B Why is KE not conserved when momentum is?

[mark2142]

No.

Okay. If we throw a elastic ball to a wall its momentum will change but its kinetic energy will not. Force will affect p but to change the KE force need to do work.
I think Kinetic energy is like a scared running dog with its tail on fire but the momentum is like a trained horse which always move in one direction.

 

[jbriggs444]

Okay. If we throw a elastic ball to a wall its momentum will change but its kinetic energy will not.

If you are considering the elastic ball alone as your "system", then the momentum of the system changes. It is not conserved. That is because you are not considering a closed system. There is an interaction with something external: the wall.

Force will affect p

Yes. The external force affects momentum.

Force multiplied by (or integrated over) time yields change in momentum (impulse).

but to change the KE force need to do work.

Yes. The external force can affect kinetic energy.

Force multiplied by (or integrated over) displacement yields change in kinetic energy (work).

If we looked closely, we would discover that during the collision, the net displacement was zero during the collision. There was a distortion of the ball and a rebound, but the result left the ball the same as it began. Further, we would find that the force pattern was identical (though reversed) during the deflection and the rebound. The work done over the positive displacement during the deflection was equal but opposite to the work done over the negative displacement during the rebound. Zero net work done. Zero kinetic energy change.

If we looked closely during that same collision, the force would have been the same direction the whole time. Non-zero momentum change.

I think Kinetic energy is like a scared running dog with its tail on fire but the momentum is like a trained horse which always move in one direction.

I do not see that this mental image is of any great help when making predictions or performing calculations. But whatever floats your boat.

 

[mark2142]

I do not see that this mental image is of any great help when making predictions or performing calculations. But whatever floats your boat.

Ok :)

 

[mark2142]

Change the ball bearing to eggs. Nothing changes on the momentum end of things. But now, the kinetic energy is going get transformed completely. Some of it into the energy required to break the shell, and most of it into heat, some of it into cooking the proteins, etc. The total amount of energy in the system will remain the same, but none of that energy will be kinetic in nature.

Yeah! The total momentum cannot change until and unless there is an external force. We can see that the COM is at rest all the time. But the egg breaks and it does not rebound. The total KE mv^2 is lost to zero. That must have gone into breaking the eggs.