Page 95 - College Physics For AP Courses

Chapter 2 | Kinematics 83
elapsed time: the difference between the ending time and beginning time
free-fall: the state of movement that results from gravitational force only
independent variable: the variable that the dependent variable is measured with respect to; usually plotted along the -axis
instantaneous acceleration: acceleration at a specific point in time
instantaneous speed: magnitude of the instantaneous velocity
instantaneous velocity: velocity at a specific instant, or the average velocity over an infinitesimal time interval kinematics: the study of motion without considering its causes
model: simplified description that contains only those elements necessary to describe the physics of a physical situation position: the location of an object at a particular time
scalar: a quantity that is described by magnitude, but not direction
slope: the difference in -value (the rise) divided by the difference in -value (the run) of two points on a straight line
time: change, or the interval over which change occurs
vector: a quantity that is described by both magnitude and direction y-intercept: the value when = 0, or when the graph crosses the -axis
Section Summary
2.1 Displacement
• Kinematics is the study of motion without considering its causes. In this chapter, it is limited to motion along a straight line, called one-dimensional motion.
• Displacement is the change in position of an object.
• In symbols, displacement is defined to be

where is the initial position and is the final position. In this text, the Greek letter (delta) always means “change
in” whatever quantity follows it. The SI unit for displacement is the meter (m). Displacement has a direction as well as a
magnitude.
• When you start a problem, assign which direction will be positive.
• Distance is the magnitude of displacement between two positions.
• Distance traveled is the total length of the path traveled between two positions.
2.2 Vectors, Scalars, and Coordinate Systems
• A vector is any quantity that has magnitude and direction.
• A scalar is any quantity that has magnitude but no direction.
• Displacement and velocity are vectors, whereas distance and speed are scalars.
• In one-dimensional motion, direction is specified by a plus or minus sign to signify left or right, up or down, and the like.
2.3 Time, Velocity, and Speed
• Time is measured in terms of change, and its SI unit is the second (s). Elapsed time for an event is

where is the final time and is the initial time. The initial time is often taken to be zero, as if measured with a
stopwatch; the elapsed time is then just .
• Average velocity is defined as displacement divided by the travel time. In symbols, average velocity is

• The SI unit for velocity is m/s.
• Velocity is a vector and thus has a direction.
• Instantaneous velocity is the velocity at a specific instant or the average velocity for an infinitesimal interval.
• Instantaneous speed is the magnitude of the instantaneous velocity.
• Instantaneous speed is a scalar quantity, as it has no direction specified.