Northern Illinois University
Physics 150
Lab 1 Recitation
Basic Kinematics
Within physics, mechanics is the branch that focuses on the motion of objects and the forces that cause
changes in the motion. Within mechanics, the study of kinematics deals with the description of motion.
There are several variables that are used in the study of kinematics: distance, displacement, speed,
velocity, acceleration, and time. Correct usage of each term is necessary for an understanding of
kinematics.
First, let’s look at distance and displacement. On the surface, these terms may seem synonymous.
However, in physics, they are distinct. It is important to be clear about the distinction and about which
concept you are referring to.
Distance, in kinematics, is how far an object has traveled, measured along the path it took. Displacement
is its location, measured with respect to a reference point. So if I walk from my house to my friend’s
house, I may have walked a mile. But if I measure the distance (displacement) from my house to theirs on
a map, it may only be three-quarters of a mile (since I am not able to walk between the houses in a
perfectly straight line); in addition, to specify the displacement I need to include the direction (north, for
example). If I walk to my friend’s house and back, the total distance I traveled is two miles, but my total
displacement is zero – I am right back where I started!
The terms speed and velocity are related in a very similar manner.
Speed is a measure of how much distance you (or an object) travel in a prescribed amount of time. If it
takes you twenty minutes to walk one mile from your residence to Faraday Hall, your speed is one mile
per twenty minutes or three miles per hour. Velocity includes both speed and direction; if the direction
from your residence to Faraday is east, then your velocity was three miles per hour due east.
In most situations, velocity changes over time. This change in velocity (either magnitude, direction, or
both) is known as acceleration. If you are accelerating, you may be speeding up, slowing down, and/or
changing direction. For example, if your speed increased three miles per hour each second that you
traveled, you would be accelerating at a rate of three miles per hour per second in the direction of motion.
Note that acceleration, like displacement and velocity, must include a direction.
In this lab class, the typical units of measure will be:
distance and displacement: meters (m) or millimeters (mm)
speed and velocity: meters per second (m/s) or millimeters per second (mm/s)
acceleration: meters per second squared (m/s2)