Distance is simply a measure of how far an object traveled. It is a scalar quantity that has units such as meters, feet, kilometers, or miles.
Displacement is related to distance with some differences. Displacement is defined as the change in position. It is equal to the final position subtracted by the initial position.
Displacement is a vector quantity. It has magnitude and direction. Displacement can be positive or negative where as distance is always positive. If I described a car that traveled 300 meters, I'm referring to the distance traveled since no direction was given. If I say a car traveled 50 meters south, I'm describing the car's displacement since the direction of movement was given.
Speed is a scalar quantity like distance. It is always positive. Instantaneous Speed is the absolute value of Instantaneous Velocity. This is not always the case with Average Speed and Average Velocity which can have two completely different values.
Average speed is defined as the distance traveled divided by the time. Average Velocity is calculated by dividing the Displacement by the total time of travel. Since distance does not always equal Displacement, Average Speed will not always be equivalent to Average Velocity.
If a car travels 50 meters east for 4 seconds and 20 meters west for 6 seconds, the Total Distance traveled is 70 meters in 10 seconds which will give us an Average Speed of 7 m/s. The net displacement is +30 meters east which occurred in 10 seconds which will yield an Average Velocity of +3 m/s. In this example, the magnitudes of Average Speed and Average Velocity are different because the Distance traveled is different from the car's Displacement.
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