If there is a vacuum in space, then what are the rockets repelled from?

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We recently received this question from one of our subscribers:

We often get similar questions, so we decided to answer them in more detail in this article.

Everyone knows that the Earth, with its gravitational force, attracts us. But for many it is not obvious that we (people) also attract the Earth. Moreover, these forces are equal in magnitude, but opposite in direction. This is a consequence of Newton’s third law and before we try to answer the above question, we need to correctly understand Newton’s third law. It sounds like this:

But everyone, of course, is accustomed to hearing such a very simple formulation of the same law:

Here are some examples of the manifestation of Newton’s third law. The car accelerates as it travels because the road pushes the wheels forward as a result of the opposition that occurs when the wheels push it back. That is why when the car moves over stones, they fly away in the opposite direction from the movement of the car.

Probably the most striking example of the manifestation of Newton’s third law is the movement of a rocket. Sometimes you can come across the misconception that a rocket takes off only because it is repelled from the Earth by a jet of burning fuel, but in fact, in a vacuum, spent fuel is more easily thrown out of the nozzle, so the thrust of the jet engine is even greater there. But still, from what then is the rocket repelled?

Imagine a space rocket flying in a space vacuum, while ejecting gas heated to a temperature of about 2800 * C from a nozzle with tremendous acceleration. Let’s see what happens when this happens.

The energy that originally existed in the form of the internal energy of a chemical fuel is converted into the kinetic energy of gas molecules that are formed during combustion, which means that they acquire a certain speed, and quite high. All these gases are thrown by the rocket in one direction, and if we multiply the speed of this gas by its mass, we get an impulse, and the change in momentum over time leads to the emergence of a force. If this force is divided by the mass of the rocket, we get the acceleration with which the rocket will fly away from the gas side. By the way, this movement is called reactive.

Consequently, thus, in space, the rocket, so to speak, is repelled by the hot gas, which with acceleration flows out of the rocket nozzle in the opposite direction.

Author: Alexey Nimchuk. Edited by Fedor Karasenko.

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