PHYSICAL SCIENCE
Chapter 3
Section 1
Newton’s Second Law
3.1
Force, Mass, and Acceleration
• Newton’s first law of motion states
that an unbalanced force is
necessary to change an object’s
motion ( speed up, slow down, or
change direction = acceleration).
• Newton’s second law of motion
describes how the forces exerted on
an object, its mass, and its
acceleration are related.
Newton’s Second Law
3.1
Force and Acceleration
• What’s different
about throwing a
ball horizontally
as hard as you
can and tossing
it gently?
• When you throw
hard, you exert a
much greater force
on the ball.
Newton’s Second Law
3.1
Force and Acceleration
• The hardthrown ball has
a greater
change in
velocity, and
the change
occurs over a
shorter period
of time.
Newton’s Second Law
3.1
Force and Acceleration
• Recall that acceleration is the
change in velocity divided by the
time it takes for the change to
occur.
• So, a hard-thrown ball has a
greater acceleration than a gently
thrown ball.
Newton’s Second Law
3.1
Mass and Acceleration
• If you throw a softball
and a baseball as
hard as you can, why
don’t they have the
same speed?
• The difference is
due to their
masses.
Newton’s Second Law
3.1
Mass and Acceleration
• If it takes the same amount of time
to throw both balls, the softball
would have less acceleration.
• The acceleration of an object
depends on its mass as well as
the force exerted on it.
• Force, mass, and acceleration are
related.
 Net force acting on an object causes
object to accelerate in the direction
of net force
 Force & motion are connected
 An object will have greater
acceleration if a greater force is applied
to it.
 The mass of an object and the force
applied to it affect acceleration.
Newton’s Second Law
• Newton’s second law of motion states
that the acceleration of an object is in
the same direction as the net force on
the object, and that the acceleration can
be calculated from the following
equation:
Newton’s 2nd Law of Motion
 Newton’s 2nd law of motion connects
force, mass, and acceleration in the
equation acceleration equals net force
divided by mass
F
 Uses the equation:
 F = ma
m
a
 Friction
 Force that opposes motion between two
surfaces that are touching each other
 Microwelds
 Area where surface bump stick together
 Static Friction
 Friction between two surfaces that are not
moving past each other
 Sliding friction
 Force that opposes the motion of two surfaces
sliding past each other
 Rolling friction
 Friction between a rolling object and the surface
it rolls on
Friction
• Suppose you give a skateboard a push
with your hand.
• According to Newton’s first law of
motion, if the net force acting on a
moving object is zero, it will continue
to move in a straight line with constant
speed.
• Does the skateboard keep moving with
constant speed after it leaves your hand?
Friction
• Recall that when an object slows down it
is accelerating.
• By Newton’s second law, if the
skateboard is accelerating, there must be
a net force acting on it.
• The force that slows the skateboard
and brings it to a stop is friction.
• Friction is the force that opposes
the sliding motion of two surfaces
that are touching each other.
• The amount of friction between two
surfaces depends on two
factorsthe kinds of surfaces and
the force pressing the surfaces
together.
What causes friction?
• If two surfaces are in contact,
welding or sticking occurs where the
bumps touch each other.
• These microwelds are the source of
friction.
Sticking Together
• The larger the force pushing the two
surfaces together is, the stronger these
microwelds will be, because more of the
surface bumps will come into contact.
• To move one
surface over
the other, a
force must be
applied to
break the
microwelds.
Static Friction
• Suppose you have filled a cardboard box
with books and want to move it.
• It’s too heavy to
lift, so you start
pushing on it, but
it doesn’t budge.
• If the box
doesn’t move,
then it has zero
acceleration.
Static Friction
• According to Newton’s second law, if
the acceleration is zero, then the
net force on the box is zero.
• Another force that cancels your
push must be acting on the box.
Static Friction
• That force is the friction due to the
microwelds that have formed
between the bottom of the box and
the floor.
• Static friction is
the frictional
force that
prevents two
surfaces from
sliding past each
other.
Sliding Friction
• You ask a friend to help you move
the box.
• Pushing together,
the box moves.
Together you and
your friend have
exerted enough
force to break the
microwelds
between the floor
and the bottom of
the box.
Sliding Friction
• If you stop pushing, the box quickly
comes to a stop.
• This is because as the box slides across
the floor, another forcesliding
frictionopposes the motion of the box.
• Sliding friction is the force that
opposes the motion of two surfaces
sliding past each other.
Rolling Friction
• As a wheel rolls over a surface, the wheel
digs into the surface, causing both the
wheel and the surface to be deformed.
Rolling Friction
• Static friction acts over the deformed
area where the wheel and surface are in
contact, producing a frictional force
called rolling fiction.
• Rolling friction is the frictional
force between a rolling object and
the surface it rolls on.
 Air resistance
 Force that opposes the force of gravity
 The amount of air resistance depends on an
object’s shape, size, and speed.
 Terminal velocity
 Forces on a falling object are balanced and the
object falls with constant speed
 When a falling object no longer accelerates
 When a falling object has it’s acceleration due to
gravity equals to it’s air resistance
Air Resistance
• When an object falls toward Earth,
it is pulled downward by the force of
gravity.
• However, a fluid friction force called air
resistance opposes the motion of
objects that move through the air.
• Air resistance causes objects to fall with
different accelerations and different
speeds.
Air Resistance
• Air resistance acts in the opposite
direction to the motion of an object
through air.
• If the object is falling downward, air
resistance acts upward on the object.
• The size of the air resistance force also
depends on the size and shape of an
object.
Air Resistance
• The amount of air resistance on an
object depends on the speed, size, and
shape of the object.
• Air resistance,
not the
object’s mass,
is why feathers,
leaves, and
pieces of paper
fall more slowly
than pennies,
acorns, and
apples.
Terminal Velocity
• As an object falls, the downward force of
gravity causes the object to accelerate.
• However, as an
object falls faster,
the upward force of
air resistance
increases.
• This causes the net
force on a sky diver
to decrease as the
sky diver falls.
Terminal Velocity
• Finally, the upward air resistance force
becomes large enough to balance the
downward force of gravity.
• This means the net force on the
object is zero.
• Then the acceleration of the object is
also zero, and the object falls with a
constant speed called the terminal
velocity.
Terminal Velocity
• The terminal velocity is the
highest speed a falling object will
reach.
• The terminal velocity depends on the
size, shape, and mass of a falling object.
Section Check
3.1
Question 1
Newton’s second law of motion states
that _________ of an object is in the
same direction as the net force on the
object.
A.
B.
C.
D.
acceleration
momentum
speed
velocity
Section Check
3.1
Answer
The answer is A. Acceleration can be
calculated by dividing the net force in
newtons by the mass in kilograms.
Section Check
3.1
Question 2
The unit of force is __________.
A.
B.
C.
D.
joule
lux
newton
watt
Section Check
3.1
Answer
The answer is C.
One newton = 1 kg · m/s2
Section Check
3.1
Question 3
What causes friction?
Answer
Friction results from the sticking
together of two surfaces that are in
contact.
GRAVITY
Section 2
What is gravity?
• Gravity is an attractive force between
any two objects that depends on the
masses of the objects and the distance
between them.
 Law of gravitation
 Any two masses exert an attractive force on each other
 Four Basic Forces include: (from strongest to
weakest)




Strong nuclear force (within the nucleus of an atom)
Weak nuclear force
Electromagnetic force (repel AND attract)
Gravity (attract, ONLY)
 Gravity
 A long-range force that gives the universe its structure
 Acceleration due to Gravity
 g = 9.8 m/s2
The Law of Universal
Gravitation
• Isaac Newton formulated the law of
universal gravitation, which he published
in 1687.
• This law can be written as the following
equation.
The Law of Universal
Gravitation
• In this equation G is a constant called
the universal gravitational constant, and
d is the distance between the two
masses, m1 and m2.
• The law of universal gravitation enables
the force of gravity to be calculated
between any two objects if their masses
and the distance between them is
known.
The Range of Gravity
• According to the law of universal
gravitation, the gravitational force
between two masses decreases
rapidly as the distance between the
masses increases.
The Range of Gravity
• No matter how far apart two objects
are, the gravitational force between
them never completely goes to
zero.
• Because the gravitational force
between two objects never
disappears, gravity is called a longrange force.
Finding Other Planets
• In the 1840s the most distant planet
known was Uranus.
• The motion of Uranus calculated from the
law of universal gravitation disagreed
slightly with its observed motion.
• Some astronomers suggested that
there must be an undiscovered planet
affecting the motion of Uranus.
Finding Other Planets
• Using the law of universal gravitation and
Newton’s laws of motion, two
astronomers independently calculated
the orbit of this planet.
• As a result of
these
calculations, the
planet Neptune
was found in
1846.
Earth’s Gravitational
Acceleration
• When all forces except gravity acting on
a falling object can be ignored, the object
is said to be in free fall.
• Close to Earth’s surface, the acceleration
of a falling object in free fall is about
2
9.8 m/s .
• This acceleration is given the symbol g
and is sometimes called the acceleration
due to gravity.
 Weight
 gravitational force exerted on an
object’s mass
 decreases as an object moves away
from Earth
W = m•g
 Objects in space shuttle float
because they have no force
supporting them.
Weight and Mass
• Weight and mass are not the same.
• Weight is a force and mass is a
measure of the amount of matter
an object contains.
• Weight and mass are related.
Weight increases as mass increases.
Weight and Mass
• The weight of an object usually
is the gravitational force
between the object and Earth.
• The weight of an object can
change, depending on the
gravitational force on the
object, but mass remains
constant.
CALCULATE YOUR WEIGHT
 Convert your weight in pounds to mass in
kilograms by multiplying your weight by
0.454
pounds x 0.454 = mass (kg)
 Then multiply your mass by acceleration
due to gravity: 9.8 m/s2
 Weight (N) = mg
 What is your weight in Newtons?
Weight and Mass
• The table shows how various weights on
Earth would be different on the Moon and
some of the planets.
Weightlessness and Free Fall
• You’ve probably seen pictures of
astronauts and equipment floating
inside the space shuttle.
• They are said to be experiencing
the sensation of weightlessness.
Weightlessness and Free Fall
• However, for a typical mission, the
shuttle orbits Earth at an altitude of
about 400 km.
• According to the law of universal
gravitation, at 400-km altitude the force
of Earth’s gravity is about 90 percent as
strong as it is at Earth’s surface.
• So an astronaut with a mass of 80 kg
weighs 780 N on Earth’s surface, would
still weigh about 700 N in orbit.
Floating in Space
• So what does it mean to say that
something is weightless in orbit?
• When you stand on a
scale you are at rest
and the net force on
you is zero.
• The scale supports
you and balances
your weight by
exerting an upward
force.
Floating in Space
• The dial on the scale shows the upward
force exerted by the scale, which is your
weight.
• Now suppose you
stand on the
scale in an
elevator that is
falling.
Floating in Space
• If you and the scale were in free fall, then
you no longer would push down on the
scale at all.
• The scale dial
would say you
have zero weight,
even though the
force of gravity
on you hasn’t
changed.
Floating in Space
• A space shuttle in orbit is in free fall,
but it is falling around Earth, rather
than straight downward.
• Everything in the orbiting space shuttle is
falling around Earth at the same rate, in
the same way you and the scale were
falling in the elevator.
• Objects in the shuttle seem to be floating
because they are all falling with the same
acceleration.
Projectile Motion
• If you’ve tossed a ball to someone,
you’ve probably noticed that thrown
objects don’t always travel in straight
lines. They curve downward.
• Earth’s gravity causes projectiles to
follow a curved path.
Horizontal and Vertical Motions
• When you throw a ball, the force exerted
by your hand pushes the ball forward.
• This force gives the ball horizontal
motion.
• No force
accelerates it
forward, so its
horizontal velocity
is constant, if you
ignore air
resistance.
Horizontal and Vertical Motions
• However, when you let go of the ball,
gravity is pulling it downward, giving it
vertical motion.
• The ball has constant horizontal velocity
but increasing vertical velocity
(downward).
Horizontal motion and vertical
motion are independent of each
other!
Horizontal and Vertical Motions
• Gravity exerts an unbalanced force on
the ball, changing the direction of its
path from only forward to forward and
downward.
• The result of these two motions is
that the ball appears to travel in a
curve.
Horizontal and Vertical
Distance
• If you were to throw
a ball as hard as you
could from shoulder
height in a perfectly
horizontal direction,
would it take longer
to reach the ground
than if you dropped
a ball from the same
height?
Click image to view movie
Horizontal and Vertical Distance
• Surprisingly, it wouldn’t.
• Both balls travel the same vertical
distance in the same amount of
time.
Centripetal Force
• When a ball enters a curve, even if
its speed does not change, it is
accelerating because its direction is
changing.
• When a ball goes around a curve,
the change in the direction of the
velocity is toward the center of the
curve.
Centripetal Force
• Acceleration
toward the
center of a
curved or
circular path is
called
centripetal
acceleration.
Centripetal Force
• According to the second law of motion,
when a ball has centripetal acceleration,
the direction of the net force on the ball
also must be toward the center of the
curved path.
• The net force exerted toward the center
of a curved path is called a centripetal
force.
Centripetal Force and Traction
• When a car rounds a curve on a
highway, a centripetal force must be
acting on the car to keep it moving in a
curved path.
• This centripetal force is the
frictional force, or the traction,
between the tires and the road
surface.
Centripetal Force and Traction
• Anything that moves in a circle is doing
so because a centripetal force is
accelerating it toward the center.
Gravity Can Be a Centripetal Force
• Imagine whirling an object tied to a
string above your head.
• The string exerts a centripetal force on
the object that keeps it moving in a
circular path.
Gravity Can Be a Centripetal Force
• In the same way, Earth’s gravity exerts a
centripetal force on the Moon that keeps
it moving in a nearly circular orbit.
Summary:
 Projectiles
 Anything that is thrown
 Have horizontal and vertical velocities due to
gravity
 Follow a curved path
 Centripetal Acceleration
 Acceleration toward the center of a curved
path
 Is caused by a centripetal force, an unbalanced
force
Section Check
3.2
Question 1
Gravity is an attractive force between
any two objects and depends on
__________.
Answer
Gravity is an attractive force between any
two objects and depends on the masses
of the objects and the distance between
them.
Section Check
3.2
Question 2
Which is NOT one of the four basic
forces?
A.
B.
C.
D.
gravity
net
strong nuclear
weak nuclear
Section Check
3.2
Answer
The answer is B. The fourth basic
force is the electromagnetic force,
which causes electricity, magnetism,
and chemical interactions between
atoms and molecules.
Section Check
3.2
Question 3
Which of the following equations
represents the law of universal
gravitation?
A.
B.
C.
D.
F = G(m1m2/d2)
G = F(m1m2/d2)
F = G(m1 - m2/d2)
F = G(d2/m1m2)
Section Check
3.2
Answer
The answer is A. In the equation, G is
the universal gravitational constant
and d is the distance between the two
masses, m1 and m2.
CHAPTER 3
Section 3
rd
Newton’s 3
Law of Motion
Newton’s third law of motion describes
action-reaction pairs this way. When one
object exerts a force on a second object,
the second one exerts a force on the first
that is equal in strength and opposite in
direction.
The Third Law of Motion
Action and Reaction
• When a force is applied in nature, a
reaction force occurs at the same time.
• When you jump on a trampoline, for
example, you exert a downward force
on the trampoline.
• Simultaneously, the trampoline exerts
an equal force upward, sending you high
into the air.
Action and Reaction Forces Don’t
Cancel
• According to the third law of
motion, action and reaction forces
act on different objects.
• Thus, even though the forces are
equal, they are not balanced
because they act on different
objects.
Newton’s 3rd Law of Motion
 For every action there is an equal but
opposite reaction
 Action-reaction forces act on different
objects and differ from balanced forces.
 States that forces come in pairs
 Rocket propulsion is based on Newton’s
third law of motion.
Action and Reaction Forces
Don’t Cancel
• For example, a swimmer “acts” on the
water, the “reaction” of the water pushes
the swimmer forward.
• Thus, a net force,
or unbalanced
force, acts on the
swimmer so a
change in his or
her motion
occurs.
Rocket Propulsion
• In a rocket engine, burning fuel
produces hot gases. The rocket engine
exerts a force on these gases and
causes them to escape out the back of
the rocket.
• By Newton’s third
law, the gases
exert a force on
the rocket and
push it forward.
 Momentum
 related to how much force is needed to change
an object’s motion
 equals mass times velocity
P=m*v
 Law of conservation of momentum
 Momentum can neither be created nor
destroyed, it just changes forms
 Momentum of a system before a collision is the
same as the momentum of a system after the
collision http://physics.uwstout.edu/physapplets/a-city/physengl/newtonscradle.htm
Momentum
• A moving object has a property called
momentum that is related to how much
force is needed to change its motion.
• The momentum of an object is the
product of its mass and velocity.
Momentum
• Momentum is given the symbol p and
can be calculated with the following
equation:
• momentum = mass x velocity
• P = mv
• The unit for momentum is kg · m/s.
Notice that momentum has a direction
because velocity has a direction.
Force and Changing Momentum
• Recall that acceleration is the
difference between the initial and
final velocity, divided by the time.
• Also, from Newton’s second law, the
net force on an object equals its mass
times its acceleration.
Force and Changing Momentum
• By combining these two
relationships, Newton’s second law
can be written in this way:
• In this equation mvf is the final
momentum and mvi is the initial
momentum.
Law of Conservation of Momentum
• The momentum of an object doesn’t
change unless its mass, velocity, or both
change.
• Momentum, however, can be transferred
from one object to another.
• The law of conservation of momentum
states that if a group of objects exerts
forces only on each other, their total
momentum doesn’t change.
When Objects Collide
• The results of a collision depend on the
momentum of each object.
• When the first
puck hits the
second puck
from behind, it
gives the second
puck momentum
in the same
direction.
When Objects Collide
• If the pucks are speeding toward each
other with the same speed, the total
momentum is zero.
Section Check
3.3
Question 1
According to Newton’s third law of motion,
what happens when one object exerts a force
on a second object?
Answer
According to Newton’s law, the second object
exerts a force on the first that is equal in
strength and opposite in direction.
Section Check
3.3
Question 2
The momentum of an object is the
product of its __________ and
__________.
A.
B.
C.
D.
mass, acceleration
mass, velocity
mass, weight
net force, velocity
Section Check
3.3
Answer
The correct answer is B. An object’s
momentum
is the product of its mass and velocity, and is
given the symbol p.
Section Check
3.3
Question 3
When two objects collide, what happens
to their momentum?
Section Check
3.3
Answer
According to the law of conservation of
momentum, if the objects in a collision
exert forces only on each other, their
total momentum doesn’t change, even
when momentum is transferred from
one object to another.