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 = llCos(è)
= l(1cosè)
(it is 1cosè becaus cos 90=1.. and its cos 90cosè)
so
ÄE = mgÄh
= .47Kg * 9.81m/s² * 1.2m * (1cos31)
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 ll cos delta?