Physics
Describing Motion: Velocity
Introduction
Everything around us is in motion. People
walk, run, ride bikes, or drive. The earth
rotates and orbits the sun.
The sun is also moving around the Milky
Way Galaxy which is also moving through
space.
Even the particles of atoms are moving.
Since motion is common to everything in
the universe, it is essential to study how it
works.
Three Ways to Describe Motion
1. Language—Common words can
describe motion in sentences.
2. Mathematical equations.
3. Graphs show how quantities
involving motion change over time.
Position and Distance
Key Terms
Frame of
Reference: Motion
is relative to where
you are in relation
to the object in
motion
Reference Point: A
point to begin in
making a
measurement (the
zero point)
Position: the
separation between
the object and the
reference point
Distance: how far
something has
moved
Scalar VS Vector
Scalar: a quantity
having only
magnitude. Ex:
distance
Vector: a quantity
having both
magnitude and
direction. Ex:
displacement or
placement
w
Examples of Scalars and Vectors
Average Velocity
Key Terms:
Instantaneous position: the position
of an object at any given time, t
Displacement: The change in
position of an object
Average Velocity is the ratio of the
change in displacement and the
change in time
Example Problem
In the 1988 Summer Olympics, Florence
Griffith-Joyner won the 100-m race in
10.54 s. Assuming the length of the race
is measured to 0.1 m, find her average
velocity in m/s and km/h.
Dd =+100.0 m 100.0 m/10.54 s
Dt = 10.54 s
= 9.488 m/s or
9.5 m/s
9.488 m/s(3600s/h)/(1000 m/km) = 34.2 km/h
Position-Time Graphs
A graph that shows how position
depends on time is called a positiontime graph.
Time is the independent variable and
is graphed on the x-axis.
Position is the dependent variable
and is graphed on the y-axis.
The slope of the graph is the average
velocity.
Constant Velocity=Uniform Velocity
Constant velocity—
the average
velocity of an
object is the same
for all time
intervals
For a position-time
graph, constant
velocity is a
straight line equal
to the slope.
Slope Review
Slope is defined as
rise over run.
The rise refers to the
numbers of the
vertical axis; the run
refers to the numbers
of the x-axis.
The slope, m, in the
equation: Y = mx + b,
is found by dividing
the change in y by the
change in x.
The slope of a
position-time graph is
velocity.
Positive and Negative Velocities
Positions can be
positive or negative.
Positive is to the right
of a reference point
and negative is to the
left of a reference
point.
Time intervals,
however, are always
positive.
Velocity can be either
positive or negative
depending on if the
position is positive or
negative.
Instantaneous Velocity
To find the instantaneous velocity,
draw a straight line tangent to that
point. The slope of
the tangent line is
the instantaneous
velocity.
Velocity-time Graph
A velocity-time
graph is used to
describe motion
with either
constant or
changing velocity.
For constant
velocity, the
graphed line is
parallel to the xaxis.