1. College Physics, 7th Edition
Lecture Outline
Chapter 2
College Physics, 7th Edition
Wilson / Buffa / Lou
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2. Chapter 2 Kinematics: Description of Motion
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3. Units of Chapter 2 Distance and Speed: Scalar Quantities
One-Dimensional Displacement and Velocity: Vector Quantities
Acceleration
Kinematic Equations (Constant Acceleration)
Free Fall
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4. 2.1 Vocabulary
Mechanics: The study of motion and what produces and affects motion.
Divided into 2 parts:
Kinematics
Dynamics

5. 2.1 Distance and Speed: Scalar Quantities
Distance is the path length traveled from one location to another. It will vary depending on the path.
Distance is a scalar quantity—it is described only by a magnitude.
Scalar?
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6. 2.1 Distance and Speed: Scalar Quantities
When something is in motion, its position changes with time.
Average speed is the distance traveled [actual path length] divided by the elapsed time:
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7. 2.1 Distance and Speed: Scalar Quantities
Since distance is a scalar, speed is also a scalar (as is time).
Instantaneous speed is the speed measured over a very short time span. This is what a speedometer reads. [It’s a particular instant in time]
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8. 2.1 Distance and Speed: Scalar Quantities
True or False Statement:
If a car travels with constant speed (speedometer doesn’t change), then the average and instantaneous speeds will be equal.
Come up with an analogy to support your decision.

9. 2.1 Distance and Speed: Scalar Quantities
Example # 1:
In January 2004, a Mars Exploration Rover touched down on the surface of Mars and rolled out for exploration. The average speed of the Rover on flat, hard ground is 5.0 cm/s.
a.) Assuming the Rover traveled continuously over this terrain at its average speed, how much time would it take to travel 2.0m nonstop in a straight line?
Now imagine the Rover was programmed to drive at its average speed for 10s, then stop and observe terrain for 20s before moving onward for another 10s and repeating the cycle. Now what would the Rover’s average speed be if traveling 2.0m?

10. Question 2.3 Position and Speed
a) yes
b) no
c) it depends on the position
If the position of a car is zero, does its speed have to be zero?
Answer:

11. Question 2.6a Cruising Along I
a) more than 40 mi/hr
b) equal to 40 mi/hr
c) less than 40 mi/hr
You drive for 30 minutes at 30 mi/hr and then for another 30 minutes at 50 mi/hr. What is your average speed for the whole trip?
Answer:

12. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
A vector has both magnitude and direction. Manipulating vectors means defining a coordinate system, as shown in the diagrams to the left.
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13. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
Displacement is a vector that points from the initial position to the final position of an object.
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14. Question 2.1 Walking the Dog
You and your dog go for a walk to the park. On the way, your dog takes many side trips to chase squirrels or examine fire hydrants. When you arrive at the park, do you and your dog have the same displacement?
a) yes
b) no
Answer:

15. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
Note that an object’s position coordinate may be negative, while its velocity may be positive; the two are independent.
Velocity is how fast something is moving AND in which direction it is moving.
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16. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
For motion in a straight line with no reversals, the average speed and the average velocity are the same. [Why?? Does this make sense?]
Otherwise, they are not; indeed, the average velocity of a round trip is zero, as the total displacement is zero!
Instantaneous Velocity is much like instantaneous speed, only with direction.
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17. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
What is uniform motion?
Different ways of visualizing uniform velocity:
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18. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
Position vs. Time Graphs
You know you loved those!! 
We had 3….what are they, draw and describe what they represent.

19. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
Example #1:
A jogger jogs from one end to the other of a straight 300m track in 2.50 min. and then jogs back to the starting point in 3.30 min. What was the joggers average velocity
A.) in jogging to the far end of the track?
B.) coming back to the starting point?
C.) for the total jog?

20. 2.2 One-Dimensional Displacement and Velocity: Vector Quantities
Most motion is non-uniform. [Different distances in different time intervals.]
This object’s velocity is not uniform. Does it ever change direction, or is it just slowing down and speeding up?
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21. Question 2.13a Graphing Velocity I
a) it speeds up all the time
b) it slows down all the time
c) it moves at constant velocity
d) sometimes it speeds up and
sometimes it slows down
e) not really sure
The graph of position versus time for a car is given below. What can you say about the velocity of the car over time? t x
Answer:

22. Question 2.13b Graphing Velocity II
a) it speeds up all the time
b) it slows down all the time
c) it moves at constant velocity
d) sometimes it speeds up and
sometimes it slows down
e) not really sure
The graph of position vs. time for a car is given below. What can you say about the velocity of the car over time? t x
Answer: b

23. Question 2.14a v versus t graphs I
a) decreases
b) increases
c) stays constant
d) increases, then decreases
e) decreases, then increases
Consider the line labeled A in the v vs. t plot. How does the speed change with time for line A? v t A B
Answer:

24. Question 2.14b v versus t graphs II
a) decreases
b) increases
c) stays constant
d) increases, then decreases
e) decreases, then increases
Consider the line labeled B in the v vs. t plot. How does the speed change with time for line B? v t A B
Answer:

25. Question 2.15a Rubber Balls Iv v a c t t v v b d t t
You drop a rubber ball. Right after it leaves your hand and before it hits the floor, which of the above plots represents the v vs. t graph for this motion? (Assume your y-axis is pointing up).
Answer:

26. Question 2.15b Rubber Balls IIv v a t c v v t d b t t
You toss a ball straight up in the air and catch it again. Right after it leaves your hand and before you catch it, which of the above plots represents the v vs. t graph for this motion? (Assume your y-axis is pointing up).
Answer:

27. Question 2.15c Rubber Balls IIIv v a t c t v v b d t t
You drop a very bouncy rubber ball. It falls, and then it hits the floor and bounces right back up to you. Which of the following represents the v vs. t graph for this motion?
Answer:

28. Question 2.7 Velocity in One Dimension
If the average velocity is non-zero over some time interval, does this mean that the instantaneous velocity is never zero during the same interval?
a) yes
b) no
c) it depends
Answer:

29. 2.3 Acceleration Acceleration is????? Vector or not? Why?
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30. 2.3 Acceleration
Acceleration means that the speed of an object is changing, or its direction is, or both.
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31. Question 2.8a Acceleration I
a) yes
b) no
c) depends on the velocity
If the velocity of a car is non-zero (v ¹ 0), can the acceleration of the car be zero?
Answer: a

32. Question 2.8b Acceleration II
When throwing a ball straight up, which of the following is true about its velocity v and its acceleration a at the highest point in its path?
a) both v = 0 and a = 0
b) v ¹ 0, but a = 0
c) v = 0, but a ¹ 0
d) both v ¹ 0 and a ¹ 0
e) not really sure
Answer:

33. 2.3 Acceleration Example # 1:
A couple in a SUV are traveling at 90 km/hr on a straight highway. The driver sees an accident in the distance and slow down to 40 km/hr in 5.0s. What is the average acceleration of the SUV?

34. 2.3 Acceleration
Acceleration may result in an object either speeding up or slowing down (or simply changing its direction).
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35. 2.3 Acceleration
If the acceleration is constant, we can find the velocity as a function of time:
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36. 2.3 Acceleration Example #2:
A drag racer starting from rest accelerates in a straight line at a constant rate of 5.5 m/s2 for 6.0s.
A.) What is the racer’s velocity at the end of this time?
B.) If a parachute deployed at this time causes the racer to slow down uniformly at a rate of 2.4 m/s2, how long will it take the racer to come to a stop?

37. 2.4 Kinematic Equations (Constant Acceleration)
From previous sections:
???????
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38. 2.4 Kinematic Equations (Constant Acceleration)
A motorboat starting from rest on a lake accelerates in a straight line at a constant rate of 3.0 m/s for 8.0s. How far does the boat travel during this time?

39. 2.4 Kinematic Equations (Constant Acceleration)
Substitution gives:
and:
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40. 2.4 Kinematic Equations (Constant Acceleration)
These are all the equations we have derived for constant acceleration. The correct equation for a problem should be selected considering the information given and the desired result.
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41. 2.4 Kinematic Equations (Constant Acceleration)
Example #1:
Two riders on dune buggies sit 10m apart on a long, straight track, facing in opposite directions. Starting at the same time, both riders accelerate at a constant rate of 2.0 m/s2. How far apart will the dune buggies be at the end of 3.0s?

42. 2.4 Kinematic Equations (Constant Acceleration)
Homework packet of Kinematic Equations
If you need help refer to your old physics notes.

43. 2.5 Free Fall
An object in free fall has a constant acceleration (in the absence of air resistance) due to the Earth’s gravity.
This acceleration is directed downward. Why?
When an object is dropped, its initial velocity is?? At a later time while falling its velocity is?
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44. Question 2.9a Free Fall I a) its acceleration is constant everywhere
You throw a ball straight up into the air. After it leaves your hand, at what point in its flight does it have the maximum value of acceleration?
a) its acceleration is constant everywhere
b) at the top of its trajectory
c) halfway to the top of its trajectory
d) just after it leaves your hand
e) just before it returns to your hand on the way down
Answer:

45. 2.5 Free Fall
Object in motion solely under the influence of gravity free fall.
What if the object is thrown upward?

46. 2.5 Free Fall
The effects of air resistance are particularly obvious when dropping a small, heavy object such as a rock, as well as a larger light one such as a feather or a piece of paper.
However, if the same objects are dropped in a vacuum, they fall with the same acceleration.
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47. 2.5 Free Fall
Is the acceleration independent or dependent upon mass/weight?
It was once thought, heavier bodies accelerate faster than light bodies. Who said this?
David Scott – 1979 [Got idea from Galileo]
What was Galileo’s idea?

48. Question 2.9b Free Fall II Bill Alice vA vB
Alice and Bill are at the top of a building. Alice throws her ball downward. Bill simply drops his ball. Which ball has the greater acceleration just after release?
a) Alice’s ball
b) it depends on how hard the ball was thrown
c) neither—they both have the same acceleration
d) Bill’s ball v0
Bill
Alice vA vB
Answer:

49. 2.5 Free Fall
Here are the constant-acceleration equations for free fall:
The positive y-direction has been chosen to be upwards. If it is chosen to be downwards, the sign of g would need to be changed.
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50. 2.5 Free Fall
Example #1:
A boy on a bridge throws a stone vertically downward with an initial speed of 14.7 m/s toward the river below. If the stone hits the water 2.00s later, what is the height of the bridge above the water?

51. 2.5 Free Fall
Example #2:
A Lunar Lander makes a descent toward a level plain on the Moon. It descends slowly by using retro (braking) rockets. At a height of 6.0m above the surface, the rockets are shut down with the Lander having a downward speed of 1.5 m/s. What is the speed of the Lander just before touching down? [g of moon = 1.6 m/s2]

52. Question 2.10b Up in the Air II
Alice and Bill are at the top of a cliff of height H. Both throw a ball with initial speed v0, Alice straight down and Bill straight up. The speeds of the balls when they hit the ground are vA and vB. If there is no air resistance, which is true?
a) vA < vB
b) vA = vB
c) vA > vB
d) impossible to tell v0
Bill
Alice H vA vB v0
Bill
Alice H vA vB
Answer:

53. Question 2.12b Throwing Rocks II
You drop a rock off a bridge. When the rock has fallen 4 m, you drop a second rock. As the two rocks continue to fall, what happens to their velocities?
a) both increase at the same rate
b) the velocity of the first rock increases faster than the velocity of the second
c) the velocity of the second rock increases faster than the velocity of the first
d) both velocities stay constant
Answer:

54. Summary of Chapter 2
Motion involves a change in position; it may be expressed as the distance (scalar) or displacement (vector).
A scalar has magnitude only; a vector has magnitude and direction.
Average speed (scalar) is distance traveled divided by elapsed time.
Average velocity (vector) is displacement divided by total time.
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55. Summary of Chapter 2
Instantaneous velocity is evaluated at a particular instant.
Acceleration (vector) is the time rate of change of velocity.
Kinematic equations for constant acceleration:
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56. Summary of Chapter 2 An object in free fall has a = –g.
Kinematic equations for an object in free fall:
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