# K.E. and P.E. in a Pendulum and Spring

Haseeb Ali
Can anyone explain the conversion of K.E and P.E during compression and expansion of spring and during the motion of pendulum ?

Homework Helper
Gold Member
Start with the pendulum. Which bit don't you understand? Do you know what PE and KE are?

Haseeb Ali
Start with the pendulum. Which bit don't you understand? Do you know what PE and KE are?
Yes .. i know that. I just want you to explain the interconversion of K.E and P.E as pendulum moves to and fro ,

Mentor
We're not going to do that. We require our members to put more effort than that into their own learning.

• Haseeb Ali
Homework Helper
Gold Member
At maximum deflection the bob has maximum Potential Energy. Call that PEmax.

Using Basic trig. you can calculate the bob height and hence the PE at any angle of deflection. Lets call that PE(theta) where theta is the instantaneous angle of deflection.

Then applying conservation of energy...

PE(theta) + KE(theta) = PEmax
or rearrange that to give
KE(theta) = PEmax - PE(theta)

Edit: Sorry russ_watters is right. That's why I haven't calculated PE(theta) for you.

• Haseeb Ali