The Mysterious 8.88 ft-lbs: A Man in Space Story

[Doc Al]

Yes, you showed that the second firing did 3 times more work, but did you show why 3 times more work didn't add 3 times more velocity to the pellet? No, the only answer you could give was that because K.E. is defined to be 1/2mv^2.

Huh? Given the definition of work ($\vec{F}\cdot \vec{s}$), and an understanding of Newton's laws, one derives the fact that $W = \Delta (1/2mv^2)$. Yes, we call that quantity "Kinetic Energy", but regardless of the name it is still $1/2mv^2$.

If K.E. was undefined and you started from rest and applied a constant force over 1 distance unit to achieve a velocity v for a particular mass, wouldn't it be reasonable to assume that if you applied the same force over 2 distances, you should achieve a final velocity of 2v?

Not if you understand Newton's laws. A force can both do work on a particle leading to a change in KE and exert an impulse ($\vec {F}\Delta t$) on a particle leading to a change in momentum.

 

[bruce2g]

...You thought the right brick came to rest but it didn't since it has a final kinetic energy of -1.0mv^2.

In my universe, macroscopic kinetic objects cannot have negative energy; therefore I will waste no more time on this matter since you appear to be inhabiting some other universe in which bricks can have negative energy.

Bruce