Chapter 12 - Matter in Motion - Velocity
The words speed and velocity are often used interchangeably, but
they don’t mean exactly the same thing. Velocity has direction, and
so it is a vector. 50 mph northward is a different velocity than 50
mph southward. Speed is merely the magnitude, or numerical value,
without regard to direction, so in both these cases the speed is just
50 mph. The speedometer in an automobile measures speed, not
velocity, because it doesn’t know in what direction you are going.
Velocity can be a negative number. For example, if the motion is
constrained along a line (such as on a straight north/south road),
we might define one direction to be positive (north) and the other
negative (south). Speed, on the other hand, is always a positive
number (unless it is zero).
|On the weather channel a hurricane's path is
explained in terms of velocity. People want to know
how fast a hurricane is traveling but they also need
to know in what direction it is traveling so they can
evacuate if necessary. For instance, the weatherman
will report a hurricane's velocity as five kilometers
per hour moving in a northeasterly direction.
|One of the dangers of hurricanes is that they can change their direction very
quickly and unpredictably, as seen in this map of the path of hurricane Jeanne in
2004 . Any change in the velocity of an object is called acceleration.