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# Simple pendulum kinetic energy help watch

1. Hi guys, I'll just go straight to the point.
I seem to not be able to get the kinetic energy part correct. I can get the GPE correct easily. I also want to find KE without having to minus max GPE with curren GPE.
I know to find kinetic energy is 0.5*m*v^2.
v should be "+-2πf(A^2-x^2)" where A is amplitude (length of string), x is the current displacement and f is 1/tp (tp=2*pi*sqrt(l/g))
This means "KE=0.5*m*(+-2πf(A^2-x^2))^2" right?
I'll give a question for you to solve (if you want ) that i found on another website
mass=0.34kg
gravity=9.8ms^-2
length of string=0.75m
angle of drop is 8.4
the max energy should be 0.027 (3 d.p)
2. (Original post by hllo123)
Hi guys, I'll just go straight to the point.
I seem to not be able to get the kinetic energy part correct. I can get the GPE correct easily. I also want to find KE without having to minus max GPE with curren GPE.
I know to find kinetic energy is 0.5*m*v^2.
v should be "+-2πf(A^2-x^2)" where A is amplitude (length of string), x is the current displacement and f is 1/tp (tp=2*pi*sqrt(l/g))
This means "KE=0.5*m*(+-2πf(A^2-x^2))^2" right?
I'll give a question for you to solve (if you want ) that i found on another website
mass=0.34kg
gravity=9.8ms^-2
length of string=0.75m
angle of drop is 8.4
the max energy should be 0.027 (3 d.p)
A isn't the length of the string, it's the Amplitude of the oscillation... the peak distance from the equilibrium point.

fwiw pendulums that show SHM always have small amplitudes in comparison to length - because SHM requires a restoring force that is proportional in size to the displacement... and this is approximately true for small angles, but not large angles.
3. (Original post by hllo123)
Hi guys, I'll just go straight to the point.
I seem to not be able to get the kinetic energy part correct. I can get the GPE correct easily. I also want to find KE without having to minus max GPE with curren GPE.
I know to find kinetic energy is 0.5*m*v^2.
v should be "+-2πf(A^2-x^2)" where A is amplitude (length of string), x is the current displacement and f is 1/tp (tp=2*pi*sqrt(l/g))
This means "KE=0.5*m*(+-2πf(A^2-x^2))^2" right?
I'll give a question for you to solve (if you want ) that i found on another website
mass=0.34kg
gravity=9.8ms^-2
length of string=0.75m
angle of drop is 8.4
the max energy should be 0.027 (3 d.p)
(Original post by Joinedup)
A isn't the length of the string, it's the Amplitude of the oscillation... the peak distance from the equilibrium point.

fwiw pendulums that show SHM always have small amplitudes in comparison to length - because SHM requires a restoring force that is proportional in size to the displacement... and this is approximately true for small angles, but not large angles.
The angle of oscillation is never more than about 10 degrees.

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