>> It seems like shining a flashlight in one direction should produce thrust in the other. Is that not true or is it something different at work here?
That is absolutely true. Electromagnetic radiation effectively has momentum. Take the energy in joules and convert it to mass via E=mc^2 and then plug it in the momentum equation p = mv but using c as the velocity and you get p = E/c. Change in momentum over change in time is force, so we can then get to F = P/c. I believe the units are Newtons, Watts, and meters/sec. So with 300 Megawatts you can produce a whole Newton of force!
I used this on a physics final once, but hid the work because the prof expected us to use formulas for radiation pressure derived from electromagnetics and I had missed that lecture. Later I derived the general case of my approach and it matched the EM derived formulas. 50/50 if he would have appreciated it or marked it down. The question was "a 50mW laser is reflected off a mirror, what force is exerted on the mirror due to the beam?". Note that the change in momentum is doubled for reflection vs emission or absorption.
> the prof expected us to use […] and I had missed that lecture
You just gave me a wave of nostalgia. That’s how I survived all of my math and physics classes. If you develop an intuitive understanding of the material, you can derive things as needed from a few core principles you do remember. A bit sad when learning is the way to subvert an educational system, though.
I am not sure that I buy that. Charging the battery moves molecules from one electrode to the other. The number of atoms in the battery doesn't change. Do they really get heavier by virtue of changing their arrangement?
If you use that logic (and maybe it is right) then it seems to me as though pushing a rock up a hill makes it ever so slightly heavier too.
Maybe that's my complete lack of intuition about non-classical stuff though.
Yes, they do get heavier by virtue of changing their arrangement. And yes, pushing a rock up a hill makes it - well, the rock will get slightly lighter in weight by virtue of being further from the Earth (classical inverse square law of gravity) - but the total mass of the rock-Earth system (assume you used solar power so you didn't take the energy from a terrestrial source) will slightly increase.
This is my understanding: say you have a spring which is just two positive particles. When you compress it, bringing the particles closer, you're storing energy in the EM field (E²+B²) near the particles. When you accelerate the spring, you're also accelerating the energy, which requires extra energy. This feels the same as the spring becoming heavier.
This is just a microscopic explanation for the general principle that energy has mass. I've never done the math to confirm that the extra energy to accelerate the compressed spring corresponds to the extra mass from stored energy.
When oxygen combines with hydrogen to form water, it gives off energy equivalent to the change in mass (water molecules are oh-so-slightly less massive than their constitute oxygen and hydrogen atoms). A battery produces power by converting molecules into different molecules with more binding energy (i.e. lighter molecules that have given up more energy/mass).
That is absolutely true. Electromagnetic radiation effectively has momentum. Take the energy in joules and convert it to mass via E=mc^2 and then plug it in the momentum equation p = mv but using c as the velocity and you get p = E/c. Change in momentum over change in time is force, so we can then get to F = P/c. I believe the units are Newtons, Watts, and meters/sec. So with 300 Megawatts you can produce a whole Newton of force!
I used this on a physics final once, but hid the work because the prof expected us to use formulas for radiation pressure derived from electromagnetics and I had missed that lecture. Later I derived the general case of my approach and it matched the EM derived formulas. 50/50 if he would have appreciated it or marked it down. The question was "a 50mW laser is reflected off a mirror, what force is exerted on the mirror due to the beam?". Note that the change in momentum is doubled for reflection vs emission or absorption.