Kinetic & Potential Energy of a Pendulum

AI Thread Summary
When a pendulum is released, it starts with zero kinetic energy and zero potential energy at the lowest point. The potential energy reference level affects calculations, as it can be set at different heights. After the pendulum hits the rod, it retains a speed that can be calculated, which allows for the analysis of its motion as a shorter pendulum with half the original length. The subsequent motion is determined by this new length and the speed at the lowest point. Understanding these principles is essential for analyzing the energy transformations in pendulum dynamics.
VicGong
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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.
 
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VicGong said:
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,
:welcome: ##\qquad## !​
VicGong said:
what happens after it hits the rod
Can you describe it in words ?
Perhaps do the experiment :smile: ?

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

[edit] Ah! Oro was a fraction of a second faster

##\ ##
 
VicGong said:
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.
 
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