In physics, the motion of an object is often analyzed in terms of its distance traveled, velocity, and acceleration. The distance traveled is measured from a starting point that makes sense for the problem. For a car trip the starting point might be one’s home. Distance is measured in units such as feet, miles, meters, and kilometers.
The speed of an object measures how much distance it covers in a given unit of time, usually seconds, minutes, or hours. Some examples of units for speed are miles per hour (mi/hr), kilometers per second (km/sec), or feet per minute (ft/min). The rate of speed is determined by dividing the distance traveled by the amount of time it took to travel that distance.
An object’s velocity is a vector quantity, that is, the direction of travel is part of the measurement. The velocity includes the speed and direction of motion. Often motion to the right or to the east is considered a positive direction and motion to the left or the west is a negative direction. An object that travels 5 miles east in an hour has the same speed as an object that travels 5 miles west in an hour, but the velocity of the first object is +5 mi/hr and the velocity of the second object is -5 mi/hr.
Finally, the acceleration of an object is a measure of how much the velocity changes over a given time interval. Measurements and units of acceleration might look like 5 m/sec2 or 8 mi/hr-sec. In the first case the object’s speed changes by 5 m/sec every second. If it starts at 0 mi/sec, after 1 second its velocity is 5 m/sec, after 2 seconds its velocity is 10 m/sec, and so on. The second object is increasing speed at a rate of 8 mi/hr every second. If this object starts at 0 mi/hr, after one second it would be traveling at 8 mi/hr, at 2 seconds it is traveling at 16 mi/hr, and so on.
Acceleration is also a vector quantity. A positive acceleration means that the velocity is increasing and a negative acceleration means that velocity is decreasing. A negative acceleration is in the opposite direction to the direction that the object is traveling.
The graph shows the distance travelled by a vehicle versus time during an experimental test of the vehicle.