/sci/ - Science & Math

>>11580688
A rocket is a closed system if you always draw your bubble to contain everything you start with.

Imagine a rocket floating in a universe unaffected by gravity wells. We, the observers, watch from an inertial reference frame which is stationary to the rocket before it fires its engines. The rocket is pointed along the positive x axis ftom the origin (us).

If a 101 kg rocket fires its engines at ~1 G of acceleration, for one second in a universe with nothing else in it and attains a velocity of 9.81 meters per second such that it lost a kg of propellant in the process, it will have a momentum of 981 kg*m/s because it's 100 kgs going 1 meter per second.

There will now also be a cloud of propellant moving opposite this rocket which, when the velocity of every bit of that gas cloud is integrated with respect to mass, will have a net momentum of negative 981 kg*m/s (negative because of our arbitrarily-chosen convention for motion).

The rocket at the beginning of the experiment had a momentum of zero kg*m/s relative to our reference frame. At the end of the experiment, we still have a system with net zero momentum (a 981 kg*m/s rocket and a -981 kg*m/s gas cloud)- thus, momentum is conserved. The system ceases to be closed when you define the boundary to include only the rocket and in that case momentum is not conserved.

It is also interesting to note that so long as the rocket was lined up and is not moving on the Y or Z axes at all, you can add up the velocity-mass components of the gas cloud and will find that its net Y and Z momentum will be zero (even though individual particles fly off in all directions).

My final argument against all this dipshittery is firearms. Do you really think all that recoil is developed by the bullet pushing on the two grams of air in the barrel?