Rebuttal to Flat Earth argument: Space travel impossible since rockets have nothing to push against in space

WOULD ROCKET ENGINES FIRING IN SPACE NOT BE ABLE TO AFFECT THE MOTION OF THE SPACECRAFT SINCE THEY HAVE NOTHING TO PUSH AGAINST IN THE VACUUM OF SPACE?
PROBLEM STATEMENT AND RESPONSE by T. Mark Hightower 9/20/2017

SUMMARY OF THIS ARGUMENT USED IN FAVOR OF FLAT EARTH THEORY
This argument describes physical behavior familiar to most on the earth where it is either clear that motion is achieved by pushing against something or it appears that this is the case. I will describe two examples. A car pushes against the road with its drive wheels powered by the car's engine or motor in a direction opposite to the direction of the car's motion. A jet aircraft in motion in the air (atmosphere) appears to be achieving its forward motion by pushing against the air behind it with the thrust of its jet engines. In space there is no air, but instead a vacuum. A spacecraft in space firing it rocket engine would have nothing to push against, therefore the firing of its rocket engine would have no effect on its motion. Therefore space travel as commonly believed to be possible is actually impossible. Therefore the space program and space travel are fake and a complete hoax.

T MARK HIGHTOWER'S RESPONSE
This argument appears to make sense but it does not take into account the proper application of basic principles of physics that are well established by experiments that can be done here on the earth. The main principle of physics here is known as conservation of momentum. Linear momentum is the product of mass and velocity. Let me illustrate this with a simple thought experiment that could easily be demonstrated with an actual experiment. Let's say you have two ice skaters who weigh the same and are at rest in the middle of an ice rink facing each other holding hands. They each weigh 150 lbs. The skaters then simultaneously push against each other as hard as they can and release their hands, while keeping their skates parallel and in the direction of their pushing. These are skilled skaters able to maintain their position with skates parallel traveling backwards so that they travel in a straight line in the opposite direction of the mutual push against each other. What will happen? They will end up traveling in opposite directions to each other at the same speed. Eventually they will both slow down to a complete stop due to the slight friction between their skates and the ice as well as slight air drag due to their motion in the air. But the principle being illustrated depends on looking at their velocities right after they push away from each other, and not how they would eventually slow down. The total momentum they had before they pushed was zero, (300)(0), because their velocity was zero. Let's say the magnitude of their push was enough to give them each the same speed of 5 mph. So once they pushed off from each other, one had a momentum of (150)(5) and the other had a momentum of (150)(-5), their momentums being equal in magnitude but in opposite directions, which is taken care of mathematically by the negative sign. So their total momentum is still zero, 750 - 750 = 0.

Let's imagine a similar experiment. In this case let's say one skater weighs twice the weight of the other skater. Keeping the total weight of the skaters the same, one skater would weigh 100 lbs and the other 200 lbs. If they then push off from each other in the necessary magnitude of impulse to give the same individual magnitudes of momentum as the previous example, the 100 lb skater would be traveling at 7.5 mph and the 200 lb skater would be traveling at 3.75 mph. The math works out like this. (100)(7.5) + (200)(-3.75) = 750 - 750 = 0. So the skaters are not pushing against the ice or against the air in order to affect their motion. They are pushing against each other.

So for a spacecraft in space firing its rocket engine you have the mass of the spacecraft and the mass of the reacted rocket fuel that is ejected at high velocity upon firing. In this case the spacecraft is quite heavy compared to the very light rocket fuel, but the rocket fuel is expelled at such a high velocity that it pushes against the rocket, and the rocket pushes against it as they part ways. The total momentum of the rocket fuel system looked at as a whole will remain the same, the heavier space craft will go off in one direction at a lower velocity, while the much lighter rocket fuel will go off in the other direction at a much higher velocity. So this is how rockets work in space so as to change the motion or velocity of the spacecraft.

But as the rocket engine is firing, it is like, if you were on the ice, for the large skater, having a huge number of tiny skaters pushing off from the large skater in rapid succession over a period of time. The longer this process continues, the faster the large skater will be traveling. But eventually the tiny skaters will run out, and this would be equivalent to the spacecraft running out of rocket fuel. There are more details about how you actually calculate the behavior of rocket engines where you get into defining thrust and if I remember right something called specific impulse, but you do not need to get into that level of detail in order to understand the basic principle of how a rocket works in space according to well established laws of physics, as I have illustrated by these simple thought experiments. Space travel is indeed consistent with the commonly understood and accepted laws of physics. Therefore, space travel is possible.

ELABORATION ON THE JET EXAMPLE AND THE CAR EXAMPLE
In reality the jet engine is based on the same principle of conservation of momentum as the rocket engine in space. It creates its thrust by expelling mass behind it at high velocity. Here's the difference. The jet engine brings air in at the front to provide oxygen for the combustion of the jet fuel, so a portion of the mass the jet engine ejects at high velocity comes from the air. The rocket engine carries its own oxygen in addition to its fuel, so the mass that it ejects comes entirely from on board fuel and oxygen tanks.

The jet engine is pushing against the exhaust gases that it is expelling at high velocity, some of which comes from the air. The expelled exhaust gases are pushing against the engine. Forces always occur in pairs. Every action has an equal and opposite reaction.

In the case of the car, it is definitely pushing against the road in the direction opposite its motion with its drive wheels. Any exhaust it expels is not contributing to its motion.
END OF PROBLEM STATEMENT AND RESPONSE by T. Mark Hightower 9/20/2017