I have been doing research and the explanations don’t make sense. Kinetic energy is based off of velocity, and so is momentum. If the kinetic energy decreases, the momentum would also have to decrease, because the objects in the collision have less energy and therefore move at slower rate, and therefore have less momentum. It seems like momentum and energy are one, but but in reality it isn’t? And here’s my thought experiment:
Two objects collide and lock together. They maintain their original momentum, but are now locked together as one. According to what I am currently learning, they have lost kinetic energy, but they have maintained their total momentum. Now, the locked objects hit an equal copy of themselves in front of them, except the copies are unlocked. Due to conservation of momentum, the locked object stops, and the copies are now at the same point the two objects were at the start of the experiment, before they collided. How can energy be lost, and then gained? Where did they go? If someone could simply explain what I’m missing, that would be very helpful as I have a test in 12 hours. Thanks!!
In: Physics
Clap your hands together in front of you, trying to be as symmetrical as possible. The moment before they connect, they have the same non-zero kinetic energy (same mass, same speed); the moment after they connect, they have zero kinetic energy. Where’d the energy go? Where’d the momentum go?
I think what you’re missing is that momentum is technically based off of velocity *in a specific direction*, while kinetic energy is a scalar (aka just a number). Momentum to the left is different than momentum to the right. When you clapped your hands, your leftward momentum plus your rightward momentum always added to zero.
In your thought experiment, if I’m reading it right, I think you’re combining an inelastic collision to start with an elastic collision at the end, so you’re going to end up seeing some weird results when you flip from one to the other.
Say you’ve got a big block of concrete moving to the right. It’s got a mass M and a velocity V, and that calculates out to kinetic energy K and momentum P. Then you’ve got a snowball that you’ve thrown to the left with mass m, velocity v, kinetic energy k, and momentum p. Because it’s ELI5 we’ll ignore all those pesky things like friction and air resistance.
Snowball hits concrete, and the combined object keeps moving to the left. Turns out a snowball isn’t enough to stop a concrete block. However, because of Newton’s Laws of Motion, we know that the snowball applied a force to the block, which accelerated it – changed its velocity. Meanwhile, the combined object as a whole now has mass M+m, velocity V1 (which is slightly slower than the block’s original V), kinetic energy K1 (which is lower than the original K), and momentum P+p. Total mass is conserved, total kinetic energy is conserved, the rest change.
The key there is that kinetic energy isn’t typically conserved. It just doesn’t work that way. Imagine dropping a buttered piece of bread onto the floor, it just goes thwack and stops moving.
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TLDR: Momentum is conserved, but kinetic energy is not; since KE is just a measurement of how much moving is happening it can appear and disappear without causing problems. (There’s typically something converting to or from KE, such as potential energy turning into kinetic energy when you drop a ball, but it can also just be that it makes heat or a sound – like clapping hands – but that aspect is often ignored in physics homework).
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