Calculating Gravitational Potential Energy in a Swinging Child

[floydzeppelinfan]

Ok, this question is really making me mad... Keep in mind I just started to learn physics, so I'm not exactly a genious or anything...
but anyway here is the question:

A 40.0 kg child in a swing that is attatched to ropes 2.00m long. Find the gravitational potential energy associated with the child relative to the child's lowest position under the following conditions...

A. when the ropes are horizontal
B. at the bottom ofa circular arc


Now i understand that the formula is Pe = mass x gravity x height

BUT HOW MUST I FIND THE HEIGHT? i am just going back and forth on this one, any help is appreciated..

 

[Ambitwistor]

Um, draw a picture. The swing at the end of the rope traces out a circle of radius 2 meters.

If they're asking about the energy relative to the lowest position, then they mean that the height at the bottom of the arc is zero. So right there, you have the answer to B. For A, if the ropes are horizontal, then the swing is at the same height as the center of the circle. How far is that above the bottom of the circle, given the circle's radius is 2 meters?

 

[floydzeppelinfan]

wouldn't the height in part A be 1 ?

 

[Ambitwistor]

No. The vertical distance from the bottom of the circle to its center (and thus, to any point on a horizontal line passing through its center) is the radius of the circle.

 

[floydzeppelinfan]

OOOOOOOO that just hit me like a wall, thank you... just to confirm this , the height is 2 on A and 0 on B

 

[Ambitwistor]

Yes.