Solving Rotation Question: Find Angular Speed After Collision

[sicrayan]

Homework Statement

the small block is heading towards the uniform vertical rod. the rod is pivoted at its midpoint and is free to rotate without friction.

the block collides with the rod, which is at rest, and sticks to it at its lower point.
find the angular speed of rod immediately after collision.

Homework Equations

we can use L_i=L_f and it will give the right answer.

The Attempt at a Solution

but when i try to use the conservation of energy, it doesn't work.
KE_block=(Rotational KE)_{rod and block}
why not?

 

[Beaker87]

The Attempt at a Solution

but when i try to use the conservation of energy, it doesn't work.
KE_block=(Rotational KE)_{rod and block}
why not?

This is an inelastic collision.

In an inelastic collision, momentum is conserved, but energy is not.

If the block had bounced off of the rod, energy could have been conserved (if the collision were perfectly elastic.)

 

[tiny-tim]

welcome to pf!

yup … in a collision exam question, energy is never conserved unless the question says so!

 

[sicrayan]

all right, thank you both :)