# What Are the Units for Speed, Velocity and Momentum?

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To understand the units for speed, velocity and momentum, it is essential to know what each of these is and how each differs from the others. All three involve mass in motion. Mass may be described as moving with no reference to any particular direction, or it may be described as associated with a particular direction. Directionless properties are termed scalar, while the properties referencing a particular direction are called vector properties.

## Speed

• Speed refers to the distance traveled per unit of time, without regard to direction. It is possible to describe speed in a number of ways. Some examples are miles per hour, centimeters per second and furlongs per fortnight. Some units are more practical than others. Automobiles travel in miles per hour. The crawl of an earthworm, however, would never be measured in miles per hour. Feet per hour would make more sense. But direction does not matter. A car that can go 127 mph south could also go 127 mph north.

## Velocity

• Speed is often called velocity though the two are not technically identical. Velocity implies a direction. An automobile going 60 miles per hour north-northwest has a number plus a direction for its velocity. This might matter little to the average person, but to the scientist, it represents an important distinction. For instance, a neutron is accelerated and then it crashes into an atom, which splinters. The speeds of the pieces are important, but so is the direction of each piece.

## Momentum

• Momentum is measured in units of mass-times-velocity. Notice that momentum is a vector quantity. One example of this is kilogram-meters per second. Another is gram-centimeters per second. Momentum is primarily of interest to the scientist, and most scientists use metric terms; however, other terms can be used, such as pound-feet per hour. Momentum is considerably different from either speed or velocity. It includes motion, but also the idea of how "unstoppable" that motion is. A straw blown by a breeze into a tree possesses little momentum and bounces harmlessly off. That same straw, hurled by a tornado, has a much larger momentum and can penetrate the tree.

## Concept of a Negative Vector Quantity

• Vector properties include a directional aspect. This explains why a projectile fired to the east at 120 mph has a positive velocity toward the object at which it is aimed. The same bullet, however would have a negative velocity -- minus 120 mph -- toward an object in the west. The scalar quantity of speed at which both projectiles are traveling is greater than 120 mph.

## References

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