Home » Newton’s Laws of Motion: For My edx Course From Goddard to Apollo: The History of Rockets, Part 1

Newton’s Laws of Motion: For My edx Course From Goddard to Apollo: The History of Rockets, Part 1

How Newton’s Laws of Motion Apply to Rockets.

Newtons first law states that a object in motion will remain in motion and a object at rest will remain at rest unless acted upon by an outside force. This is easier to observe in objects with a large mass like a train for example. A train has a lot of inertia so when it is moving it would like to remain moving and it takes a great effort to stop it. The word inertia comes from Latin and literately means laziness. Things than have inertia are lazy. Lazy things like to keep doing what there doing.

Whether or not a rocket should have inertia depends on what you are trying to do. If the rocket has been launched then inertia is good because the rocket needs to overcome drag from the air. But before you launch inertia is bad because a force (thrust) must be applied to overcome the inertia and the more you have the greater the force required to overcome it. The best rockets have a small mass and are aerodynamic, this gives them a high thrust to mass ratio. Once the rocket is moving the drag should be minimized by the shape of the leading edge of the rocket, reducing the rate at which the rocket is slowed by the air.

Newtons Second Law.

There are many forces that act upon a rocket. A propulsive force is required to move the rockets while it is siting on the launch pad. The rocket can be made to move faster by either increasing the force and/or decreasing the mass. How quickly a rocket increases speed off the launch pad is called acceleration. A rocket’s acceleration is equal to the force applied divided by the mass (a=f/m). As force increases acceleration increases, as mass increases acceleration decreases. Re-arranging the formula gives you the classic way of describing Newton’s second law, force equals mass times acceleration (f=ma).

Newtons Third Law.
For every action there is an equal and opposite reaction. For example, if you hit the table with your hand the table hits back with an equal and opposite force. A rocket is able to lift off the launch pad because the acceleration caused by the expanding exhaust gas is able to overcome the rockets inertia. The same principle applies to airplanes, the only difference is that rockets are continuously accelerating as they burn fuel and their mass decreases.

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