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.

7 answers

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

mgh*

cos theta*

sorry

also Deta E*

Delta* wow i cannot type

@kelly..may i know why have to l-l cos delta?

thanks

cos 90 is not equal to 1