1. II. Describing Motion Motion Speed & Velocity Acceleration
Motion & Forces
II. Describing Motion
Motion
Speed & Velocity
Acceleration

2. A. Motion Problem: We need a reference point... Is your desk moving?
nonmoving point from which motion is measured

3. Motion

4. A. Motion Reference point Motion Motion
Change in position in relation to a reference point.
Reference point
Motion

5. Reference Point

6. A. Motion
Problem:
You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward.
You have mistakenly set yourself as the reference point.

7. Motion
You can describe the direction of an object in motion with a reference direction.
north, south, east, west, up, or down

8. Distance
Displacement

9. Distance Distance – the length traveled by an object
Practice – What is the distance?
Liz walks 4 km North, then 3 km East
Xavier runs 30 m East, then runs 40 meters West
Aaron jogs 4 km East, 4 km North, 4 km West, and 4km South
Answers: 7 km ; 70 meters ; 16 km

10. Distance It is a scalar quantity
Scalar quantities are fully described by a magnitude (or number value) alone.
I travelled a distance of 100 miles.
No direction needs to be stated.

11. Distance vs. Displacement
Distance – length
Displacement – distance AND direction of a movement from the starting point

12. Displacement vector quantity such as displacement is direction-aware
When an object changes its direction of motion, displacement takes this direction change into account; heading the opposite direction effectively begins to cancel whatever displacement there once was.

13. Displacement
If an object moves in a single direction, the displacement equals the distance + the direction
4 km
total displacement =
4km North
start here 

14. Displacement
If an object moves in two opposing directions, the displacement is the difference between the two.
total distance = = 7 km
3 km
4 km
Displacement can be positive or negative. A negative direction can be either the opposite of the original movement, or can follow the sign of a typical graph [North/East vs South/West]
start here 
total displacement =
4 km North + -3 km South = 1 km North
(of original starting point)

15. Practice problems
Distance and displacement

16. Displacement – cont’d
If an object moves in two directions, a triangle will be formed. If the angle is 90º , use a2 + b2 = c2 to solve.
3 km
4 km
displacement
start here 

17. Displacement – cont’d
If an object moves in two directions, a triangle will be formed. If the angle is 90º , use a2 + b2 = c2 to solve.
3 km
displacement =
= c2
c2 = (16) + (9) = 25
c2 = √(25)
c = 5km
5 km
4 km
start here 

18. Displacement – cont’d units and direction
Displacement can be given in:
units and direction
example: 10 meters West
A person jogs 30 meters East then turns around and jogs 40 meters West
displacement = 10 meters West

19. Displacement Practice – What is the displacement?
Liz walks 8km North, then 3 km East
8.5 km Northeast displacement
Aaron jogs 4 km East, 4 km North, 4 km West, and 4km South
0 meters displacement from the start

20. v d t B. Speed & Velocity Speed rate of motion
distance traveled per unit time

21. B. Speed & Velocity Instantaneous Speed Average Speed
speed at a given instant
Average Speed

22. B. Speed & Velocity It depends on the storm’s direction! Problem:
A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried?
It depends on the storm’s direction!

23. B. Speed & Velocity Velocity speed in a given direction
can change even when the speed is constant!

24. Practice
Find the velocity in m/s of a swimmer who swims 110 m toward the shore in 72 s.
v = d t
v = 110m = 1.5 m/s toward
72 s shore

25. t a C. Acceleration a: acceleration vf: final velocity
vf - vi t
C. Acceleration
Acceleration
the rate of change of velocity
change in speed or direction
a: acceleration
vf: final velocity
vi: initial velocity
t: time

26. C. Acceleration “speeding up” Negative acceleration “slowing down”
Positive acceleration
“speeding up”
Negative acceleration
“slowing down”

27. Other ways to accelerate
Acceleration can also be a change in direction.
You standing on Earth (changing direction).
You are skiing down a hill and turning to avoid all the snowboarders falling in front of you. (velocity changed and you changed directions)
You are slowing your car for a stop sign. (decreased in speed along the road – negative acceleration)

28. t d v D. Calculations d = 100 m v = d ÷ t t = 20 s
Your neighbor skates at a speed of 4 m/s towards home. You can skate 100 m in 20 s. Who skates faster?
GIVEN:
d = 100 m
t = 20 s
v = ?
WORK:
v = d ÷ t
v = (100 m) ÷ (20 s)
v = 5 m/s
You skate faster! v d t

29. t a D. Calculations a = (vf - vi) ÷ t t = 3 s
A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration?
GIVEN:
vi = 10 m/s
t = 3 s
vf = 32 m/s
a = ?
WORK:
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2 a
vf - vi t

30. t d v D. Calculations v = 330 m/s t = d ÷ v d = 1km = 1000m
Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it?
GIVEN:
v = 330 m/s
d = 1km = 1000m
t = ?
WORK:
t = d ÷ v
t = (1000 m) ÷ (330 m/s)
t = 3.03 s = 3 s v d t

31. t a D. Calculations t = ? t = (vf - vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2)
How long will it take a car traveling 30 m/s to come to a stop if its acceleration is m/s2?
GIVEN:
t = ?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
WORK:
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
t = -30 m/s ÷ -3m/s2
t = 10 s a
vf - vi t

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

33. E. Graphing Motion Distance-Time Graph Who started out faster?B
Who started out faster?
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

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

35. E. Graphing Motion acceleration slope = +v = speeds up -v = slows down
Velocity-Time Graph
slope =
straight line =
flat line =
acceleration
+v = speeds up
-v = slows down
constant accel.
no accel. (constant velocity)

36. E. Graphing Motion Specify the time period when the object was...
slowing down
8 to 10 seconds
speeding up
3 to 5 seconds
moving at a constant speed
5 to 8 seconds

37. III. Defining Force Force Newton’s First Law Friction
Motion & Forces
III. Defining Force
Force
Newton’s First Law
Friction

38. Balanced and Unbalanced forces

39. A. Force Fkick Fgrav Force
Anything that changes the state of rest or motion of an object. It has a magnitude and a direction.
What forces are being exerted on the football?
Fkick
Fgrav

40. Force
Net force
the combination of all the forces acting on an object

41. A. Force Balanced Forces
forces acting on an object that are opposite in direction and equal in size
no change in velocity

42. A. Force Unbalanced forces
unbalanced forces that are not opposite and equal
velocity changes (object accelerates)
Fnet
Ffriction
Fpull N W

43. Balanced Forces
Subtract opposite forces to find net force on an object.
Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium. There is no unbalanced force acting upon the book and thus the book maintains its state of motion.

44. The force of gravity pulling downward and the force of the table pushing upwards on the book are of equal magnitude and opposite directions. These two forces balance each other. Yet there is no force present to balance the force of friction. As the book moves to the right, friction acts to the left to slow the book down.

45. Free body diagrams
Free-body diagrams are diagrams used to show the relative magnitude (strength) and direction of all forces acting upon an object in a given situation.
The size of the arrow in a free-body diagram is shows the magnitude of the force. The direction of the arrow reveals the direction in which the force acts.

46. Free body diagrams
It is customary in a free-body diagram to represent the object by a box or a small circle and to draw the force arrow from the center of the box or circle in an outward direction.

47. Free body diagrams Fgrav = gravity Fair = air resistance
F = force Fnorm = normal force
Ffrict = friction Fapp = applied force
Fgrav = gravity Fair = air resistance
Ftens = tension

48. Example
Object lies motionless on a surface.

49. Example
A rightward force is applied to a book in order to move it across a desk at a constant velocity. Consider frictional forces. Neglect air resistance.

50. Example
Object slows due to friction (rough surface).

51. Example An object is suspended from the ceiling.
A new force is tension.

52. Example
A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance.
A new force is air resistance. It pushes an object up.

53. http://www.mrwaynesclass.com/freebodies/r eading/index01.html
Free body diagrams with movement

54. Resultant forces
A stationary object remains stationary if the sum of the forces acting upon it - resultant force - is zero.
A moving object with a zero resultant force keeps moving at the same speed and in the same direction.

55. Resultant forces
If the resultant force acting on an object is not zero, a stationary object begins to accelerate in the same direction as the force. A moving object speeds up, slows down or changes direction.

56. Resultant forces Add forces going in the same direction.
Subtract forces going in opposite directions.

57. Resultant Forces

58. Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.
ConcepTest 1
TRUE or FALSE? The object shown in the diagram must be at rest since there is no net force acting on it.
FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.
Taken from “The Physics Classroom” © Tom Henderson,

59. https://www.youtube.com/watch?v=VutAx3R DFbI

60. C. Friction Friction force that opposes motion between 2 surfaces
depends on the:
types of surfaces
force between the surfaces

61. Friction Friction is greater when - a surface is rough (not smooth)
- an object moves faster

62. Friction
Static Friction: friction between surfaces that are stationary
Kinetic Friction: friction between moving surfaces
The force necessary to make a stationary object start moving is usually greater than the force needed to keep it moving.

63. Kinetic Friction How can a car minimize its fluid friction?
smooth surface – wax car surface
shape of the car -streamlining

64. http://www.mrwaynesclass.com/freebodies/r eading/index01.html
Free body diagrams with movement

65. Chapter 11 Newton’s Laws

66. B. Newton’s First Law
Newton’s First Law of Motion
An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

67. B. Newton’s First Law Newton’s First Law of Motion “Law of Inertia”
tendency of an object to resist any change in its motion
increase mass increase inertia
Think of the paper and book activity.

68. ConcepTest 2
You are a passenger in a car and not wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation? ...

69. ConcepTest 2
1. Before and after the collision, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. both of the above 4. neither of the above
2. Starting at the time of collision, the door exerts a leftward force on you.

70. To Summarize
Newton’s First Law describes when the net force acting on an object is zero.
Think of a car crash; the coin, index card, and cup experiment

71. Motion & Forces IV. Force & Acceleration Gravity Air Resistance
Newton’s Second Law
Gravity
Air Resistance
Calculations

72. A. Newton’s Second Law Newton’s Second Law of Motion
Describes the effect of an unbalanced force on the motion of an object.
The unbalanced force acting on an object equals the object’s mass times its acceleration.

73. F = ma F m a F m A. Newton’s Second Law F: force (N) m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2

74. Newton Force is measured in Newtons (N).
A Newton is equal to 1kg x m/s2
A mass x acceleration.

75. Practice Newton’s Second Law
A baseball accelerates downward at 9.8 m/s2 . If the gravitational force is the only force acting on the baseball and it is 1.4N, what is the baseball’s mass?

76. Newton’s Second Law F = m = a = m = F/a
First set up problem and rearrange equation
Put in numbers and solve.

77. Summary of Newton’s Second Law
Newton’s Second Law of Motion deals with unbalanced forces.
Unbalanced forces on an object cause two things:
1. Change in motion
2. Causes acceleration

78. B. Gravity
Gravity
force of attraction between any two objects in the universe
increases as...
mass increases
distance decreases

79. B. Gravity
Who experiences more gravity - the astronaut or the politician?
Which exerts more gravity - the Earth or the moon?
more mass
less distance

80. Gravity Can gravity occur in space? Yes. Gravity does NOT require air.
Does gravity continue acting on a fallen object after it hits Earth’s surface?
Yes. Gravity keeps objects on Earth’s surface.

81. W = mg B. Gravity Weight the force of gravity on an object MASS WEIGHT
W: weight (N)
m: mass (kg)
g: acceleration due to gravity (m/s2)
MASS
always the same
(kg)
WEIGHT
depends on gravity
(N)

82. B. Gravity Would you weigh more on Earth or Jupiter?
Jupiter because...
greater mass
greater gravity
greater weight

83. you tube video on newton's 2nd law
Dhvw

84. Animation from “Multimedia Physics Studios.”
B. Gravity
Accel. due to gravity (g)
In the absence of air resistance, all falling objects have the same acceleration!
On Earth: g = 9.8 m/s2
elephant
feather
Animation from “Multimedia Physics Studios.”

85. Free fall
The motion of a body when only gravity is acting on it. (No air resistance.)

86. With air resistance
Without air resistance

87. physics classroom

88. Go to the astronaut’s, David Scott, demonstration
Go to the astronaut’s, David Scott, demonstration

89. C. Air Resistance Air Resistance a.k.a. “fluid friction” or “drag”
force that air exerts on a moving object to oppose its motion
depends on:
speed
surface area
shape
density of fluid

90. C. Air Resistance Fair Fgrav Terminal Velocity
maximum velocity reached by a falling object
reached when… Fgrav = Fair
Fair
no net force
 no acceleration
 constant velocity
Fgrav

91. Animation from “Multimedia Physics Studios.”
C. Air Resistance
Terminal Velocity
increasing speed  increasing air resistance until…
Fair = Fgrav
Animation from “Multimedia Physics Studios.”

92. Animation from “Multimedia Physics Studios.”
C. Air Resistance
Falling with air resistance
heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity
Fgrav = Fair
larger Fgrav
 need larger Fair
 need higher speed
Animation from “Multimedia Physics Studios.”

93. a F m D. Calculations F = ? F = ma m = 40 kg F = (40 kg)(4 m/s2)
What force would be required to accelerate a 40 kg mass by 4 m/s2?
GIVEN:
F = ?
m = 40 kg
a = 4 m/s2
WORK:
F = ma
F = (40 kg)(4 m/s2)
F = 160 N m F a

94. a F m D. Calculations m = 4.0 kg a = F ÷ m F = 30 N
A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration?
GIVEN:
m = 4.0 kg
F = 30 N
a = ?
WORK:
a = F ÷ m
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2 m F a

95. a F m D. Calculations F(W) = 557 N m = F ÷ a m = ?
Mrs. J. weighs 557 N. What is her mass?
GIVEN:
F(W) = 557 N
m = ?
a(g) = 9.8 m/s2
WORK:
m = F ÷ a
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
m = 57 kg m F a

96. ConcepTest Is the following statement true or false?
An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.
False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

97. Conservation of Momentum
Motion & Forces
VI. Action and Reaction
Newton’s Third Law
Momentum
Conservation of Momentum

98. A. Newton’s Third Law
Newton’s Third Law of Motion
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

99. A. Newton’s Third Law Problem:
How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?
Aren’t these “balanced forces” resulting in no acceleration?
NO!!!

100. A. Newton’s Third Law Explanation:
forces are equal and opposite but act on different objects
they are not “balanced forces”
the movement of the horse depends on the forces acting on the horse

101. 3rd Law of Motion

102. A. Newton’s Third Law Action-Reaction Pairs
The hammer exerts a force on the nail to the right.
The nail exerts an equal but opposite force on the hammer to the left.

103. A. Newton’s Third Law Action-Reaction Pairs
The rocket exerts a downward force on the exhaust gases.
The gases exert an equal but opposite upward force on the rocket. FG FR

105. A. Newton’s Third Law F m Action-Reaction Pairs
Both objects accelerate.
The amount of acceleration depends on the mass of the object. F m
Small mass  more acceleration
Large mass  less acceleration

106. Momentum Mass in motion
Since all objects have mass, if an object is moving it has momentum
The amount of momentum depends on two variables:
mass
velocity

107. Momentum Momentum = mass x velocity
p = mv
Momentum units are mass units times velocity units:
kg m/s is the standard but can have a other unit combinations

108. Momentum Example
Two football players of equal mass are traveling towards each other, one is moving at 5 m/sec and the other at 8 m/sec. The one moving with the faster velocity has a greater momentum and will knock the other one backwards.

109. Momentum Example
A four-wheeler moving at a relatively fast velocity has a smaller momentum than the semi-truck because of its small mass and will stop much faster.

110. Momentum practice problem
A 1000 kg car moving at 15 m/sec has what amount of momentum?
15,000 kg•m/sec as a result of multiplying the mass and the velocity.

111. http://www. physicsclassroom. com/Class/mo mentum/u4l1a
mentum/u4l1a.cfmphysics classroom examples

112. I. Newton’s Laws of Motion
Motion & Forces
I. Newton’s Laws of Motion
“If I have seen far, it is because I have stood on the shoulders of giants.”
- Sir Isaac Newton
(referring to Galileo)

113. A. Newton’s First Law Newton’s First Law of Motion
An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

114. F = ma B. Newton’s Second Law Newton’s Second Law of Motion
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
F = ma

115. C. Newton’s Third Law Newton’s Third Law of Motion
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.