1. Table of Contents Section 1 Laws of Motion Section 2 Gravity
Chapter 11
Forces
Table of Contents
Section 1 Laws of Motion
Section 2 Gravity
Section 3 Newton’s Third Law

2. Chapter 11
Section 1 Laws of Motion
Objectives
Identify the law that says that objects change their motion only when a net force is applied.
Relate the first law of motion to important applications, such as seat belt safety issues.
Calculate force, mass, and acceleration by using Newton’s second law.

3. Bellringer
Chapter 11
Section 1 Laws of Motion
In some cases, an applied force is balanced by an opposite force, and there is no change in motion. In other cases, an applied force is not balanced by an opposite force, and the result is acceleration in the direction of the applied force. Look at the following illustrations, and identify the forces and motion in
each one.
(Illustrations are shown on the next slide.)

4. Bellringer Chapter 11 1. In one drawing, no motion is likely to occur.
Section 1 Laws of Motion
1. In one drawing, no motion is likely to occur.
Which drawing is it?
2. In which diagram are the forces clearly balanced?
How does this relate to your answer to item 1?
If more force is exerted by the person, does the opposite force increase to match the new force, stay the same or decrease?

5. Newton's Laws of Motion

6. Chapter 11
Section 1 Laws of Motion
Newton’s First Law
Newton’s first law of motion states that an object at rest remains at rest and an object in motion stays in motion unless it experiences an unbalanced force.
Objects tend to maintain their state of motion.
Skateboard demo

8. Inertia is the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an unbalanced force acts on the object.
Basically inertia is the name given to describe Newton’s 1st law.

9. Newton’s First Law Inertia is related to an object’s mass.
Chapter 11
Section 1 Laws of Motion
Newton’s First Law
Inertia is related to an object’s mass.
Mass is a measure of inertia.
The bigger the mass the bigger the objects inertia.

11. Seat belts and car seats provide protection.
Because of inertia, you slide toward the side of a car when the driver makes a sharp turn.
When the car you are riding in comes to a stop, your seat belt and the friction between you and the seat stop your forward motion.

12. Video – Newtons 1st Law 6min

14. Chapter 11
Section 1 Laws of Motion
Newton’s Second Law
Newton’s second law of motion states that the unbalanced force acting on an object equals the object’s mass times its acceleration.
Force equals mass times acceleration.
Force = mass  acceleration
F = ma
Force is measured in newtons (N).
1 N = 1 kg  1 m/s2
Video – Newtons 2nd Law 1min

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

16. Chapter 11
Section 1 Laws of Motion
Newton’s Second Law

17. Math Skills
Chapter 11
Section 1 Laws of Motion
Newton’s Second Law Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s upward acceleration is m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and the stretcher?
1. List the given and unknown values.
Given: mass, m = 175 kg
acceleration, a = m/s2
Unknown: force, F = ? N

18. Math Skills 2. Write the equation for Newton’s second law.
Chapter 11
Section 1 Laws of Motion
Math Skills
2. Write the equation for Newton’s second law.
force = mass  acceleration
F = ma
3. Insert the known values into the equation, and solve.
F = 175 kg  m/s2
F = 115 kg  m/s2 = 115 N

19. Newton’s Second Law Newton’s second law can also be stated as follows:
Chapter 11
Section 1 Laws of Motion
Newton’s Second Law
Newton’s second law can also be stated as follows:
The acceleration of an object is proportional to the net force on the object and inversely proportional to the object’s mass.

20. Mike's car, which weighs 1,000 kg, is out of gas
Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using Newton's Second Law, you can compute how much force Mike is applying to the car.
Answer = _____________
50 newtons

21. How much force would the lady have to apply to the van?
Video- Newtons 2nd law 2min

22. Consider the motion of a Hot Wheels car down an incline
This animation depicts some additional information about the car's motion. The velocity and acceleration of the car are depicted by vector arrows. The direction of these arrows are representative of the direction of the velocity and acceleration vectors. Note that the velocity vector is always directed in the same direction which the car is moving

23. Objectives
Chapter 11
Section 2 Gravity
Explain that gravitational force becomes stronger as the masses increase and rapidly becomes weaker as the distance between the masses increases.
Evaluate the concept that free-fall acceleration near Earth’s surface is independent of the mass of the falling object.
Demonstrate mathematically how free-fall acceleration relates to weight.
Describe orbital motion as a combination of two motions.

24. Law of Universal Gravitation
Chapter 11
Section 2 Gravity
Law of Universal Gravitation
Sir Isaac Newton (1642–1727) generalized his observations on gravity in a law now known as the law of universal gravitation.
Universal Gravitation Equation
m1 and m2 are the masses of the two objects
d is the distance between the two objects
G is a constant for gravity (6.67 X N m2/kg2)
Video- universal gravitation Bill Nye- 2min
Video – universal gravitation- 2min

25. Got Gravity???

26. Law of Universal Gravitation
Chapter 11
Section 2 Gravity
Law of Universal Gravitation
All matter is affected by gravity.
Two objects, whether large or small, always have a gravitational force between them.
When something is very large, like Earth, the force is large.
Gravitational force increases as mass increases. More Mass= More Gravity
Video – gravitation 2min
Gravitational force decreases as distance increases. More distance = less gravity

27. Kite flying link- 3.5 min

28. Law of Universal Gravitation
Chapter 11
Section 2 Gravity
Law of Universal Gravitation

29. Greater distance from Earth= Less Gravity
Weight
The weight of an object is the result of the gravitational force between the object and the Earth. The greater the mass of an object, the greater its weight. An elephant has more mass than a mouse, so it has a greater weight. Weight is measured in newtons or N. Notice that the unit of weight is the same as the unit of force (see page 1 of this Revision Bite). On Earth, an object with a mass of 1 kg has a weight of 10 N. To convert from kilograms to newtons, we just times by ten. An object's weight can change if it goes into space or to another planet. This is because gravity may be weaker or stronger there than it is on the Earth. Mass and weight
Remember that mass is measured in kilograms, kg, and weight is measured in newtons, N. The mass of an object stays the same wherever it is, but the weight of the same object can change. This happens if the object goes somewhere where gravity is stronger or weaker, such as into space. The Moon has less mass than the Earth, so its gravity is less than the Earth's gravity. This means that objects weigh less on the Moon than they do on the Earth. For example, a 120 kg astronaut weighs 1200 N on Earth (remember to times kg by 10 to get N). The same astronaut would weigh just 200 N on the Moon, because the Moon's gravity is one sixth the Earth's gravity. Note that the astronaut's mass stays the same.

32. Chapter 11
Section 2 Gravity
Free Fall and Weight
Free fall is the motion of a body when only the force of gravity is acting on the body.
Free-fall acceleration near Earth’s surface is constant.
If we disregard air resistance, all objects near Earth accelerate at 9.8 m/s2.
Freefall acceleration is often abbreviated as the letter g, so g = 9.8 m/s2.

33. Elephant and feather falling With air resistance and without

34. Chapter 11
Section 2 Gravity
Free Fall and Weight
Weight is equal to mass times free-fall acceleration.
weight = mass x gravity
w = mg

36. Weight is different from mass.
Mass is a measure of the amount of matter in an object. Mass of an object is always the same anywhere in the universe.
Weight is the pull of gravity on an objects mass.
Gravity can change so weight can change.

37. Weight of 150 lb person on Earth (At sea level)
Mercury- 55 lbs
Venus lbs
Jupiter- 380 lbs
Top of Mt. Everest
= 148 pounds

38. Bellringer
Chapter 11
Section 2 Gravity
Recall that weight is defined as a measure of the gravitational force exerted on an object. Use knowledge you have about gravity to answer the questions in the following situations:
1. Elvis is a student whose mass is 70 kg. On Earth’s surface, Elvis weighs about 690 N. Suppose Elvis could stand on the surface of the following bodies in the solar system. In the blanks provided, match Elvis’ weight with the letter of the appropriate body. (Note that Earth has a mass of 6.0 x 1024 kg.)
Planet Elvis’ weight
a. Jupiter (m = 1.9 x 1027 kg) 780 N _______
b. Venus (m = 4.9 x 1024 kg) N _______
c. Neptune (m = 1.0 x 1026 kg) N _______
d. Mercury (m = 3.3 x 1023 kg) N _______
e. Earth’s moon (m = 7.4 x 1022 kg) 620 N _______

39. Bellringer, continued Chapter 11
Section 2 Gravity
Bellringer, continued
2. Suppose Elvis is in orbit around Venus at a distance twice as far from the planet’s center as the surface of Venus is. Would you expect his weight to be greater than, less than, or equal to his weight on the surface of the planet?

41. Chapter 11
Section 2 Gravity
Free Fall and Weight
Velocity is constant when air resistance balances weight.
The constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity is called the terminal velocity.
Video- falling objects 6min

42. Chapter 11
Section 2 Gravity
Terminal Velocity

43. Question
When a skydiver jumps from a helicopter, his terminal velocity before opening the parachute reaches approximately 320 km/h.
Why does the rate of descent of the skydiver slow when the parachute opens? After all, the skydiver still has the same mass.

44. Answer
When the skydiver’s parachute is unopened, his surface area is much less than when the parachute is open. Therefore, the air resistance is much less and the skydiver will fall at a faster rate. Once the parachute opens, the surface area of the parachute creates far more resistance with the air and the skydiver’s descent is slowed. The force of gravity is exactly the same, and the mass has not changed, but the air resistance is greater.

45. Observe the motion of the skydiver below
Observe the motion of the skydiver below. As the skydiver falls, he encounters the force of air resistance. The amount of air resistance is dependent upon two variables:
The speed of the skydiver As a skydiver falls, he accelerates downwards, gaining speed with each second. The increase in speed is accompanied by an increase in air resistance (as observed in the animation below). This force of air resistance counters the force of gravity. As the skydiver falls faster and faster, the amount of air resistance increases more and more until it approaches the magnitude of the force of gravity. Once the force of air resistance is as large as the force of gravity, a balance of forces is attained and the skydiver no longer accelerates. The skydiver is said to have reached a terminal velocity.
The cross-sectional area of the skydiver A skydiver in the spread eagle position encounters more air resistance than a skydiver who assumes the tuck position or who falls feet (or head) first. The greater cross-sectional area of a skydiver in the spread eagle position leads to a greater air resistance and a tendency to reach a slower terminal velocity. The importance of cross-sectional area to skydiving is also demonstrated by the use of a parachute. An open parachute increases the cross-sectional area of the falling skydiver and thus increases the amount of air resistance which he encounters (as observed in the animation below). Once the parachute is opened, the air resistance overwhelms the downward force of gravity. The net force and the acceleration on the falling skydiver is upward. An upward net force on a downward falling object would cause that object to slow down. The skydiver thus slows down. As the speed decreases, the amount of air resistance also decreases until once more the skydiver reaches a terminal velocity.

46. Observe the motion of the skydiver below
Observe the motion of the skydiver below. As the skydiver falls, he encounters the force of air resistance. The amount of air resistance is dependent upon two variables:
The speed of the skydiver As a skydiver falls, he accelerates downwards, gaining speed with each second. The increase in speed is accompanied by an increase in air resistance (as observed in the animation below). This force of air resistance counters the force of gravity. As the skydiver falls faster and faster, the amount of air resistance increases more and more until it approaches the magnitude of the force of gravity. Once the force of air resistance is as large as the force of gravity, a balance of forces is attained and the skydiver no longer accelerates. The skydiver is said to have reached a terminal velocity.
The cross-sectional area of the skydiver A skydiver in the spread eagle position encounters more air resistance than a skydiver who assumes the tuck position or who falls feet (or head) first. The greater cross-sectional area of a skydiver in the spread eagle position leads to a greater air resistance and a tendency to reach a slower terminal velocity. The importance of cross-sectional area to skydiving is also demonstrated by the use of a parachute. An open parachute increases the cross-sectional area of the falling skydiver and thus increases the amount of air resistance which he encounters (as observed in the animation below). Once the parachute is opened, the air resistance overwhelms the downward force of gravity. The net force and the acceleration on the falling skydiver is upward. An upward net force on a downward falling object would cause that object to slow down. The skydiver thus slows down. As the speed decreases, the amount of air resistance also decreases until once more the skydiver reaches a terminal velocity.

47. Free Fall and Motion Orbiting objects are in free fall.
Chapter 11
Section 2 Gravity
Free Fall and Motion
Orbiting objects are in free fall.
The moon stays in orbit
around Earth because Earth’s gravitational force provides
a pull on the moon.
Two motions combine to cause orbiting.
Orbiting =Forward motion + gravity.

48. Two Motions Cause Orbiting
Chapter 11
Section 2 Gravity
Two Motions Cause Orbiting

49. Projectile Motion and Gravity
Chapter 11
Section 2 Gravity
Projectile Motion and Gravity
Projectile motion is the curved path an object follows when thrown, launched, or otherwise projected near the surface of Earth.

51. Chapter 11
Section 2 Gravity
Projectile Motion
Projectile motion has two components—horizontal(Forward) and vertical motion(gravity/ Free fall). The 2 motions combine to form the actual curved path of the object.
The two components are independent of each other.

52. Projectile Motion and Gravity
Chapter 11
Section 2 Gravity
Projectile Motion and Gravity

53. What is projectile motion?

54. Periodic Motion
Periodic motion is motion that repeats itself. (A projectile motion, since it doesn't repeat, is not periodic). Periodic motion is performed, for example, by a rocking chair, a bouncing ball, a vibrating tuning fork, a swing in motion, and the Earth in its orbit around the Sun.

55. Rotation vs. Revolution
Rotation means an object spinning on an axis.
Ex- top spinning or the Earth rotates 1 time
in 24 hours.
Revolution- is the elliptical path an object takes. Ex- Earth travels around Sun 1 time in 365 days.

56. Chapter 11
Section 3 Newton’s Third Law
Objectives
Explain that when one object exerts a force on a second object, the second object exerts a force equal in size and opposite in direction on the first object.
Show that all forces come in pairs commonly called action and reaction pairs.
Recognize that all moving objects have momentum.

57. Chapter 11
Section 3 Newton’s Third Law
Bellringer
You have learned that forces account for changes in the motion of objects. Using what you have learned, explained what happens in the following situation:
An ice skater holding a basketball is standing on the surface of a frozen pond. The skater throws the ball forward. At the same time, the skater slides on the ice in the opposite direction.

58. Bellringer, continued Chapter 11
Section 3 Newton’s Third Law
Bellringer, continued
1. Is the force on the ball greater than, less than, or equal to the opposite force on the skater?
2. Is the acceleration of the ball greater than, less than, or equal to the acceleration of the skater? (Hint: Remember Newton’s Second Law.)
3. Explain your answers.

59. Action and Reaction Forces
Chapter 11
Section 3 Newton’s Third Law
Action and Reaction Forces
Newton’s third law of motion states that for every action force, there is an equal and opposite reaction force.
Forces always occur in action-reaction pairs.
Action-reaction force pairs are equal in size and opposite in direction.
Video- Newtons 3rd Law 1min

61. Action and Reaction Forces
Chapter 11
Section 3 Newton’s Third Law
Action and Reaction Forces
Force pairs do not act on the same object.
When one object exerts an action force on a second object, the second object exerts a reaction force on the first object.
Video- Newtons 3rd law 3min

62. Equal forces don’t always have equal effects.
For example, the action force of Earth pulling on an object and causing it to fall is much more obvious than the equal and opposite reaction force of the falling object pulling on Earth.
Earth has a much bigger mass and therefore is effected very little by the smaller objects gravity pulling on the Earth.
Video- Newtons 3 laws of motion 5min

63. Chapter 11
Section 3 Newton’s Third Law
Momentum
Momentum is a quantity defined as the product of the mass and velocity of an object. Momentum(p) = mass  velocity
p = mv
All moving objects have momentum.
For a given velocity, the more mass an object has, the greater its momentum is.
Likewise, the faster an object
is moving, the greater its
momentum is.

64. p = mv v Momentum p m p: momentum (kg ·m/s) m: mass (kg)
quantity of motion
p = mv m p v
p: momentum (kg ·m/s)
m: mass (kg)
v: velocity (m/s)

65. Question??
Which is more important for you to wear your seatbelt, on a bus or in a car?
Why??
Bus has more mass and therefore cannot be moved as easily in an accident.
Car has less mass and therefore is easier to move in a collision.

66. Chapter 11
Section 3 Newton’s Third Law
Math Skills
Momentum Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley toward the pins.
1. List the given and unknown values.
Given: mass, m = 6.00 kg
velocity, v = 10.0 m/s down the alley
Unknown: momentum, p = ? kg • m/s (and direction)

67. Math Skills, continued 2. Write the equation for momentum.
Chapter 11
Section 3 Newton’s Third Law
Math Skills, continued
2. Write the equation for momentum.
momentum = mass x velocity
p = mv
3. Insert the known values into the equation, and solve.
p = mv = 6.00 kg  10.0 m/s
p = 60.0 kg • m/s down the alley

68. Momentum Force is related to change in momentum.
Chapter 11
Section 3 Newton’s Third Law
Momentum
Force is related to change in momentum.
When you force an object to change its motion, you force it to change its momentum.
Momentum is conserved in collisions.
The law of conservation of momentum states that the total amount of momentum in an isolated system is conserved.

69. Rocket Propulsion Conservation of momentum explains rocket propulsion.
Chapter 11
Section 3 Newton’s Third Law
Rocket Propulsion
Conservation of momentum explains rocket
propulsion.

70. Momentum Momentum is transferred. Pool table- Cue ball is
Chapter 11
Section 3 Newton’s Third Law
Momentum is transferred.
When a moving object hits a second object, some or all of the momentum of the first object is transferred to the second object.
Momentum can be transferred in collisions, but the total momentum before and after a collision is the same.
Pool table- Cue ball is
moving fast and transfers
its momentum to the 2nd ball.

71. pbefore = pafter

72. pbefore = pafter

73. Conservation of Momentum
A 5-kg cart traveling at 4.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision.
BEFORE
Cart 1:
m = 5 kg
v = 4.2 m/s
Cart 2 :
m = 2 kg
v = 0 m/s
AFTER
Cart 1 + 2:
m = 7 kg
v = ?
p = 21 kg·m/s m p v
p = 0
v = p ÷ m
v = (21 kg·m/s) ÷ (7 kg)
v = 3 m/s
pbefore = 21 kg·m/s
pafter = 21 kg·m/s

74. Conservation of Momentum
So…now we can solve for velocity.
GIVEN:
p = kg·m/s
m = 250 kg
v = ?
WORK:
v = p ÷ m
v = (-1000 kg·m/s)÷(250 kg)
v = - 4 m/s (4 m/s backwards) m p v

75. Chapter 11
Section 3 Newton’s Third Law
Concept Mapping

76. Understanding Concepts
Chapter 11
Standardized Test Prep
Understanding Concepts
1. What is the net force on a 2.0 kg weight hanging motionless on a string?
A. 0.0 N
B. 2.0 N
C. 9.8 N
D N

77. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
1. What is the net force on a 2.0 kg weight hanging motionless on a string?
A. 0.0 N
B. 2.0 N
C. 9.8 N
D N

78. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
2. What is the source of the force that causes a jet airplane to accelerate forward?
F. gravitational pull
G. air pressure on the wings
H. exhaust gases pushing against the engine
I. exhaust gases pushing against the atmosphere

79. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
2. What is the source of the force that causes a jet airplane to accelerate forward?
F. gravitational pull
G. air pressure on the wings
H. exhaust gases pushing against the engine
I. exhaust gases pushing against the atmosphere

80. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
3. Why does a skydiver not accelerate downward after reaching terminal velocity?
A. The force of gravity is inactive on the skydiver at terminal velocity.
B. Air resistance exceeds the force of gravity.
C. Air resistance balances the force of gravity.
D. The force of gravity decreases as the skydiver descends.

81. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
3. Why does a skydiver not accelerate downward after reaching terminal velocity?
A. The force of gravity is inactive on the skydiver at terminal velocity.
B. Air resistance exceeds the force of gravity.
C. Air resistance balances the force of gravity.
D. The force of gravity decreases as the skydiver descends.

82. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
4. The ancient Greek scientist, Aristotle, claimed that the speed of a falling object depends on its weight. But you can disprove his hypothesis by dropping a pen and a baseball simultaneously and observing when they hit the floor.
Why do falling objects not act as Aristotle thought they would?

83. Understanding Concepts,
Chapter 11
Standardized Test Prep
Understanding Concepts,
4. The ancient Greek scientist, Aristotle, claimed that the speed of a falling object depends on its weight. But you can disprove his hypothesis by dropping a pen and a baseball simultaneously and observing when they hit the floor.
Why do falling objects not act as Aristotle thought they would?
Answer:The acceleration due to gravity depends on the total masses of the object and Earth. Because Earth is so much larger than either of the objects, the acceleration depends on the mass of Earth and is the same for both objects.

84. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
5. Analyze why you would weigh less on the surface of Mars, even though your body remains exactly the same size and shape.

85. Understanding Concepts, continued
Chapter 11
Standardized Test Prep
Understanding Concepts, continued
5. Analyze why you would weigh less on the surface of Mars, even though your body remains exactly the same size and shape.
Answer: Weight is a measure of the force of gravity on an object. The mass of your body stays the same, but the force on it is less on Mars.

86. Chapter 11
Standardized Test Prep
Reading Skills
The United States Air Force trains astronauts in a large jet airplane, which is known as the “Vomit Comet” because many people get airsick during its flight. The plane accelerates upward and then falls back toward Earth, in the form of an arc. At the peak of its flight, the passengers seem to float inside the plane, and objects around them appear to be unaffected by gravity for about 20 seconds during each arc.
6. Describe the forces that are acting on the passengers as the plane begins its acceleration upward towards the top of the arc.

87. Reading Skills, continued
Chapter 11
Standardized Test Prep
Reading Skills, continued
[See previous slide for reading passage.]
6. Describe the forces that are acting on the passengers as the plane begins its acceleration upward towards the top of the arc.
Answer: The forces acting on the passengers are gravitational pull of Earth and an additional downward force against the airplane, which is moving upward.

88. Interpreting Graphics
Chapter 11
Standardized Test Prep
Interpreting Graphics
7. When a ball is falling toward Earth, what is the reaction force to the pull of gravity on the ball?
F. air pressure pushing up on the ball
G. force of the ground against the ball
H. upward pull of the ball on Earth
I. pull of gravity on the ball toward Earth

89. Interpreting Graphics, continued
Chapter 11
Standardized Test Prep
Interpreting Graphics, continued
7. When a ball is falling toward Earth, what is the reaction force to the pull of gravity on the ball?
F. air pressure pushing up on the ball
G. force of the ground against the ball
H. upward pull of the ball on Earth
I. pull of gravity on the ball toward Earth

90. The force of gravity on you from a pound of lead compared with the force from a pound of feathers?
a) The force from lead will be larger than the force from feathers.
b) The force from feathers will be larger than the force from lelad.
c) The forces will be the same.