Lecture Presentation - Calhoun Physics · What Do Forces Do? •Net Force •The unbalanced force...

Chapter 4

Lecture Presentation

Forces and Newton’s Laws of

Motion

© 2015 Pearson Education, Inc.

Slide 4-2

Chapter 4 Assignment: #’s 47, 65, & RT - 35

Chapter Goal: To establish a connection between force and

motion.


Section 4.1 Motion and Forces


Slide 4-4

Newton’s Laws

• Newton’s 1st Law

• An object at rest will stay at rest AND an object in motion

will stay in motion, UNLESS acted on by an

Unbalanced Force

• Fnet = Unbalanced Force

• Inertia

• Mass = the stuff that makes something up

• Measured in kilograms (kg)

• Does not change based on location

• An objects stubbornness to change, it wants to keep doing

what it’s doing


Slide 4-5

Newton’s Laws

• In the absence of friction (the unbalanced force), if the

sled is moving, it will stay in motion.


Slide 4-6

Newton’s Laws

• Newton’s 2nd Law

𝑎 =𝐹𝑛𝑒𝑡𝑚

• Acceleration and Fnet are directly related

• Always in same direction

• Acceleration and Mass are indirectly related

𝐹𝑛𝑒𝑡 = 𝑚𝑎


Slide 4-7

Newton’s Laws

• Newton’s 3rd Law

• For every action there is an equal but opposite reaction.

• Do it Kramer!

• Do it, you won’t.

• Don’t be a baby, just do the demonstration!

• It’s for the Youths! For the Children!

• Do it for the GODDAM PHYSICS!

• Haha! Idiot!


Slide 4-8

What Is a Force?

• A force is a push or a pull.

• An object is what the force is acting on.

• An agent is what is creating the force.


Slide 4-9

What Is a Force?

• A force is a vector

• The symbol is 𝐹

• The Unit is Newtons (N)

• Contact forces act on an object by

touching it.

• Long-range forces act on an object

without physical contact.


Slide 4-10

The Unbalanced Force

• Net Force

• The unbalanced force

• The summation of all forces acting on an object

• Must work in axis separately

𝐹𝑛𝑒𝑡 = 𝐹 = 𝐹1 + 𝐹2 + 𝐹3…



Slide 4-11

Combining Forces


Section 4.2 A Short Catalog of Forces Section 4.3 Identifying Forces


Slide 4-13

Weight or Force due to Gravity

• Weight or Force due to Gravity

• 𝑤 or 𝐹𝑔

• The gravitational pull of the earth on an object on or near

the surface of the earth.

• Always, always, always…

• You know how much I hate to say this

• Always points down to the center of the Earth

𝑤 = 𝑚𝑔 or 𝐹𝑔 = 𝑚𝑔

• Mass and Weight are different

• Weight can change based on location


Slide 4-14

Weight or Force due to Gravity


Slide 4-15

Normal Force

• The supportive force exerted by a surface on an object.

• Always, directed “up”, perpendicular to the surface.

• “Perceived” Weight

• “Apparent” Weight

• It’s what we feel as weight.

• 𝑛 or 𝐹𝑛

• DO NOT CONFUSE WITH THE NET FORCE!


Slide 4-16

Normal Force


Slide 4-17

Normal Force


Slide 4-18

Friction

• Rubbing force of two surfaces.

• Is always parallel to the surface.

• Kinetic friction

• acts on a moving object

• opposes the motion = points in opposite direction

• Static friction

• keeps an object “stuck” on a surface

• points in direction necessary to prevent motion.

• 𝑓𝑘 or 𝑓𝑠 or 𝐹𝑓


Slide 4-19

Friction


Slide 4-20

Drag

•Air Resistance

• Points opposite direction of motion.


Slide 4-21

Spring Force

• The force a spring exerts

• 𝐹 𝑠𝑝

• Push

• Pull


Slide 4-22

Tension Force

• The pull of a string or rope.

• 𝑇

• Always in the direction of string or rope.


Slide 4-23

Thrust

• A force that pushes an object.


Slide 4-24

QuickCheck 4.2

A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp?

A. The weight of the ball acting vertically down.

B. A horizontal force that maintains the motion.

C. A force whose direction changes as the direction of motion changes.

D. The weight of the ball and a horizontal force.

E. The weight of the ball and a force in the direction of motion.


Slide 4-25

QuickCheck 4.3

A steel beam hangs from a cable as a crane lifts the beam. What forces act on the beam?

A. Gravity

B. Gravity and tension in the cable

C. Gravity and a force of motion

D. Gravity and tension and a force of motion


Slide 4-26

QuickCheck 4.4

A bobsledder pushes her sled across horizontal snow to get it going, then jumps in. After she jumps in, the sled gradually slows to a halt. What forces act on the sled just after she’s jumped in?

A. Gravity and kinetic friction

B. Gravity and a normal force

C. Gravity and the force of the push

D. Gravity, a normal force, and kinetic friction

E. Gravity, a normal force, kinetic friction, and the force of the push


Slide 4-27

Identifying Forces


Text: pg. 105

Slide 4-28

Identifying Forces


Text: pg. 105

Slide 4-29

Conceptual Example 4.1: Identifying forces on a bungee jumper


Text: pg. 106

Slide 4-30

Conceptual Example 4.2: Identifying forces on a skier


Text: pg. 106

Slide 4-31

Example Problems: pgs. 120-121

• You are now able to complete

• Section 4.1

• #’s 1-6

• Section 4.2 & 4.3

• #’s 7-12

• We will not spend class time on them


Section 4.4 What Do Forces Do? Section 4.5 Newton’s Second Law


RELATIONSHIPS

• 𝐴 𝛼 𝐵

• Direct Relationship

A

B

RELATIONSHIPS

• 𝐴 𝛼 𝐵2

• Direct Exponential Relationship

A

B

RELATIONSHIPS

• 𝐴 𝛼 1

𝐵

• Indirect Relationship

A

B

RELATIONSHIPS

• 𝐴 𝛼 1

𝐵2

• Indirect Exponential Relationship

A

B

Slide 4-37

Relationships Examples

𝐹𝑔 =𝐺𝑚1𝑚2𝑟2

• What type of relationship does 𝐹𝑔 𝑎𝑛𝑑 𝑚1 have?

• Direct

• What type of relationship does 𝐹𝑔 𝑎𝑛𝑑 𝑟2 have?

• Indirect Exponential


Slide 4-38

Relationships Examples

𝐹𝑐 =𝑚𝑣2

𝑟

• What type of relationship does 𝐹𝑐 𝑎𝑛𝑑 𝑣2have?

• Direct Exponential

• What type of relationship does 𝐹𝑐 𝑎𝑛𝑑 𝑟 have?

• Indirect


Slide 4-39

Newton’s Laws

• Newton’s 2nd Law

𝑎 =𝐹𝑛𝑒𝑡𝑚

• Acceleration and Fnet are directly related

• Always in same direction

• Acceleration and Mass are indirectly related



Slide 4-40

What Do Forces Do?

• Net Force

• The unbalanced force

• The summation of all forces acting on an object

• Must work in axis separately

𝐹𝑛𝑒𝑡 = 𝐹 = 𝐹1 + 𝐹2 + 𝐹3…


• A single force does not cause an acceleration

• All the forces together, Fnet, cause an objects acceleration

• Acceleration is the bridge between forces and motion


Slide 4-41

Conceptual Example

A basketball is released from rest in a stiff breeze directed to

the right. In what direction does the ball accelerate?


Slide 4-42

QuickCheck 4.5

A cart is pulled to the right with a constant, steady force. How will its acceleration graph look?


A constant unbalanced force produces a constant acceleration.

A. B. C.

Slide 4-43

QuickCheck 4.6

A constant force causes an object to accelerate at 4 m/s2. What is the acceleration of an object with twice the mass that experiences the same force?

A. 1 m/s2

B. 2 m/s2

C. 4 m/s2

D. 8 m/s2

E. 16 m/s2


Slide 4-44

QuickCheck 4.7

An object, when pushed with a net force F, has an acceleration of 2 m/s2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be

A. ½ m/s2

B. 1 m/s2

C. 2 m/s2

D. 4 m/s2


Slide 4-45

QuickCheck 4.8

A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the net force greater?

A. The train

B. The skier

C. The net force is the same on both.

D. There’s not enough information to tell.


Slide 4-46

QuickCheck 4.9

An object on a rope is lowered at constant speed. Which is true?

A. The rope tension is greater than the object’s weight.

B. The rope tension equals the object’s weight.

C. The rope tension is less than the object’s weight.

D. The rope tension can’t be compared to the object’s weight.


Constant velocity

Zero acceleration

Slide 4-47

QuickCheck 4.10

An object on a rope is lowered at a steadily decreasing speed. Which is true?

A. The rope tension is greater than the object’s weight.

B. The rope tension equals the object’s weight.

C. The rope tension is less than the object’s weight.

D. The rope tension can’t be compared to the object’s weight.


Decreasing downward velocity

Acceleration vector points up

points up

Slide 4-48

Skeleton Questions

• If we change one thing, how will it change another thing?

• Choose a formula


• Rearrange (if necessary) 𝐹𝑛𝑒𝑡𝑚= 𝑎

• Create “Skeleton” or “Framework”

( )

( )𝑎

• Input multipliers

( 2 )

( 1 )𝑎 = 2𝑎


Slide 4-49

Example Problem: pg. 121


Slide 4-50



Section 4.6 Free-Body Diagrams


Slide 4-52

Free-Body Diagrams


Text: pg. 112 Box

Slide 4-53

Free-Body Diagrams


Text: pg. 112

Slide 4-54

QuickCheck 4.11

An elevator, lifted by a cable, is moving upward and slowing. Which is the correct free-body diagram?


A. B. C. E. D.

Slide 4-55

QuickCheck 4.12

A ball has been tossed straight up. Which is the correct free-body diagram just after the ball has left the hand? Ignore air resistance.


No points of

contact.

Gravity is

the only

force. A. B. C. D.

Slide 4-56

QuickCheck 4.13

A ball, hanging from the ceiling by a

string, is pulled back and released.

Which is the correct free-body

diagram just after its release?


A. B. C. D. E.

Slide 4-57

QuickCheck 4.14

A car is parked on a hill. Which is

the correct free-body diagram?


C.

Slide 4-58

QuickCheck 4.15

A car is towed to the right at

constant speed. Which is the

correct free-body diagram?


D.

Slide 4-59

Example Problems: pg. 122

Problems 27 through 29 show a free-body diagram. For

each problem, (a) redraw the free-body diagram and (b)

write a short description of a real object for which this is the

correct free-body diagram.


Slide 4-60







Slide 4-61







Slide 4-62


Problems 30 through 38 describe a situation. For each

problem, identify all the forces acting on the object and

draw a free-body diagram of the object.

30. Your car is sitting in the parking lot.

31. Your car is accelerating from a stop.

32. Your car is skidding to a stop from a high speed.


Lab


Design and execute an experiment that will demonstrate the relationships in

Newton’s Second Law.


Slide 4-65

Lab: Prove Newton’s Second Law

• What do you think will happen to the mass-cart system

once the mass is released?

• Use Physics Terms

• Will it accelerate at 9.8 m/s2 ?

• Yes/No? – Defend/Refute


Slide 4-66

Lab: Prove Newton’s Second Law

• What can we measure?

• Time, Δx, Mass

• What can we calculate?

• Acceleration, Force

• What can we change?

• Mass, Force

• How many things should we change at a time?

• One

• How SHOULD it affect acceleration?


Section 4.7 Newton’s Third Law


Slide 4-68

Newton’s 3rd Law

• Motion often involves two or more objects interacting

with each other.


Slide 4-69

Interacting Objects

• An action/reaction pairs of forces exists as a pair, or not at

all.


Slide 4-70

Reasoning with Newton’s Third Law


Text: pg. 115

Slide 4-71

Runners and Rockets


Text: pg. 116

Slide 4-72

Runners and Rockets


Text: pg. 116

Slide 4-73

QuickCheck 4.16

10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is

A. Greater than the force of Sarah on Jack.

B. Equal to the force of Sarah on Jack.

C. Less than the force of Sarah on Jack.


Slide 4-74

QuickCheck 4.17

A mosquito runs head-on into a truck. Splat! Which is true during the collision?

A. The mosquito exerts more force on the truck than the truck exerts on the mosquito.

B. The truck exerts more force on the mosquito than the mosquito exerts on the truck.

C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito.

D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck.

E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito.


Slide 4-75


#42 Squid use jet propulsion for rapid escapes. A squid

pulls water into its body and then rapidly ejects the

water backward to propel itself forward. A 1.5 kg

squid (not including water mass) can accelerate at

20 m/s2 by ejecting 0.15 kg of water.

a. What is the magnitude of the thrust force of

the squid?

b. What is the magnitude of the force on the

water being ejected?

c. What acceleration is experienced by the

water?


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Lecture Presentation - Calhoun Physics · What Do Forces Do? •Net Force •The unbalanced force...

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