forces and the laws of motion (chapter 4) 4 (forces and the laws of...topics • changes in motion...

Everglades High SchoolPhysics Honors

Forces and the Laws of Motion

(Chapter 4)

Alberto Dominguez

Updated for 2018-9 Edition of HMH Physics

Topics

• Changes in motion

• Force

• Force diagrams (free-body diagrams)

• Newton’s First Law

• Inertia

• Equilibrium

• Newton’s Second and Third Laws

• Newton’s Second Law

• Newton’s Third Law

• Everyday Forces

• Weight

• The Normal Force

• The Force of Friction

Section 1 Objectives

• Describe how force affects the motion of an

object.

• Interpret and construct free-body diagrams.

• SC.912.P10.10 – Compare the magnitude and

range of the four fundamental forces

Section 1 Changes in Motion

p. 120

Force

• Force = an interaction between two objects

• Forces cause acceleration

• A force can cause a stationary object to move (e.g.,

throwing a ball)

• A force can cause a moving object to slow down or stop

(e.g., catching a ball)

• A force can cause a moving object to change direction

• The SI unit of force is the Newton (N)

• 1 N = 1 kg x 1 m/s2

• 1 pound = 4.448 N


p. 120

Force

• Forces can act through contact or at a

distance

• Contact forces – forces that result from two objects

physically touching

• Field forces – forces that do not involve contact

• Gravity, Electromagnetic force, Weak force, Strong force

• All macroscopic contact forces are actually

due to microscopic field forces

• Examples: Friction, Collision forces

• Ultimately, every force can be categorized as

one of the four fundamental field forces


p. 121

Force Diagrams

• Force is a vector

• A free-body diagram helps

analyze a situation


pp. 122-124

Practice A – Drawing Free-Body Diagrams

#1, #2

p. 146


Chapter Review

#7, #8, #9


• Explain the relationship between the motion of

an object and the net external force acting on

the object.

• Determine the net external force on an object.

• Calculate the force required to bring an object

into equilibrium.

• SC.912.P12.3 – Interpret and apply Newton’s

three laws of motion

Section 2 Newton’s First Law

p. 125

Newton’s First Law

• “An object at rest remains at rest, and an object in

motion continues in motion with constant velocity,

unless the object experiences a net external force.”

• Known as the Law of Inertia

• Net force = the vector sum of all forces acting on an

object

• Inertia = the tendency of an object not to accelerate

• Mass is a measure of inertia

• When the net force is zero, the acceleration is zero

• Objects moving with constant velocity (this includes

objects at rest as just a special case) are in equilibrium

• Equilibrium ⬄ constant v⬄ a = 0⬄ Net force = 0


p. 125

Practice B


#3

Practice B – Determining Net Force

Chapter Review

#10, #11, #12

p. 128



• Describe an object’s acceleration in terms

of its mass and the net force acting on it.

• Predict the direction and magnitude of the

acceleration caused by a known net

force.

• Identify action-reaction pairs.

• SC.912.P12.3 – Interpret and apply

Newton’s three laws of motion

Section 3 Newton’s Second and Third Laws

p. 130

Newton’s Second Law

• Net force is proportional to mass and

acceleration

• The acceleration of an object is

directly proportional to the net force

acting on the object and inversely

proportional to the object’s mass

• Net force = Mass x Acceleration

• ∑F = ma


p. 131

Practice C #1

The net force on the

propeller of a 3.2 kg model

airplane is 7.0 N forward.

What is the acceleration of

the airplane?


p. 132

Practice C #4

A soccer ball kicked with

a force of 13.5 N

accelerates at 6.5 m/s2

to the right. What is the

mass of the ball?


p. 132

Practice C #5

A 2.0 kg otter starts from rest

at the top of a muddy incline

85 cm long and slides down to

the bottom in 0.50 s. What

net force acts on the otter

along the incline?


p. 132

#2, #3

Practice C – Newton’s Second Law

Chapter Review

#19, #20, #21, #22, #40, #41, #42, #43, #44

p. 132


Newton’s Third Law

• Forces always exist in pairs

• “For every action, there is an equal and opposite

reaction.”

• If two objects interact, the magnitude of the force

exerted on object 1 by object 2 is equal to the

magnitude of the force simultaneously exerted

on object 2 by object 1, and these two forces are

opposite in direction

• The action and reaction forces each act on different

objects


p. 133

Section Review #3

Identify the action-reaction pairs in the

following situations:

a)A person takes a step

b)A snowball hits someone in the back

c) A baseball player catches a ball

d)A gust of wind strikes a window


p. 134

Section Review #2

A child causes a wagon to

accelerate by pulling it with a

horizontal force. Newton’s Third

Law says that the wagon exerts

an equal and opposite force on

the child. How can the wagon

accelerate?


p. 134


• Explain the difference between mass and

weight.

• Find the direction and magnitude of

normal forces.

• Describe air resistance as a form of

friction.

• Use coefficients of friction to calculate

frictional force.

• SC.912.P10.10 – Compare the magnitude

and range of the four fundamental forces

Section 4 Everyday Forces

p. 135

Weight and the Normal Force

• Weight = the gravitational force

exerted by an astronomical body on

an object near the surface of that

body (e.g., on the Earth’s surface)

•Fg = W = mg

• Normal Force = the perpendicular

force exerted by a surface on an

object on the surface


p. 135

Friction

• Friction generally opposes an applied force

• Static friction = frictional force that prevents an object

at rest from moving

• Kinetic friction = frictional force on an object in motion

• Kinetic friction is less than static friction

• It is more difficult to start the object moving than to keep

it moving afterwards

• The frictional force is proportional to the normal force

• Coefficient of friction = ratio of the frictional force to

the normal force

• Air resistance is a form of kinetic friction


p. 136

Friction

Figure 4.5, p. 137

Equations of Friction

• Coefficient of friction 𝝁 = f / N• Coefficient of friction = ratio of the frictional force to the normal force

• f = 𝝁N• The frictional force is proportional to the normal force

• More specifically,

• fs = 𝝁sN

• fk = 𝝁kN

• In simple problems, N = Fg = mg, in which case

• f = 𝝁mg


p. 138

Coefficients of Friction

Figure 4.7, p. 138

Practice D #2

A 25 kg chair initially at rest on a

horizontal floor requires a 165 N

horizontal force to set it in motion with a

constant velocity. Once the chair is in

motion, a 127 N horizontal force keeps it

moving at a constant velocity. Find the

coefficients of static friction and kinetic

friction between the chair and the floor.


p. 139

#1, #3

Practice D – Coefficients of Friction

Chapter Review

#35, #49

p. 139


Practice E #1 – Modified Version 1

A physics student pulls on a rope

attached to a box of books and

moves the box down the hall. The

student pulls with a force of 185 N

horizontally. The box has a mass of

35.0 kg. Find the acceleration of the

box.


p. 141

Practice E #1 – Modified Version 2 (with Friction)

The same physics student pulls on a

rope attached to a box of books and


student pulls with a horizontal force

of 185 N. The box has a mass of

35.0 kg and 𝝁k between the box and

the floor is 0.27. Find the

acceleration of the box.


p. 141

Practice E #1 – Modified Version 3 (with Friction and an Angle)

The same physics student pulls on a

rope attached to a box of books and


student pulls with a force of 185 N at

an angle of 25.0° above the

horizontal. The box has a mass of

35.0 kg and 𝝁k between the box and

the floor is 0.27. Find the

acceleration of the box.


p. 141

Practice E #3

A 75 kg box slides down a

ramp that makes an angle

of 25.0° with the horizontal,

with an acceleration of 3.60

m/s2. Find 𝝁k between the

box and the ramp.


p. 141

#2, #4

Practice E – Overcoming Friction

Chapter Review

#29, #36, #37, #38, #39, #47, #50, #52

p. 139


Additional Practice

• Mixed Review #45, #46, #48, #51, #53, #54

• Standardized Test Prep #1–#17

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