Momentum problem

[drgshahbaz1193]

I have a question about the problem having to do with momentum in the 4th physics video. In the video the block moves to a height because of the momentum provided by the bullet. The problem is solved with the work energy theorem W =(1/2)mv2 + mgh. But the kinetic energy equation is on the opposite side and without the negative value that would come from moving it. I am a little confused. Thanks.

[mcat_premed3832]

Actually, in the video, Conservation of Energy was used to determine the height h.

Since total energy is conserved at any point in the event . . .

Bottom Top
Ek + Ep = Ek + Ep
Ek + 0 = 0 + Mgh


The Work Energy theorem could have been used as an alternate method of calculation.

W = Ep + Ek

Recall that W = 0 and if we take down as positive then moving up 'h' will be negative:

W = 0 = - Ep + Ek

- Ek = - Ep
Ek = Ep which is the same result as using the Conservation of Energy.

Note: since gravity and the movement up to height h are in opposite directions, no matter the sign convention, you will get the same result.