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Finding Potential Energy of a Swing When the Ropes Are Horizontal?

by falconpunch
Tags: energy, horizontal, potential, ropes, swing
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falconpunch
#1
Apr14-12, 12:29 PM
P: 1
1. The problem statement, all variables and given/known data

A 45.0 kg child is in a swing that is attached
to ropes 1.90 m long.
The acceleration of gravity is 9.81 m/s2 .
Find the gravitational potential energy as-
sociated with the child relative to the child’s
lowest position under the following condi-
tions:
a) when the ropes are horizontal.
Answer in units of J.

2. Relevant equations

Height of a pendulum:
h=L(1-cos(θ))

Potential Energy

PE=mgh

3. The attempt at a solution

So first I found the height of the swing at a horizontal position which I assumed is as at 90.

h=L(1-cos(θ))
2.75m=1.9(1-cos(90))

Then I used that height and put it into the formula for potential energy

PE=mgh

1213.99 J? = (45)(9.81)(2.75)

but the online homework interface I have to use to input my answer tells me this isn't correct. I was wondering if maybe my answer isn't in Joules, do I need to convert something. Any help would be greatly appreciated.

I realized my problem, i had my calculator in radian's sorry for the stupid question, I'd delete the question if i can find out how
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rock.freak667
#2
Apr15-12, 08:25 AM
HW Helper
P: 6,204
The ropes are 1.9 m long while the height you used is 2.75 m. How exactly did you get 2.75 m using h = L(1-cosθ)?


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