# Kinetic & Potential Energy of a Pendulum

• VicGong

#### VicGong

Homework Statement
Assume a pendulum of length L is released from angle theta. When it swings to its lowest point (at the point where the string is vertical). the string hits a rod that is perpendicular to the plane of the swing and positioned at 1/2 L. Find an expression for the angle to which the pendulum will swing after hitting the bar.
Relevant Equations
PE = mgh
KE = 1/2 mv^2
TME = PE + KE
When the pendulum is released, the Kinetic Energy should be 0. When the pendulum is at the bottom/hits the rod, it should have 0 potential energy. However, I don't quite understand what happens after it hits the rod.

When the pendulum is at the bottom/hits the rod, it should have 0 potential energy.
This depends on where you put the reference level for your potential.

What conservation laws are applicable?

Hello @VicGong,
what happens after it hits the rod
Can you describe it in words ?
Perhaps do the experiment ?

Note that "it should have 0 potential energy" defines a zero-point for the potential energy.

 Ah! Oro was a fraction of a second faster

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However, I don't quite understand what happens after it hits the rod.
When the string hits the rod, the pendulum bob is moving at some speed ##v_0## which you can easily calculate. The subsequent motion will be that of a pendulum of length ##\frac{1}{2}L## that has speed ##v_0## at the lowest point of its motion.

VicGong and Lnewqban