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 every action there is an equal and opposite reaction. So if something goes shooting out the back of a rocket then there is an equal force in the opposite direction.
This is how the laws of the universe work.
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
The opposing force comes from the object pushed away from you. Consider:
What is the difference between pushing an object away from you with your legs, and jumping off that object?
If you were in space, in free fall, and asked to do those two tasks what to you do differently? Besides your point of view they are the same motions and the same result.
Now light having momentum is a bit weird since it has no mass, but I don’t think that is central to this question.
Think about a rifle. When you fire a bullet, there is recoil. This recoil is the force of the gas pushing against the gun with an equal force to how its pushing against the bullet.
In a rocket nozzle the same thing happens, the gases are ignited in the combustion chamber, which heats them up and they expand. This makes them accelerate through the throat (the narrowest part of the rocket nozzle) and out of the end of the nozzle. When this gas expands, it pushes on the walls of the combustion chamber which accelerates the rocket with the same force as the gas exiting the engines.
[Feynman speaks about the why question](https://www.youtube.com/watch?v=36GT2zI8lVA) in physics.
We can explain phenomena by other phenomena (The sun looks red in a sunset because of the atmosphere scattering light) but follow the why chain and you get to a certain point where.. it’s just how things are. We live in a world with certain properties. Matter has properties like inertia, temperature, etc.. Fundamental forces exist, like electromagnetism and gravity.
Asking “why?” to any of those fundamental things is delving into metaphysics. Philosophy merging with physics. Why do time and space exist? Why do we exist? What does existing mean? The question *Why do objects resist forces by applying an opposite force* is part of that realm.
The better question to ask in physics is always **how**, not why.
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