Newton's Laws and Flight

Newton's Laws and Flight:

Flight Notes

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Law 1: Inertia: The law of inertia has two parts. The first part states that an object at rest will stay at rest unless acted on by a force. A force is a push or a pull. Inertia can be seen when someone pulls a tablecloth out from under dinner plates and the plates stay on the table; the plates stay at rest. Likewise, a plane sitting on the runway will stay at rest until the engine forces it to move.

The second part of Newton's first law states that an object in motion will stay in motion in a straight line, unless acted on by a force. You experience inertia every time a car stops and your body continues to move forward, pulling against the seatbelt and shoulder strap. Were it not for very powerful brakes, a fast-moving plane would continue to roll right off of the runway.

Inertia is a property of mass; massive planes have more inertia than smaller planes. Therefore, massive planes require much more force to speed up or slow down. See inertia for more information.

Law 2: F = ma: The net force acting on an object is equal to the product of its mass and acceleration. A force is a push or pull. If a net force acts on a mass, it will accelerate in the direction of the force. Acceleration is a decrease in speed, an increase in speed, or a change in direction.

Newton's 2nd law states that the acceleration of an object is inversely proportional to its mass. In other words, it is difficult to change the speed of massive objects and it is easier to change the speed of smaller objects. For instance, a full shopping cart is difficult to speed up and slow down. In contrast, if the same amount of force is applied to an empty shopping cart, its speed can be changed more easily.

Here is another example: imagine a motorcycle, a car and a big truck at a stoplight, each with the same horsepower motor. Which vehicle will have a greater acceleration? The motorcycle will have a greater acceleration because its mass is smaller. (Acceleration = Force/Mass) As compared to a massive plane, a small plane is easier to accelerate.

Newton's 2nd law: also states that the rate at which an object changes speed is proportional to the force that is exerted. Engines provide thrust and accelerate a plane forward along the runway. If the engines supply a small force, only a small acceleration will result. If a larger force is generated, a larger acceleration will result. Newton's 2nd law can be used to calculate the force required to change the speed of a plane.

Law 3: F = -F:    Newton's 3rd Law states that forces always come in equal and opposite pairs. Squeeze your index finger and thumb together. Which pushes with more force? No matter how hard you squeeze, the forces are equal an opposite. Your thumb and finger interaction is an example of an action-reaction pair. If you push on a door, the door pushes back with an equal and opposite force. This is also an action-reaction pair. As you slide a sled down a hill, the frictional forces of the sled are equal and opposite to the frictional forces of the snow. This is another example of an action-reaction pair.

Consider some examples of action-reaction pairs associated with a plane: As a plane sits on the runway, it applies a force on the earth and the earth pushes back with an equal and opposite force. As a plane flies, the force of the air hitting the plane is always equal and opposite to the force of the plane pushing against the air. The force generated by the engine pushes against air while the air pushes back with an equal and opposite force. See Newton's 3rd Law for more information.