Speed

You are probably familiar with the concept of speed. Speed is a type of rate. A rate tells you the amount of something that occurs during a given unit of time. If you know the distance an object travels in a certain amount of time, you know the speed of the object. For example, if a car travels 70 miles in one hour, the car is traveling at a speed of 70 miles/hour (miles per hour).

If the speed of an object does not change during its motion, the object is traveling at a constant speed. Most cars are equipped with cruise control. Cruise control allows a car to travel at a constant speed.

Most objects do not move at a constant speed very long. Most moving objects change speed many times while in motion. The marathon runners pictured here do not run at a constant speed during the entire race. At the starting line, there are many runners crowded together, so the runners are forced to move slowly. As the crowd thins out, a runner can increase his or her speed. The runner may slow down as he or she climbs a hill and increases speed going down the hill. When the race is over, a runner may calculate his or her average speed. The average speed of an object tells you the average rate at which it covers distance.

Velocity

Have you heard the term velocity? Many people often use the terms speed and velocity interchangeably, but speed and velocity are different.

Look at the statements below. What is the difference in the statements on the cards below?

One of the statements describes the speed of the airplane, and the other one describes the velocity. When you know both the speed and direction of an object's motion, you know the velocity of the object. When you know both the speed and direction of an object's motion, you know the velocity of the object. The statement in the orange box describes the airplane’s speed, while the statement in the green box describes the airplanes velocity.

Acceleration

Remember, the speed at which an object travels can change. An object can increase in speed or decrease in speed. The direction an object travels can also change. A car can travel north for 20 meters and then turn right and travel west for 50 meters. When an object changes speed or direction, we call this acceleration. Acceleration is the rate at which velocity changes. Remember, velocity has two components, speed, and direction.