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This page is intended for college, high school, or middle school students. For younger students, a simpler explanation of the information on this page is available on the Kids Page.

Computer Drawing of a falling ball which is used to explain
Newton's First Law of Motion

Sir Isaac Newton first presented his three laws of motion in the "Principia Mathematica Philosophiae Naturalis" in 1686. His first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object will maintain a constant velocity. If that velocity is zero, then the object remains at rest. And if an additional external force is applied, the velocity will change because of the force.

An object falling through the atmosphere is a good example of this principle. Just prior to release, the velocity of the object is zero, the object is at rest. The weight force acting on the object is balanced by the tension force in the rope holding the object. There is no net force on the object, and the object would remain at rest indefinitely. If the rope is cut, the tension force in the rope no longer acts on the object; the object is subjected to a single force, the gravitational attraction of the earth. Since there is no initial air resistance, the object begins to free fall and accelerate. But as the object velocity increases, it encounters air resistance, or drag, which opposes the motion. The magnitude of the drag depends on the square of the velocity. The drag increases until it is equal to the weight. At that point, there is again no net external force on the object, the acceleration goes to zero, and the body falls at a constant terminal velocity. The magnitude of the terminal velocity depends on the relative magnitude of the weight, the drag coefficient, the air density, and the size of the object.

Now, as the object continues to fall at the terminal velocity, it encounters air with an ever-increasing density. The density is a function of the air pressure and temperature, which are themselves functions of altitude. So, the terminal velocity decreases slightly as the object falls to earth; it is not truly a constant value. You can study this effect with an interactive simulator at this web site.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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