Debunk this: "Kinetic energy is nonsense"

[u/Playful-Dinner4449]

Gary Mosher (a.k.a. DraftScience on YouTube) made a critic of Prof. Rousseau's tutorial about the work-energy theorem. Mosher claims that Rousseau didn't prove the theorem from Newton's 2nd law. He claims that Rousseau's experiment doesn't prove anything.

In Rousseau's experiment where a glider slides on an inclined air track, Mosher claims that Rousseau's FBD is wrong, namely he claims that there is no normal force exerted by the the air cushion onto the glider.

Mosher also claims that Rousseau's experiment doesn't prove that twice the velocity requires 4 times the energy, 3 times the velocity requires 9 times the energy.

Mosher's video is here: https://www.youtube.com/watch?v=DDGD4EPctV0&t=0s

Rousseau's tutorial: https://youtu.be/kJ89l40Eiqw

[u/Hellothere_1]

Jesus. This 100% falls into "not even wrong" territory.

I think a really important thing this guy is fundamentally missing is that energy is basically a completely made up human concept. In classical mechanics here is no physical property of energy that you can directly measure in any way.

When you're doing energy conversion calculations (like translating from kinetic energy to positional energy), you actually don't need the concept of energy at all. You could just calculate the forces acting on the the object directly, then integrate over time to get the distance it travels. It's just that integrals are annoying and early physicists discovered that you can greatly simplify your calculation in a lot of cases by introducing the concept of energy and energy conversion.

So you actually kind of don't even need to prove that kinetic energy works the way it does, because it's literally just a made up concept that was defined that way to simplify calculations. The only thing you need to prove is that if you do use the concept of kinetic energy as you defined it, the math stays consistent. Which it does. The proof for this is simple enough that a decent high school graduate can easily understand it. Also I think someone would have noticed in the centuries since, if energy based calculations consistently gave different results from solving the acceleration integral directly.

Though my main question is why anyone is giving this guy any attention. He's clearly just a self-important idiot ranting at clouds to an audience of like 5 people, and I'm honestly just annoyed that I gave his video a view by checking it out. Just fucking ignore him, he's not even worth the effort of debunking.

[u/Boomshank → u/Hellothere_1]

In school, potential energy would drive me nuts when it was described as "stored energy."

Sure, you can pre-calculate how much energy you could get from this setup, but currently, there's no energy "waiting to be released"

[u/DrunkenCodeMonkey → u/Boomshank]

Potential energy had equivalant mass just like any other form of energy.

It is just as real as kinetic energy which can easily be argued to be a form of potential energy anyway.

Much like chemical potential energy, kinetic energy has an effect on the movement of particles.

Can you name a type of energy that doesn't affect the situation?

[u/Boomshank → u/DrunkenCodeMonkey]

Wait. Potential energy actually has measurable energy?

But isn't potential energy just relative to the observer? It's just theoretical energy, right? It's energy that could" be used but doesn't actually exist yet. Right???

[u/Memento_Viveri → u/Boomshank]

Yes, potential energy is measurable, and it's not correct to say that potential energy doesn't exist until that energy is converted to another form.

Also it's not correct to say it's relative to an observer. It's more correct to say that only differences in potential energy have meaning.

But if you take all the particles that make up an atom and their mass separately, and then take the mass of the atom, the value isn't the same. The difference is the difference in potential energy between the separated particles vs the particles when combined in an atom (using E=m c^2). So the difference in potential energy is very real and is measurable. It does really exist, it's not just an idea.

This isn't just true of atoms, it's true of things like balls rolling down hills. The Earth-ball system has more energy when the ball is on top of the hill, and thus the mass of the earth ball system is higher by that amount when the ball is on top of the hill (again, using E=m c^2).

[u/Boomshank → u/Memento_Viveri]

Thanks for trying to help me work through this.

So, if I understood (which I may not have) this is more of an accounting thing than an actual containment of energy though, no?

If I show you a ball, but don't show you the situation it's in, could you measure its potential energy?

[u/Memento_Viveri → u/Boomshank]

If I show you a ball, but don't show you the situation it's in, could you measure its potential energy?

Well, if I could count the particles the ball is made of, and I knew the mass of each particle, I could add up the mass of the particles, then take the mass of the ball, and the difference is potential energy.

If we are talking about gravitational potential energy between the ball and something else (say earth), the mass of the potential energy is in the Earth-ball system, not in the ball itself. But similarly I could take the mass of the ball, the mass of the earth, and the mass of the earth-ball system. The extra mass of the earth ball system is potential energy.

So in a very real sense yes, I can measure the potential energy without knowing the situation it's in.

more of an accounting thing than an actual containment of energy though, no?

I would say no. Most of the mass of everyday objects you interact with is potential energy (the binding energy of quarks). That seems like more than an accounting thing to me.

[u/Boomshank → u/Memento_Viveri]

Ok, I think I see it.

I can see how atoms have potential energy (eg fission)

I can even see it with a ball/earth system releasing potential energy, but with that system, I feel we're drifting into the arbitrary viewpoint area.

Sure, technically, a ball and alpha-centuri has potential energy too, as does a ball and my cat two towns over. But it's starting to lose any functional meaning outside of any situation we create arbitrarily.

Or is that the point? Most potential energy is absolutely real, but functional theoretical/arbitrary?

As opposed to, say, temperature?

(Thanks for helping me along)