# Force and Acceleration

The velocity of a moving object is the rate at which its position or location changes and is measured in units of length per time -- for example, meters per second or miles per hour.

The acceleration of a moving object is the rate at which its velocity changes and is measured in units of length per time2 -- for example, miles per hour2 or meters per second2.

The mass of an object measures its resistance to acceleration or its inertia. In the metric system mass is measured in units of grams or kilograms.

When a force is applied to an object it produces an acceleration according to the equation

This equation is known as Newton's Second Law of Motion. Thus,

and we see that force is measured in units of mass times length per time2 -- for example, gram-meters per second2.

Two common units of force are the Newton and the dyne. One Newton is one kilogram-meter per second2 and one dyne is one gram-centimeter per second2. Thus, one Newton is 105 dynes.

In the United States we often use the pound as a unit of force. This unit is the source of much confusion. It is important to make a distinction between two concepts -- mass and weight. Mass is an intrinsic property of an object -- its resistance to acceleration. An object always has the same mass regardless of where it is. An object's weight, however, depends on its mass and its location. An object on the surface of the moon weighs one-sixth of its earth-weight and an object in space has no weight. The pound is a unit of weight but some people confuse it with a unit of mass. A pound is equivalent to 4.448 Newtons.

Copyright c 1995 by Frank Wattenberg Department of Mathematics, Montana State University, Bozeman, MT 59717.