Velocity
Velocity, the speed of a body in a particular direction. The term speed refers only to the body's rate of motionthat is, how fast the body movesregardless of the direction. A body with a constant velocity travels with unchanging speed in an unchanging direction. When the velocity changes in some way, the body is said to undergo an acceleration.
The instantaneous speed and instantaneous velocity of a body are its speed and velocity at any given instant of time. The average speed is the distance traveled divided by the time taken to travel that distance. The average velocity, on the other hand, is the direct distance traveled between two points divided by the time taken to travel between the two points.
IllustrationA car travels from one city to another city 70 miles (112.6 km) due north. The two cities are connected by a winding road 90 miles (144.8 km) long. If the trip takes 2 hours, the car will have an average speed of 45 miles per hour (72.4 km/h) and an average velocity of 35 miles per hour (56.3 km/h) north.
AccelerationAccelerations may be made in speed, in direction, or in both speed and direction. An increase in speed is referred to as positive acceleration; a decrease, as negative acceleration. A change in direction is called radial acceleration.
Acceleration is measured in terms of the rate at which the velocity changes with time. This measurement is expressed in units of velocity divided by units of time, such as feet per second per second or meters per second per second.
IllustrationAn object is moving at a velocity of 2 feet (0.6 m) per second in a given direction and speeds up to 8 feet (2.4 m) per second in 2 seconds, still in the same direction. Its average acceleration is found by subtracting the old velocity (2 feet per second) from the new velocity (8 feet per second) and dividing the result by time (2 seconds), giving an acceleration of 3 feet (0.9 m) per second per second. This means that the velocity increased by 3 feet per second during each second of acceleration.