The modern science of spaceflight rests on Newton’s Three Laws of Motion, a set of ideas and equations developed in the 1600s. In particular, the Third Law, which relates the action and reaction of forces, is essential to explain how a rocket accelerates. To make a rocket move forward, its engines point rearward. The action of the engine thrust produces a reaction on the rocket, accelerating it.
Newton’s Third Law
Newton’s Third Law of motion is all about how forces on objects balance each other, whether the objects are moving or not. A force acting on an object in one direction is always balanced by an equal force acting in the opposite direction. For example, a chair rests on a floor. Gravity acts on the chair, producing a downward force on the floor. The floor reacts with its own equally-strong force, pushing upward. If the floor didn’t balance the downward force, the chair would crash through the floor.
Rocket Mass and Inertia
A rocket’s mass has the property of inertia -- a resistance to changes in speed or direction. The inertia is tiny for a toy rocket and large for rockets used for space missions. In order for a rocket to accelerate, some force must overcome the inertia. In space or on the Earth, the rocket remains motionless until a force acts to move it. The rocket’s engines supply that force in the form of thrust. Depending on the rocket’s environment and circumstances, other forces may also be involved. For example, a rocket on a launchpad is weighed down by gravity. Engine thrust must overcome the rocket’s weight in addition to its inertia in order to achieve liftoff. As the rocket gains speed, it also encounters air resistance -- another force with which the engines contend.
A typical rocket produces thrust by chemical reactions in the rocket engine; the chemicals ignite, producing hot jet of gas. The heat of the reaction forces the gas out the rocket nozzle at great speed. According to Newton’s Third Law, the force that pushes the gas backwards from the rocket is balanced by an equal and opposite force acting in the forward direction. Non-chemical types of spacecraft use moving ions or even beams of light to produce thrust, but regardless of the technology, the Third Law of Motion still applies.
The forward force from the rocket engine’s thrust acts against the rocket’s inertia and other forces, producing a forward acceleration. Due to Newton’s Third Law, the forward and backward forces are always in balance. The larger the rocket’s mass, the more thrust it needs to achieve a given amount of acceleration. When the rocket runs out of fuel, thrust disappears, and acceleration becomes zero. If it happens in space, no air resistance slows it down; the rocket moves forward at the same speed indefinitely until it encounters an object or external force, affecting the rocket’s motion. If the rocket has engines for steering and slowing, the force from these will change its motion also.
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