What are rockets pushing against in space?

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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

15 Answers

Anonymous 0 Comments

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)

Anonymous 0 Comments

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.

Anonymous 0 Comments

>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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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).

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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Anonymous 0 Comments

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