## Displacement, velocity, acceleration

• Kinemetics is the description of motion; it concerns only the accurate description of the positions of objects, and the change in their positions. It does not deal with the sources of their motion; we'll discuss dynamics in a few weeks.
• Displacement is a vector which points from the initial position of an object to its final position. The standard units of displacement are meters.
• Velocity is a vector which shows the direction and rate of motion. The standard units of velocity are meters per second.
• Speed and velocity are not the same thing: speed is a scalar, whereas velocity is a vector. One must use different rules when combining speeds and combining velocities.
• The average velocity of an object is the total displacement during some extended period of time, divided by that period of time.
• Instantaneous velocity, on the other hand, describes the motion of a body at one particular moment in time.
• Acceleration is a vector which shows the direction and magnitude of changes in velocity. Its standard units are meters per second per second, or meters per second squared.
• Average acceleration is the total change in velocity (magnitude and direction) over some extended period of time, divided by the duration of that period.
• Instantaneous acceleration is the rate and direction at which the velocity of an object is changing at one particular moment.
• In everyday English, we use the term decelerate to describe the slowing of a body, but physicists use the word accelerate to denote both positive and negative changes in speed.

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