4 ms·

It's not at all counterintuitive. It's just poorly explained. Energy is 1/2mv^2, and momentum (related to force) is mv. Anytime you have different speeds, you

It's not at all counterintuitive. It's just poorly explained. Energy is 1/2mv^2, and momentum (related to force) is mv. Anytime you have different speeds, you can arbitrage. If I am pushing against still air, I can generate nearly-infinite force for arbitrarily little energy. That's why you use wings instead of pointing jet engines downwards. You push a lot of air by a little bit (lots of mv, not a lot of 1/2mv^2, since v is small). If I am going an epsilon faster than the wind, with very little energy, I can push hard on the air.

ses1984 5y ago
Ok I have taken college level mechanics (after physics 1-2) and I have no idea what you’re saying.
- gibba999 5y ago
  Perhaps I can't explain it well either, without pictures. But it's not all too complicated. With pencil and paper, I could explain to a high school student. If you've done college level mechanics, that's on me, or on the medium. Perhaps I need to make a video explaining this clearly, since people are running in circles around this and it's just not very hard.
- yodelshady 5y ago
  With due respect to you, and assuming you passed, that's pretty damning of whatever college you went to. I'd want a refund. The definitions of kinetic energy and momentum and some basic differentiation, that should all be thoroughly covered.
whatsakandr 5y ago
I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each other, energy can be extracted. Doesn't matter how fast you're going relative to the two surfaces.
- gibba999 5y ago
  I actually find the limit to be somewhat less insightful. It sorta shows that it's possible, but it doesn't really show why it works in this case. 1. Air isn't solid. 2. There aren't two rollers. The analogy is a little bit distant. I just need to find time to make mine more eloquent.
  - ummonk 5y ago
    A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.
    - gibba999 5y ago
      I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant. For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss: - In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me. - In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency. Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However: - Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head. - And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding. I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of: 1. Being 100% precise 2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head

ses1984 5y ago

Ok I have taken college level mechanics (after physics 1-2) and I have no idea what you’re saying.

gibba999 5y ago
Perhaps I can't explain it well either, without pictures. But it's not all too complicated. With pencil and paper, I could explain to a high school student. If you've done college level mechanics, that's on me, or on the medium. Perhaps I need to make a video explaining this clearly, since people are running in circles around this and it's just not very hard.
yodelshady 5y ago
With due respect to you, and assuming you passed, that's pretty damning of whatever college you went to. I'd want a refund. The definitions of kinetic energy and momentum and some basic differentiation, that should all be thoroughly covered.

whatsakandr 5y ago

I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each other, energy can be extracted. Doesn't matter how fast you're going relative to the two surfaces.

gibba999 5y ago
I actually find the limit to be somewhat less insightful. It sorta shows that it's possible, but it doesn't really show why it works in this case. 1. Air isn't solid. 2. There aren't two rollers. The analogy is a little bit distant. I just need to find time to make mine more eloquent.
- ummonk 5y ago
  A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.
  - gibba999 5y ago
    I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant. For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss: - In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me. - In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency. Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However: - Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head. - And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding. I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of: 1. Being 100% precise 2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head