What are rockets pushing against in space?



In order to move, you have to apply an equal force to something else, right? So how can rockets move in space where there are no particles?

In: Physics

The force doesn’t have to push against anything. For example, gravity pulls us towards the earth without pushing against anything else. Strictly speaking, the thrust from a rocket engine is pushing against the rocket.

We tend to think that we need to push against something because that’s how our bodies work when we walk or swim, but a sturdy balloon deflating in a vacuum would fly around just as easily as one flying around in the air (more easily, in fact, because there’s no wind resistance)

The force isn’t applied to something. Force is applied in a direction. You’re applying a force in one direction (the rocket engines), so there must be an opposite reaction. The opposite action is the rocket moving forward.

>In order to move, you have to apply an equal force to something else, right? So how can rockets move in space where there are no particles?

They’re pushing “against” the same thing against which they’re pushing within an atmosphere: their exhaust gases.

Rockets don’t push against anything. They throw burning gas behind them. If you apply newton’s third law of motion to the situation then the effect of throwing the gas out behind the rocket applies the same force to the rocket in the opposite direction, causing it to accelerate.

When a rocket fires its engines, it is “pushing” against the gases that exit. Newton’s 3rd law, which you seem to cite, states that every action has an equal and opposite reaction. Think of this as forces being mutual. Multiply a force by the amount of time that force is applied, and you’ve got the change in momentum. So a rocket “pushes” its exhaust gases out very quickly out the back, applying force over some time. Those exhaust gases have momentum in that backwards direction. The universe conserves momentum, however, so the rocket has to have equal and opposite momentum (so the momenta cancel each other out). This moves the rocket in the forward direction. Since the rocket is more massive than the exhaust gases, the rocket will move more slowly but have the same momentum (which is equal to the product of mass and velocity).

Wrong. Newton’s third law states that for every action there is an equal and opposite reaction. Imagine floating in space. If you throw a baseball you will drift in the opposite direction, but not as fast as your mass is a lot greater than the baseball. But the energy imparted to the ball, will be equal to the energy applied to your body.

Now instead of a baseball, use one of those huge medicine balls they made us toss around in gym class. The ball will move slower away from you, but you will move faster than you did with the baseball. The energy of the medicine ball is equal to the energy of your body.

Now instead of balls, we use a chemical explosion, that converts liquid fuel to fast expanding gas, you need to throw away a lot of gas very fast to move the heavy rocket quickly.

Despite a couple other responses you correctly identified that there are two objects involved in any force (Newton’s third law).

In this case you fell into the common blind spot of forgetting that a gas is an object. The rocket engine pushes the exhaust backwards hard, and that pushes the rocket forward.

And this is what it does the entire time, even down here in the atmosphere. The air here is actually slowing the rocket, as it’s in front of the rocket and has to be pushed out of the way for the rocket to ascend.

Think of it this way: it’s the exhaust flame of the rocket engine pushing itself off of the rocket, and pushes the rocket away.

Second sentence in case I get moderated for a single sentence reply.


They don’t push against anything it is just the conservation of momentum, throw something out the back and the rest goes forward. https://youtu.be/fsxxKNpaSz0

In a few words: they are simply splitting in half

The solid part (what we call the rocket) goes one way, the gas part (what we call rocket plume) goes the other way. That’s why a rocket is almost entirely made of rocket fuel that burns almost explosively, because gas isn’t really heavy by itself, so you need a lot of it and you need to throw it really really fast.

Think of it like kickback from a gun. If you’ve never fired one, I’m sure you’ve seen it on TV. You shoot a bullet, the bullet leaves the gun in one direction really fast and the gun pushes back the other direction (at a lower speed because it weighs a lot more).

A rocket is essentially doing the same thing. It’s shooting stuff out behind it at a really fast speed. The rocket is pushed in the opposite direction. This is true both in the atmosphere and in space.

Rockets move by ejecting a mass (exhaust gasses) at high speed opposite the direction they want to go.

Rockets actually work better in space because there is no air slowing down the exhaust gasses from the rocket engine – and of course there is no air in front of the rocket slowing it down either.

A simple experiment you can do to see this effect;

Use a garden hose with nozzle that you can lock in the “jet” setting. You can hold the garden hose a little ways back from the nozzle using the thrust from the water jet to hold itself up.

Now dunk it into bucket of water, or pool and see what happens to the thrust the water jet is producing. It decreases dramatically even though it now has water to “push” against.

Raise it back out of the water and the thrust returns.

Imagine shooting a rifle. The bullet being shot forward out of the gun pushes the gun, and you, backwards. The atmosphere around you and the rifle has a negligible impact on the amount of recoil you feel. You could shoot a gun in the vacuum of space and the recoil would be the same. Now imagine you have a machine gun and are sitting on a rolling chair. Firing the machine gun would propel you backwards. Rockets work the same way except instead of riding the recoil from bullets being fired, they are riding the recoil from the exploding hot gases inside the combustion chamber being expelled out the nozzle.

In fact, the atmosphere has a negative effect on a rocket’s performance. The rocket exhaust has to push the atmosphere out of the way on it’s way out of the engine, slowing it down and reducing performance. While we can optimize an engine to lose less performance as sea level, (the space shuttle main engines are a great example of this) at best we can only reduce the performance lost.

The ignited gases in a rocket engine ARE pushing against something else. That something is the rocket. Gasses are expelled in one direction, the rocket goes in the other. Balanced, as all things should be….

You don’t need any “additional particles”.