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Newton's laws of motion and force
The three Laws of Motion were propounded by Sir Isaac Newton*. These principles summarize the behavior of moving objects. The Laws of Motion are fairly easy to understand by a layperson. These laws are valuable tool in explaining basic concepts in aerodynamics.

First Law of Motion
Newton's first law of motion states: "A body at rest will remain at rest and a body in motion will remain in motion in a straight line at a constant velocity in the absence of any interaction with the rest of the universe." This law is well demonstrated by inertia that is the resistance a body offers to any change in its state of rest or uniform motion.


(a) When the car suddenly begins to move, the inertia of the car's occupants pushes them back against their seats. As the car moves at a constant velocity, the occupants tend to remain at rest.
(b) When the car comes to a sudden stop, the inertia of the occupants tends to keep them in motion. Both effects illustrate the first law of motion.

Second Law of Motion
Newton's Second law of motion states: "When a body acted upon by a constant force, its resulted acceleration is inversely proportional to the mass of the body and directly proportional to the applied force." In equation form, the second law of motion is represented by:
F = ma   or     Force = Mass x Acceleration

Basically the second law of motion involves factors that overcome inertia (the first law of motion).

The force and the acceleration it produces are always in the same direction.

Third Law of Motion
Newton's Third law of motion states: "When a body exerts a force on another body, the second body exerts a force on the first body of the same magnitude but in the opposite direction."

Action and Reaction forces - When body A exerts force (Action) upon another body B, the body B exerts force (Reaction) on body A. The Reaction is of the same magnitude of the Action but in the opposite direction.

(a) The pushed car exerts an equivalent force to the pushing car but in the opposite direction.
(b) Both the towing and the towed car exert the same force on the tow bar but in the opposite direction.
(c) The weight (gravity) of the traffic light exerts the same force on the hanging bracket as does the suspension cable but in the opposite direction.

* Sir Isaac Newton (1642-1727), a philosopher and mathematician.

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Last update May 17, 2005
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