Part II.   Newton’s Laws of Motion and Concepts of Universal Gravitation
K.   What Happens When all Three Laws of Motion Are Combined?

Newton’s third law may be thought of, not as a separate and independent law, but rather as an extension of Newton’s first two laws to the actions and motions that result when all three laws are applied together. For example, let us again take three situations that exemplify all three laws of motion.

When a rifle at rest is fired, an explosive force occurs in the chamber, the small mass of a bullet acts and accelerates out of the muzzle at great speed in one direction, and the much larger mass of the rifle, its stock and the shoulder of the rifleman, accelerate, recoil and react at a much lesser speed in the opposite direction. The firing of a bazooka is somewhat similar.

When a stationary jet airplane takes off, the sustained forceful explosions in its turbo jet engines cause a continuous mass of air molecules to act and accelerate out of the rear of the engines at great speed in one direction, and the much greater mass of the airplane slowly accelerates and reacts in the other direction as it lifts into the sky.

Likewise, when a stationary 100 kg astronaut (M2) in space pulls with a force on a rope attached to a 500 kg barrel (M1) relatively at rest in space, the barrel acts and is slowly accelerated (a1) toward the astronaut, and the astronaut lurches, reacts and is accelerated (a2) toward the barrel at five times the speed of the barrel[1] (Figure 10). All of the above relationships are completely relative (not absolute).

Except for the resistance of friction and the force of the Earth’s gravity, all three of Newton’s laws apply in substantially the same way in empty space as on the Earth, and whether the force applied is pushing or pulling, or is gravitational, mechanical, chemical, electromagnetic or inertial. All of the aforementioned empirical laws of motion will become very important for our later discussions concerning “relative gravitational accelerations.”