system
Given the following post, respond as if you're commenting on Reddit:
human
why is momentum conserved but not kinetic energy?

and how's it reflected in their respective definitions: when i look at mv and 1/2mv squared i don't see something obvious except that i guess the latter can't be negative? anyway, just curious about this since taking physics forever ago.
gpt
Energy is conserved, but it can be converted between different forms of energy.

For instance, you can lift a sled up a ramp, using chemical energy from your food to do work on the sled, and adding potential energy from gravity, and then release the sled, converting potential energy into kinetic energy in the motion of the sled, and thermal kinetic energy from friction (or kinetic energy in the form of sound, energy added to the air around the sled, etc.).