Posted by **Carden** on Monday, March 29, 2010 at 9:46pm.

A 0.47 kg pendulum bob is attached to a string 1.2 m long. What is the change in the gravitational potential energy of the system as the bob swings from point A to point B where θ = 31°?

I've tried using PE = mgh but I don't know where the 31 degrees comes in there. I'm confused as to what formula to use.

- Physics -
**Kelly**, Tuesday, March 30, 2010 at 5:01pm
for simplicity i am going to do this symbolically.

m = .47Kg

l = 1.2m

g = 9.81m/s²

è = 31 degrees

the change in potential energy is

mgÄh

in this problem,

Äh = l-lCos(è)

= l(1-cosè)

(it is 1-cosè becaus cos 90=1.. and its cos 90-cosè)

so

ÄE = mgÄh

= .47Kg * 9.81m/s² * 1.2m * (1-cos31)

and the answer is in units of joules

answer: .79

- Physics -
**Kelly**, Tuesday, March 30, 2010 at 5:02pm
mgh*

cos theta*

sorry

- Physics -
**Kelly**, Tuesday, March 30, 2010 at 5:03pm
also Deta E*

- Physics -
**Kelly**, Tuesday, March 30, 2010 at 5:03pm
Delta* wow i cannot type

- Physics -
**lk**, Tuesday, September 11, 2012 at 11:52pm
@kelly..may i know why have to l-l cos delta?

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