2.2. Average Velocity http://www.ck12.org
2.2 Average Velocity
- Explain the difference between speed and velocity.
- Define the concept of average velocity.
- Determine how to calculate average velocity using the equation,vav=∆∆xt
Test Pilot Neil Armstrong (later to become a famous astronaut) is seen here next to the X-15 ship after a research
flight. The servo-actuated ball nose, seen at Armstrong’s right hand, provided accurate measurement of air speed at
hypersonic speeds. The X-15 was flown over a period of nearly 10 years, and set the world’s unofficial speed record
of 4,250 mph.
Average Velocity
In ordinary language, the wordsspeedandvelocityboth refer to how fast an object is moving, and are often used
interchangeably. In physics, however, they are fundamentally different. Speedis the magnitude of an object’s
motion, with no regard for the direction.Velocity, on the other hand, includes direction. It is a vector, and thus must
have a magnitude and a direction.
Average speed is calculated by dividing the total distance travelled by the time interval. For example, someone who
takes 40 minutes to drive 20 miles north and then 20 miles south (to end up at the same place), has an average
speed of 40 miles divided by 40 minutes, or 1 mile per minute (60 mph). Average velocity, however, involves total
displacement, instead of distance. It is calculated by dividing the total displacement by the time interval. In this
example, the driver’s displacement is zero, which makes the average velocity zero mph.
Neither average speed nor average velocity implies a constant rate of motion. That is to say, an object might travel
at 10 m/s for 10 s and then travel at 20 m/s for 5 s and then travel at 100 m/s for 5 s. This motion would cover a