2.21 - Summary
Position is the location of an object relative to a reference point called the origin,
and is specified by the use of a coordinate system.
Displacement is a measure of the change in the position of an object. It includes
both the distance between the object’s starting and ending points, and the direction
from the starting point to the ending point. An example of displacement would be
“three meters west” or “negative two meters”.
Similarly, velocity expresses an object’s speed and direction, as in “three meters
per second west.” Velocity has a direction. In one dimension, motion in one direction
is represented by positive numbers, and motion in the other direction is negative.
An object’s velocity may change while it is moving. Its average velocity is its
displacement divided by the elapsed time. In contrast, its instantaneous velocity is
its velocity at a particular moment. This equals the displacement divided by the
elapsed time for a very small interval of time, as the time interval gets smaller and
smaller.
Acceleration is a change in velocity. Like velocity, it has a direction and in one
dimension, it can be positive or negative. Average acceleration is the change in
velocity divided by the elapsed time, and instantaneous acceleration is the
acceleration of an object at a specific moment.
There are four very useful motion equations for situations where the acceleration is constant. They are the last four equations shown on the
right.
Free-fall acceleration, represented by g, is the magnitude of the acceleration due to the force of Earth’s gravity. Near the surface of the Earth,
falling objects have a downward acceleration due to gravity of 9.80 m/s^2.