Section 2.1 of Holt Physics

1. Displacement and Velocity
p. 40-46

2. Objectives
The student will be able to:
• Describe motion in terms of displacement, time, and
velocity
• Calculate the displacement of an object traveling at a
known velocity for a specific time interval
• Construct and interpret graphs of position versus time

3. Displacement
Displacement: the straight-line change in position of an
object
• Units: meters (m)
• Symbol: Δx
When an object is at rest, its displacement is zero because
its position does not change.

4. Displacement vs. Distance
• If you were to describe the trip from your home to school,
you would describe all the twists and turns in your journey.
• However, the displacement is the same no matter what route
you take to school.
• Any path connecting your house and the school will result in
the same displacement.

5. What’s the football player’s displacement?
What’s the football player’s distance traveled?

6. Trying out Socrative for 1st Time
Poll will go here – conceptual challenge ?’s from p.41

7. Coordinate Systems
Coordinate System: a system the uses coordinates to establish
position
• In this second chapter, we will be working in one dimension,
which means we'll only be measuring in one direction (left
and right, up and down, north and south, etc.)

8. Displacement Equation
Δx = xf - xi
• The values xf and xi represent the final position and initial
position.
• The xf and xi are determined by their positions on the
coordinate system.

9. Negative and Positive
• Displacement can be positive or negative.
• Displacement is an example of a quantity that has both a
magnitude (size) and a direction.
• The direction of the displacement is designated by the sign
(positive or negative.)
• We will say that displacements to the left and down are
negative and displacements to the right and up are positive.

11. Average Velocity
Average Velocity: total displacement divided by the time
interval during which the displacement occurred
• Units: meters/second (m/s)
• Symbol: 푣 or vavg

12. Velocity Equation
푣푎푣푔 =
Δ푥
Δ푡
=
푥푓−푥푖
푡푓−푡푖
Like displacement, velocity has a magnitude and a
direction. The sign of the displacement indicates direction.
An object traveling to the left or down has a negative
velocity, and an object travelling to the right or up has a
positive velocity.

13. Velocity vs. Speed
Velocity indicates how fast something is traveling, and the
direction in which it is traveling.
Speed, however, has no associated direction.
The average velocity depends on displacement, but the average
speed is equal to the distance traveled divided by the time
interval.

14. Sample Problem 2A
p.43 – During a race on level ground, Andra covers 825 m in
137 s while running due west.
Find Andra’s average velocity.

15. Practice Problems
Practice 2A – p.44 #1-4

16. Quick Note on Average Velocity
• Average velocity is not always equal to the average of the
initial and final velocities.
• For instance, if you drive first at 40 km/h west and later at
60 km/h west, your average velocity is not necessarily at
50 km/h west.

17. Graphing Velocity
On a position versus time graph, the slope of the line
connecting one point and the next indicates the average
velocity.
slope =
change in vertical
change in horizontal
푣푎푣푔 =
Δ푥
Δ푡

19. Describe the velocity of each object.

20. Instantaneous Velocity
• For an object that is not traveling at constant velocity, the
average velocity will depend on the time interval we choose.
• To find the velocity of the object at a certain instant of time,
we study a small time interval near that instant. As the
intervals become smaller and smaller, the vavg approaches
the exact velocity at that time.

21. Instantaneous Velocity
Instantaneous velocity: velocity of an object at some instant or
at a specific point in the object’s path.
You can determine the instantaneous velocity for a position vs.
time graph by drawing a straight line that is tangent to your
graph at that instant. The slope of your tangent line is the
instantaneous velocity.

23. Homework
p. 69-70 # 1, 2, 6, 9-11