In addition to direction and distance, motion is also described by how fast an object is moving. There are two measurements needed to describe speed - distance and elapsed time. Speed is defined as the distance traveled in a given amount of time. If you are able to walk 4.8 km (3mi) in an hour, you are moving at a speed of 4.8 km/h (3mph).
Average Speed = Total Distance/Total Time
Average Speed = D(distance)/time
The term velocity is used to describe speed and direction. Acceleration is used to describe the rate at which velocity changes. On a steep water slide your speed will change quickly; you will accelerate rapidly. On a water slide with a gentler slope your speed will not increase quickly; therefore your acceleration will be less. Even when your speed is constant, you are accelerating if you change direction. An object accelerates if it speeds up, slows down (decelerates) or changes direction.
Acceleration = (speed 2 - speed 1)time.
Objects in motion in one direction usually slow down because of friction. This decrease in speed is sometimes called deceleration. Since any change in speed or direction is defined as acceleration, deceleration is negative acceleration. Many things can cause an object to decelerate. In a car, brakes are applied to cause the car to decelerate. Gravity can cause deceleration. A ball thrown upward begins to decelerate because of gravity and friction in the air.