1. Kinematics: The Fancy Word for Motion in One Dimension

2. 1. Displacement & Velocity Learning Objectives
By the time we are done you 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
Draw and understand graphs of position versus time

3. Essential Concepts We’ll Learn
Frames of reference
Vector vs. scalar quantities
Displacement
Velocity
Average velocity
Instantaneous velocity
Acceleration
Graphical representation of motion

4. Reference Frames Motion is relative
When we say an object is moving, we mean it is moving relative to something else (reference frame)

5. Scalar Quantities & Vector Quantities
Scalar quantities have magnitude
Example: speed 15 m/s
Vector quantities have magnitude and direction
Example: velocity 15 m/s North

6. Displacement

7. Displacement is change in position

8. Displacement vs. Distance
Distance is the length of the path that an object travels
Displacement is the change in position of an object

9. Describing Motion Describing motion requires a frame of reference

10. Determining Displacement
In these examples, position is determined with respect to the origin, displacement wrt x1

11. Indicating Direction of Displacement
Direction can be indicated by sign, degrees, or geographical directions.
When sign is used, it follows the conventions of a standard graph
Positive
Right Up
Negative
Left
Down

12. Reference Frames & Displacement
Direction is relative to the initial position, x1
x1 is the reference point

13. Average Velocity
Speed: how far an object travels in a given time interval
Velocity includes directional information:

14. Average Velocity

15. Velocity
Example
A squirrel runs in a straight line, westerly direction from one tree to another, covering 55 meters in 32 seconds. Calculate the squirrel’s average velocity
vavg = ∆x / ∆t
vavg = 55 m / 32 s
vavg = 1.7 m/s west

16. Velocity can be represented graphically: Position Time Graphs

17. Velocity can be interpreted graphically: Position Time Graphs
Find the average velocity between t = 3 min to t = 8 min

18. Calculate the average velocity for the entire trip

19. Graphing Motion speed slope = steeper slope = straight line =*
07/16/96
Graphing Motion
Distance-Time Graph A B
slope =
steeper slope =
straight line =
flat line =
Single point = instantaneous speed
speed
faster speed
no motion
constant speed *

20. Graphing Motion Who started out faster? Who had a constant speed?
A (steeper slope)
Who had a constant speed? A
Describe B from min.
B stopped moving
Find their average speeds.
A = (2400m) ÷ (30min) A = 80 m/min
B = (1200m) ÷ (30min) B = 40 m/min
Distance-Time Graph A B

21. Graphing Motion
Distance-Time Graph
Acceleration is indicated by a curve on a Distance-Time graph.
Changing slope = changing velocity

22. Formative Assessment: Position-Time Graphs
Object at rest?
Traveling slowly in a positive direction?
Traveling in a negative direction?
Traveling quickly in a positive direction?
dev.physicslab.org

23. Average vs. Instantaneous Velocity
Velocity at any given moment in time or at a specific point in the object’s path

24. Position-time when velocity is not constant

25. Average velocity compared to instantaneous velocity
Instantaneous velocity is the slope of the tangent line at any particular point in time.

26. Instantaneous Velocity
The velocity at a given moment in time
The instantaneous velocity is the velocity, as Δt becomes infinitesimally short,
i.e. limit as Δt 0

27. Acceleration!

28. 2. Acceleration – a change in velocity Learning Objectives
By the time we are done you will be able to…
Describe motion in terms of changing velocity
Compare graphical representations of accelerated and non-accelerated motions
Apply kinematic equations to calculate distance, time, or velocity under conditions of constant acceleration

29. Acceleration
Acceleration is the rate of change of velocity.

30. Acceleration: Change in Velocity
Acceleration is the rate of change of velocity
a = ∆v/∆t
a = (vf – vi) / (tf – ti)
Since velocity is a vector quantity, velocity can change in magnitude or direction
Acceleration occurs whenever there is a change in magnitude or direction of movement.

31. Acceleration Because acceleration is a vector, it must have direction
Here is an example of negative acceleration:

32. x-t graph when velocity is changing

33. Calculating Acceleration
a = ∆v/∆t
= m/s/s
= m/s2
Sample problem.
A bus traveling at 9.0 m/s slows down with an average acceleration of -1.8 m/s. How long does it take to come to a stop?

34. Negative Acceleration
Both velocity & acceleration can have (+) and (-) values
Negative acceleration does not always mean an object is slowing down (it can be changing direction!)

35. Is an object speeding up or slowing down?
Depends upon the signs of both velocity and acceleration
Velocity
Accel
Motion +
Speeding up in + dir -
Speeding up in - dir
Slowing down in + dir
Slowing down in - dir

36. Velocity-Time Graphs Is this object accelerating? How do you know?
What can you say about its motion?

37. Velocity-Time Graph Is this object accelerating? How do you know?
What can you say about its motion?
What feature of the graph represents acceleration?

38. Velocity-Time Graph
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39. Displacement with Constant Acceleration (C)

40. Displacement on v-t Graphs
How can you find displacement on the v-t graph?

41. Displacement on v-t Graphs
Displacement is equal to the area under the line! Whoa!

42. Graphical Representation of Displacement during Constant Acceleration

43. Displacement on a Non-linear v-t graph
If displacement is the area under the v-t graph, how would you determine this area?

44. Determining the area under a curve with rectangles

45. Displacement with initial velocity

46. Final velocity of an accelerating object

47. Displacement During Constant Acceleration (D)

48. Graphical Representation

49. Final velocity after any displacement (E)
A baby sitter pushes a stroller from rest, accelerating at m/s2. Find the velocity after the stroller travels 4.75m. (p. 57)
Identify the variables.
Solve for the unknown.
Substitute and solve.

50. Kinematic Equations

51. 3. Falling Objects Objectives
Relate the motion of a freely falling body to motion with constant acceleration.
Calculate displacement, velocity, and time at various points in the motion of a freely falling object.
Compare the motions of different objects in free fall.

52. Motion Graphs of Free Fall
What do motion graphs of an object in free fall look like?

53. Motion Graphs of Free Fall
What do motion graphs of an object in free fall look like?
x-t graph
v-t graph

54. Do you think a heavier object falls faster than a lighter one?
Why or why not?
Yes because ….
No, because ….

55. Free Fall
In the absence of air resistance, all objects fall to earth with a constant acceleration
The rate of fall is independent of mass
In a vacuum, heavy objects and light objects fall at the same rate.
The acceleration of a free-falling object is the acceleration of gravity, g
g = 9.81m/s2 memorize this value!

56. Free Fall
Free fall is the motion of a body when only the force due to gravity is acting on the body.
The acceleration on an object in free fall is called the acceleration due to gravity, or free-fall acceleration.
Free-fall acceleration is denoted with by ag (generally) or g (on Earth’s surface).

57. Free Fall Acceleration
Free-fall acceleration is the same for all objects, regardless of mass.
This book will use the value g = 9.81 m/s2.
Free-fall acceleration on Earth’s surface is –9.81 m/s2 at all points in the object’s motion.
Consider a ball thrown up into the air.
Moving upward: velocity is decreasing, acceleration is –9.81 m/s2
Top of path: velocity is zero, acceleration is –9.81 m/s2
Moving downward: velocity is increasing, acceleration is –9.81 m/s2

58. Sample Problem Falling Object
A player hits a volleyball so that it moves with an initial velocity of 6.0 m/s straight upward.
If the volleyball starts from 2.0 m above the floor,
how long will it be in the air before it strikes the floor?

59. Sample Problem, continued
1. Define
Given: Unknown:
vi = +6.0 m/s Δt = ?
a = –g = –9.81 m/s2
Δ y = –2.0 m
Diagram:
Place the origin at the
Starting point of the ball
(yi = 0 at ti = 0).

60. 2. Plan Choose an equation or situation: Both ∆t and vf are unknown.
We can determine ∆t if we know vf
Solve for vf then substitute & solve for ∆t
3. Calculate
Rearrange the equation to isolate the unknowns:
vf = m/s
Δt = 1.50 s

61. Graphing Motion Specify the time period when the object was...
slowing down
5 to 10 seconds
speeding up
0 to 3 seconds
moving at a constant speed
3 to 5 seconds
not moving
0 & 10 seconds
Speed-Time Graph

62. Is there another way?
Is there another equation that would answer the question in a single step?

63. Summary of Graphical Analysis of Linear Motion
This is a graph of x vs. t for an object moving with constant velocity. The velocity is the slope of the x-t curve.

64. Comparison of v-t and x-t Curves
On the left we have a graph of velocity vs. time for an object with varying velocity; on the right we have the resulting x vs. t curve. The instantaneous velocity is tangent to the curve at each point.

65. Displacement and v-t Curves
The displacement, x, is the area beneath the v vs. t curve.

66. Displacement and v-t Curves