How does emitting light cause you to get pushed backwards? Same with gasses escaping a rocket’s nozzle; how does that propel the rocket? Like, where does this opposing force come from?
In: 5
For rockets, it’s Newton’s 3rd law of motion, which states every action must have a reaction that is equal in magnitude, and opposite in direction. A rocket burns fuel, creating hot, expanding gas, which is directed out the nozzle / bell. The mass of this gas times its acceleration away from the rocket generates an equal “push” in the opposite direction, propelling the rocket.
Because the formula relates mass and acceleration in an inverse proportion, you can have a low mass moving very fast (rocket propellant), acting on a large mass moving much slower (the rocket), with equal force.
Newton’s 2nd: Force = Mass x Acceleration
Newton’s 3rd: F1 = -F2
Another way to imagine this is if you push your friend, it’s not just your friend who gets pushed backwards, you do, too. This is equal in magnitude, and opposite in direction.
For photons, it’s a bit trickier. They have zero mass at rest (when stationary), and so zero mass times any acceleration would equal zero force. However, physics tells us that photons gain a tiny amount of mass whenever they’re moving, ~~proportional to their velocity~~. So, just like a rocket, when photons zoom away, a tiny amount of force is generated in the opposite direction.
Edit: correction
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