Intuitively I think that a motor can make a rig move based on two principles. (i) Either the law of action and reaction is at work. Eg. an aircraft’s exhaust exerts force on the air molecules, causing a recoil reaction, so that the aircraft moves forward. Or the violent forces released during the launch of a rocket causes the rocket to initially push against the launch pad and, once airborne, against the air molecules. (ii) Either an ignition causes a rotational movement of cogwheels, which is transferred to a wheel, which causes a forward movement through the rotational movement against a static surface (the roadway). But a rocket in space cannot push against air molecules when firing its engines – there are none – let alone cause a rotational movement against a roadway! How can it ever accelerate without using the gravitational pull of a celestial body? I seem to remember that the Apollo rockets ignited the engine in the empty space between the earth and the moon, for example to be able to start or adjust. Or can they resist (action-reaction) against a gravitational field of a celestial body?
Answer
It is perfectly possible to accelerate in a vacuum, without needing anything “static” to counteract. The principle also applies within the atmosphere. A rocket that takes off, but also a jet plane in the sky also uses them, they do not resist air molecules.
The principle is an important law in physics: the conservation of momentum. The momentum of a moving mass is equal to its mass x its speed. Think of it as the “amount of movement”. You understand, there is “more” motion present when the speed is greater, but you can also say that there is “more” motion present when the mass is greater. A truck driving at 40 km/h represents more movement than a cyclist at the same speed.
How does conservation of momentum work? Suppose you have two masses that are first together next to each other, one of 5 kg and one of 2 kg. The total momentum is then 0, because both masses are stationary. Through some mechanism you now let those two masses push themselves against each other. (So ​​assume the friction with the ground is zero). Then you will see that as the masses move away from each other the ratio of their velocities is also 5/2 in favor of the light mass. If the mass of 5 kg has a speed of 10 m/s, then the lighter mass of 2 kg will have a speed of 25 m/s. Or no, actually from -25 m/s because the two speeds are opposite and so you have to give one of the two a negative value. Those speeds are therefore measured against the stationary position.
You can see that these values ​​satisfy conservation of momentum. After the separation, the total momentum is:
5 . 10 + 2 . (-25) = 0 just like before !
And if everything is already moving at a constant speed? Let’s say at 30 m/s?
Total momentum for the separation : ( 5 + 2 ) . 30 = 210 kg.m/s
Afterwards: the mass of 5 kg now moves at 30 + 10 m/s, and the mass of 2 kg at 30 – 25 m/s
so : total momentum after the separation : 5 . 40 + 2 * 5 = 210, same thing again!
Admit it, the laws of physics are nicely put together. Conservation of momentum applies not only when the center of mass is stationary, but also when it moves, which is precisely the case with a rocket:
In a rocket (in a vacuum or in the atmosphere, it doesn’t matter), fuels are used that, when they come into contact or are ignited after mixing, undergo an extremely violent reaction that transforms the fuels into a hot gas. Enormous heat in a gas means enormous pressure. The combustion chamber of a rocket engine has one opening through which the gases escape at an enormous speed. The gases leaving the rocket at the back have a certain momentum (their mass x their velocity) and conservation of momentum propels the rest of the rocket forward. Because more and more fuel is used, less and less residual mass has to be accelerated, so that the rocket motor becomes more and more efficient by that alone.
Answered by
prof.dr. Paul Hellings
Department of Mathematics, Fac. IIW, KU Leuven
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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