos{30}[/tex]m from the ground. However the guy is hanging down from a swing that's 1.8m long.

 

[bearhug]

So it's the difference. Thanks for clarifying.