Relationships between Variables
Direct Variation (Directly Proportional):
** As one variable increases (decreases), the other variable also
increases (decreases) while other variables remain constant.
** Graphically, it appears as a line or curve that moves upward
when the two variables are graphed together.
** In an equation written without variables located in the
denominator of a fraction, directly related variables are on
opposite sides of the equal sign.
Indirect or Inverse Variation (Indirectly or Inversely Proportional):
** As one variable increases (decreases), the other variable
decreases (increases) while other variables remain constant.
** Graphically, it appears as a line or curve that moves downward
when the two variables are graphed together.
** In an equation written without variables located in the
denominator of a fraction, indirectly related variables are on
the same side of the equal sign.
Example: Newton’s Second Law of Motion
[Force = mass * acceleration]
** Force is directly related to mass.
Force vs. Mass
35
Force (Newtons)
30
25
20
15
10
5
0
0
2
4
6
8
10
Mass (kilograms)
** Force is directly related to acceleration.
Force vs. Acceleration
25
Force (Newtons)
20
15
10
5
0
0
2
4
6
8
Acceleration (meters per second squared)
10
12
** Acceleration and mass are inversely (indirectly) related.
Mass vs. Acceleration
25
Mass (Kilograms)
20
15
10
5
0
0
2
4
6
8
Acceleration (Meters per second squared)
10
12