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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
Section 1 Changes in Motion
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
Section 1 Changes in Motion
p. 121
Force Diagrams
• Force is a vector
• A free-body diagram helps
analyze a situation
Section 1 Changes in Motion
pp. 122-124
Practice A – Drawing Free-Body Diagrams
#1, #2
p. 146
Section 1 Changes in Motion
Chapter Review
#7, #8, #9
Section 2 Objectives
• 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
Section 2 Newton’s First Law
p. 125
#3
Practice B – Determining Net Force
Chapter Review
#10, #11, #12
p. 128
Section 2 Newton’s First Law
Section 3 Objectives
• 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
Section 3 Newton’s Second and Third Laws
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?
Section 3 Newton’s Second and Third Laws
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?
Section 3 Newton’s Second and Third Laws
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?
Section 3 Newton’s Second and Third Laws
p. 132
#2, #3
Practice C – Newton’s Second Law
Chapter Review
#19, #20, #21, #22, #40, #41, #42, #43, #44
p. 132
Section 3 Newton’s Second and Third Laws
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
Section 3 Newton’s Second and Third Laws
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
Section 3 Newton’s Second and Third Laws
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?
Section 3 Newton’s Second and Third Laws
p. 134
Section 4 Objectives
• 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
Section 4 Everyday Forces
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
Section 4 Everyday Forces
p. 136
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
Section 4 Everyday Forces
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.
Section 4 Everyday Forces
p. 139
#1, #3
Practice D – Coefficients of Friction
Chapter Review
#35, #49
p. 139
Section 3 Newton’s Second and Third Laws
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.
Section 4 Everyday Forces
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
moves the box down the hall. The
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.
Section 4 Everyday Forces
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
moves the box down the hall. The
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.
Section 4 Everyday Forces
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.
Section 4 Everyday Forces
p. 141
#2, #4
Practice E – Overcoming Friction
Chapter Review
#29, #36, #37, #38, #39, #47, #50, #52
p. 139
Section 3 Newton’s Second and Third Laws