How Does Bullet Speed Change After Hitting a Pendulum?

[gneill]

because with that I am not mixing bullet with the pendulum, and there is a form of energy that did give the pendulum height, and that's its initial velocity.

Yes.

 

[zaddyzad]

Because energy is not conserved over the collision. Before and after the collision it is, for both objects separately.?

Can you explain this please ?

 

[zaddyzad]

I got the answer though :D

 

[zaddyzad]

And what are some other possibilities of finding the initial velocity of the pendulum. why didnt 300(0.01) = (2.21)V' work? Because for a brief instant that the bullet does hit the block or the slight second its leaving the bullet is in the block, and momentum should be transferred no ?

 

[gneill]

Because energy is not conserved over the collision. Before and after the collision it is, for both objects separately.

Can you explain this please ?

Before the collision the bullet has a constant KE assuming that there's no air friction.
After the collision the bullet has a constant KE assuming that there is no air friction.

Before collision the pendulum bob has zero velocity and a constant height, so its KE and PE are constant.
After collision the pendulum is moving in a conservative field (gravitation), so total energy is conserved.

 

[gneill]

And what are some other possibilities of finding the initial velocity of the pendulum. why didnt 300(0.01) = (2.21)V' work? Because for a brief instant that the bullet does hit the block or the slight second its leaving the bullet is in the block, and momentum should be transferred no ?

The bullet and block are never one object moving with the same velocity. Simply occupying the same space does not count

 

[zaddyzad]

I don't fully understand why EK(bullet) = EK(bullet) + EP(bullet) doesn't work. The kinetic energy before and after the collision is constant, and so it the EP that the pendulum has. Why isn't the energy of the system being conserved.

 

[issacnewton]

whats the answer you got finally ?

 

[gneill]

I don't fully understand why EK(bullet) = EK(bullet) + EP(bullet) doesn't work. The kinetic energy before and after the collision is constant, and so it the EP that the pendulum has. Why isn't the energy of the system being conserved.

Energy is only conserved for perfectly elastic collisions. Otherwise, energy is always lost in the collision to various "loss" pathways such as frictional heating, sound, plastic deformation of materials, breaking of atomic bonds(tearing, breaking), and so on.

 

[zaddyzad]

Energy is only conserved for perfectly inelastic collisions. .

Though kinetic energy is only conserved in perfect elastic collision?

 

[gneill]

Though kinetic energy is only conserved in perfect elastic collision?

D'oh! My bad. Of course it's ELASTIC collisions only. I fixed the text in the original.