1. Chapter 11 Table of Contents Section 1 Laws of Motion
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. Chapter 11 Newton’s First Law
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 maintains its velocity unless it experiences an unbalanced force.
Objects tend to maintain their state of motion.
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 outside force acts on the object.

4. Newton’s First Law, continued
Chapter 11
Section 1 Laws of Motion
Newton’s First Law, continued
Inertia is related to an object’s mass. Mass is a measure of inertia.
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.

5. Force = mass  acceleration
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

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

7. Newton’s Second Law, continued
Chapter 11
Section 1 Laws of Motion
Newton’s Second Law, continued
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.

8. Chapter 11
Section 2 Gravity
Objectives
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.

9. 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

10. Law of Universal Gravitation, continued
Chapter 11
Section 2 Gravity
Law of Universal Gravitation, continued
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 easy to detect.
Gravitational force increases as mass increases.
Gravitational force decreases as distance increases.

11. POTSDAM GRAVITY POTATO

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

13. Chapter 11 Free Fall and Weight
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.
BILL NYE GRAVITY

14. Free Fall and Weight, continued
Chapter 11
Section 2 Gravity
Free Fall and Weight, continued
Weight is equal to mass times free-fall acceleration.
weight = mass  free-fall acceleration
w = mg
Weight is different from mass.
Mass is a measure of the amount of matter in an object.
Weight is the gravitational force an object experiences because of its mass.

15. Free Fall and Weight, continued
Chapter 11
Section 2 Gravity
Free Fall and Weight, continued
Weight influences shape.
Gravitational force influences the shape of living things.
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.

16. Chapter 11
Section 2 Gravity
Terminal Velocity

17. Chapter 11 Free Fall and Motion Orbiting objects are in free fall.
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.

18. FREE FALL BOZEMAN
BUNGEE

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

20. 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.
Projectile motion applies to objects that are moving in two dimensions under the influence of gravity.
Projectile motion has two components—horizontal and vertical. The two components are independent.

21. Chapter 11
Section 2 Gravity
Projectile Motion

22. Projectile Motion and Gravity, continued
Chapter 11
Section 2 Gravity
Projectile Motion and Gravity, continued
Projectile motion has some horizontal motion.
Horizontal motion is motion that is perpendicular (90º) to Earth’s gravitational field.
The horizontal velocity is constant.
Projectile motion also has some vertical motion.
The vertical motion is the same as downward free-fall motion.

23. 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.

24. 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.

25. Action and Reaction Forces, continued
Chapter 11
Section 3 Newton’s Third Law
Action and Reaction Forces, continued
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.
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.

26. momentum = mass  velocity
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 = mass  velocity
p = mv
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.

27. 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)

28. Chapter 11 Math Skills, continued 2. Write the equation for momentum.
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

29. Chapter 11 Momentum, continued Force is related to change in momentum.
Section 3 Newton’s Third Law
Momentum, continued
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.
Conservation of momentum explains rocket propulsion.

30. Chapter 11
Section 3 Newton’s Third Law
Rocket Propulsion

31. Chapter 11 Momentum, continued Momentum is transferred.
Section 3 Newton’s Third Law
Momentum, continued
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.
Action and reaction force pairs are everywhere.

32. 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

33. 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

34. 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

35. 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

36. 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.

37. 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.

38. 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?

39. 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?
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.

40. 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.

41. 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.

42. Chapter 11 Reading Skills
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.

43. 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.

44. 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

45. 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