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Energy of Rod about pivot

Physics Asked by Dude156 on March 18, 2021

Consider a rod, with mass $m$ and length $l$, that is fixed about a pivot point at one of its ends.

Now imagine that we flick the rod so that the end of it attains a velocity of $v_{0}$.

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Now, if we look at the instant of impact, the rod appears to be moving towards the right with a velocity of $v_{0}$ or with a kinetic energy of

$KE=frac{1}{2}mv_{0}^2$

(I draw upon this idea as I often use this technique to find the velocity of a rod after a collision using conservation of momentum)

However, if we calculate the kinetic energy of the rod with respect to its moment of inertia and angular velocity, we attain a different answer:

$KE = frac{1}{2}Iomega^2$

$KE = frac{1}{2}frac{1}{3}ml^2omega^2$ (moment of inertia of a rod about a fixed end is $frac{1}{3}ml^2$)

$KE = frac{1}{2}frac{1}{3}ml^2(frac{v}{l})^2$

$KE = frac{1}{6}mv^2$

However, these two values do not equal each other. What went wrong? Is it incorrect to imagine the rod as not being attached to the pivot for an instant? If so, why does conservation of linear momentum still hold when using this trick?

2 Answers

The speed of the rod varies from $0$ at the pivot to $v_0$ at the free end.

To find the total kinetic energy of the rod via the $frac 12mv^2$ formula consider an element of the rod of length $dr$ at a distance $r$ from the pivot.

That element has a mass of $frac m L dr$ and a speed of $frac r L v_0$ and hence a kinetic energy of $frac 12 frac m L dr left(frac r L v_0 right)^2$.

Integration along the length of the rod will give you the kinetic energy of the whole rod.

Answered by Farcher on March 18, 2021

You can say that the translational K.E. Is (1/2)m$v_o^2$ where $v_o$ is the speed of the center of mass, but with that you must include the rotational energy about the center of mass.

Answered by R.W. Bird on March 18, 2021

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