Forces and Motion
Chapter 12.1 Forces
What is a Force?
What are the units of force?
 It is a push or a pull that acts
 It is measured in newtons,
on an object
 It can cause a resting object to
move, or it can accelerate a
moving object by changing
the object’s speed or
direction.
abbreviated a N.
 One newton is the force that
causes a 1-kilogram mass to
accelerate at a rate of 1 meter
per second each second (1
m/s2)
 You can use an arrow to
represent the direction and
strength of a force.
12. 1 Forces
Combining Forces
 What is net force?
 It is the overall force acting
on an object after all the
forces are combined.
 What are balanced forces?
 When the forces on an object
are balanced, the net force is
zero and there is no change in
the object’s motion.
Chapter 12.1 Forces
Combining Forces
 What are unbalanced forces?
 When an unbalanced force
acts on an object, the object
accelerates.
Chapter 12.1 Forces
Friction
 What is friction?
 What are the different types
 It is a force that opposes the
motion of objects that touch
as they move past each other.
 Friction acts at the surface
where objects are in contact.
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of Friction?
A. static friction
B. sliding friction
C. Rolling friction
D. Fluid friction
Chapter 12.1 Forces
Static Friction
 What is Static friction?
 It is the friction force that
acts on objects that are not
moving.
 It is always acts in the
direction opposite to that of
the applied force.
 Figure 5a on page 359
Chapter 12.1 Forces
Sliding Friction
 What is sliding friction?
 It is a force that opposes the
direction of motion of an
object as it slides over a
surface.
 It is a smaller friction force
that static friction
 Figure 5 B on page 359
Chapter 12.1 Forces
Rolling Friction
 What is rolling friction?
 It is the friction force that
acts on rolling objects.
 Figure 6 on page 360
Chapter 12.1 Forces
Fluid Friction
 What is fluid friction?
 It is the force that opposes the
motion of an object through a
fluid.
Chapter 12.1 Forces
Gravity
 What is gravity?
 It is a force that acts between
any two masses.
 It is an attractive force.
 Note that the force of gravity
does not require objects to be
in contact for it to act on
them.
 Earth’s gravity acts downward
toward the center of Earth.
 An upward force usually
balances the downward force
of gravity.
Chapter 12.1 Forces
Falling Objects
 What forces affect the motion
 What is Terminal Velocity?
of an object dropped from the
top of a tall building?
 Gravity causes objects to
accelerate downward,
whereas air resistance acts in
the direction opposite to the
motion and reduces
acceleration.
 It is the constant velocity of a
falling object when the force
of air resistance equals the
force of gravity.
Chapter 12.1
Projectile Motion
 What is the path of a ball
 Air resistance and gravity are
moving forward?
 It is a curved path.
 What is this curved path?
 It is the path of projectile
motion which is the motion
of a falling object (projectile)
after it is given an initial
forward velocity.
the only forces acting on a
projectile.
 The combination of an initial
forward velocity and the
downward vertical force of
gravity causes the ball to
follow a curved path.
Chapter 12.1 Assessment
 1. A force can set an object at rest into motion, or it can
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accelerate a moving object by changing its speed or
direction.
2. Static friction, sliding friction, rolling friction, and
fluid friction.
3. It acts opposite the direction of motion and slows
the acceleration of a falling object.
4. Downward toward the center of earth.
5. The combination of initial horizontal velocity and
downward vertical force causes a projectile to follow a
curved path.
Chapter 12.1 Assessment
 6. The force of static friction is greater than that of
sliding friction. The force of sliding friction is generally
greater than that of rolling friction.
 7. Varying air resistance acting on the leaf results in a
fluttering motion.
 Both coins hit the ground at the same time.
12.2 Newton’s laws of Motion
Aristotle
 Ancient Greek scientist and
philosopher
 384 B.C. to 322 B.C.
 He incorrectly proposed that
force is required to keep an
object moving at constant
speed.
12.2 Newton’s laws of Motion
Galileo
 Italian scientist
 1564- 1642
 By rolling balls down wooden
ramps, he studied how
gravity produces constant
acceleration.
 He concluded that moving
objects not subject to friction
or any other force would
continue to move indefinitely.
12.2 Newton’s laws of Motion
Isaac Newton
 English scientist
 1642 – 1727
 First to define mass and force
 Then introduced his laws of
motion
12.2 Newton’s Laws of Motion
First law of Motion
 According to Newton’s first
law of motion, the state of
motion of an object does not
change as long as the net
force acting on the object is
zero.
 Thus, unless an unbalanced
force acts, on object at rest
remains at rest, and an object
in motion remains in motion
with the same speed and
direction.
 What is inertia?
 It is the tendency of an object
to resist a change in its
motion.
 In other words, an object at
rest tends to remain at rest,
and an object in motion
tends to remain in motion
with the same direction and
speed.
12.2 Newton’s laws of Motion
Second Law of Motion
 According to Newton’s
second law of motion, the
acceleration of an object is
equal to the net force acting
on it divided by the object’s
mass.
 Mass is a measure of the
inertia of an object and
depends on the amount of
matter the object contains.
 Doubling the mass of an
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object cuts its acceleration in
half.
Double the force, the
acceleration of the ball
doubles.
Formula
A = F/m
Chart on the side board
12.2 Newton’s laws of Motion
 Math Skills
 An automobile with a mass of 1000 kilograms
accelerates when the traffic light turns green. If the net
force on the car is 4000 newtons, what is the car’s
acceleration?
 List your information
 Show your formula, substitution, work, answer
 A = 4000/1000 = 4 m/s2
12.2 Newton’s laws of Motion
 A boy pushes forward a cart of groceries with a total
mass of 4.0. kg. What is the acceleration of the cart if
the net force on the cart is 60.0 N?
 A = F/m = 60.0/40.0 = 1.5 m/s2
 What is the upward acceleration of a helicopter with a
mass of 5000 kg if a force of 10,000 N acts on it in an
upward direction?
 A = f/M = 10,000/5000 = 2 m/s2
12.2 Newton’s laws of Motion
 An automobile with a mass of 1200 kg accelerates at a
rate of 3.0 m/s2 in the forward direction. What is the
net force acting on the automobile?
 F = ma = 1200 x 3 =3600 N
 A 25-N force accelerates a boy in a wheelchair at 0.5
m/s2. What is the mass of the boy and the wheelchair?
 M = F/a = 25/.5 = 50 kg
12.2 Newton’s laws of Motion
Weight and Mass
 What is weight?
 Mass is a measure of the
 It is the force of gravity acting
inertia of an object; weight is
a measure of the force of
gravity acting on an object.
 What is your mass?
on an object.
 An object’s weight is the
product of the object’s mass
and acceleration due to
gravity acting on it.
 W = mg
 G = 9.8 m/s2
12.2 Assessment
 1. According to Newton’s first law of motion, the state
of motion of an object does not change as long as the
net force acting on the object is zero.
 2. a = F/m
 3. Mass is a measure of the inertia of an object; weight
is a measure of the force of gravity acting on an object.
12.2 Assessment
 4. Examples in which objects at rest remain at rest or
tend to continue moving (first law) and in which a net
force acting on an object causes a change in the
object’s state of motion (second law).
 5. The heavier steel ball will take longer to reach
terminal velocity. A greater speed is needed to produce
the air resistance required to balance the steel ball’s
greater weight. The steel ball must fall for a longer
period of time in order to reach this greater speed.
12.2 Assessment
 6. a = F/m = 825/75 = 11 m/s2
 7. a = (vf – vi)/t = (4 -0 )/8 = .5 m/s2
 F = ma = 85 x .5 = 42.5 N
Chapter 12.3 Third law &
Momentum
Newton’s Third Law
 What is Newton’s Third Law
of Motion?
 It is whenever one object
exerts a force on a second
object, the second object
exerts an equal and opposite
force on the first object.
 These two forces are called
action and reaction forces.
Chapter 12.3 Third law &
Momentum
Momentum
 What is momentum?
 It is the product of an object’s
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mass and its velocity.
An object has a large
momentum if the product of
its mass and velocity is large.
Momentum Formula
Momentum = Mass x Velocity
Chart on the Board
Chapter 12.3 Third Law &
Momentum
Conservation of
Momentum
 What is the law of
conservation of momentum?
 If no net force acts on a
system, then the total
momentum of the system
does not change.
 In a closed system, the loss of
momentum of one object
equals the gain in
momentum of another
object, momentum is
conserved.
Chapter 12.3 Third Law &
Momentum
Data Analysis
 Page 377
 1. At t = 0; the ball has zero
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momentum before it is
released.
2. at t = 2.5 s; about 6.5 kgm/s
3. mv = 3.25 kg=m/s,
V = 3.25/.25
V = 13 m/s upward
Chapter 12.3 Assessment
 1. Whenever one object exerts a force on a second object,
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the second object exerts an equal and opposite force on the
first object
2. Momentum = Mass x Velocity
3. The object involved must be part of a closed system.
4. Because the speeds are equal, the eagle’s greater mass
gives it more momentum.
5. It is incorrect because it is the force of the water against
the oars that propels the boat forward. The oars pushing
against the water is the action force, and the water pushing
back against the oars is the reaction force.
Chapter 12.3 Assessment
 6. As you walk, you bend your foot and push off against
the ground. This action force produces the reaction
force of the ground pushing against your shoe. The
reaction force pushes you forward.
 7. When you jump, you push against the canoe.
However, the canoe also moves in the opposite
direction of your jump. Because the canoe moves away,
it produces a smaller reaction force on you. This small
reaction force is not strong enough to propel you to
the river bank.
Chapter 12.4 Universal Forces
Universal Forces
 What are the four universal
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forces?
Electromagnetic Forces
Strong Nuclear Force
Weak Nuclear Force
Gravitational Force
Chapter 12.3 Universal Forces
Electromagnetic Forces
Electric Forces
 What are the two different
 Electric forces act between
aspects of the
electromagnetic force?
 They are Electric and
Magnetic force.
 Electric force and magnetic
force are the only forces that
can both attract and repel.
charged objects or particles
such as electrons and
protons.
 Objects with opposite charges
– positive and negative –
attract one another.
 Objects with like charges
repel one another.
Chapter 12.4 Universal Forces
Magnetic Forces
 Magnets have two poles,
north and south, that attract
each other.
 Two poles that are alike repel
each other.
 Opposite poles attract each
other.
Chapter 12.4 Universal Forces
Nuclear Forces
 What is the Strong Nuclear
Force?
 It is a powerful force of
attraction that acts only on
the neutrons and protons in
the nucleus, Holding them
together.
 It overcomes the electric force
of repulsion that acts among
the protons in the nucleus.
 This force acts over only
extremely short distances, it
is 100 times stronger than the
electric force of repulsion at
these distances.
Chapter 12.4 Universal Forces
Weak Nuclear Force
 The weak force is weaker in
strength than the strong
nuclear force.
 It is an attractive force that
acts only over a short range.
(less than the range of the
strong nuclear force)
 It is involved in certain types
of radioactive processes.
Chapter 12.4 Universal Forces
Gravitational Force
 What is gravitational force?
 It is an attractive force that
acts between any two masses.
 Newton’s law of universal
gravitation states that every
object in the universe attracts
every other object.
 Gravity is the weakest
universal force, but it is the
most effective force over long
distances.
 What are two factors
affecting gravitational force?
 Mass and Distance
 Larger the mass, larger
gravitational force
 Increasing the distance
between the masses
significantly reduces the
gravitational force.
Chapter 12.4 Universal Forces
Chapter 12.4 Universal Forces
The Earth, Moon, and
Tides
 Earth’s gravitational
 If you have spent time at the
attraction keeps the moon in
a nearly circular orbit around
Earth.
 A centripetal force is a centerdirected force causes a
continuously changes the
direction of an object to make
it move in a circle.
seashore, you have probably
noticed that the level of the
tide changes throughout the
day. The gravitational pull
from the moon produces two
bulges in Earth’s oceans.
 Side of Earth closest to moon
 Side of Earth farthest to
moon
Chapter 12 Universal Forces
 1. Electromagnetic force is the only force that vban
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both attract and repel.
2. The strong nuclear force acts within the nucleus to
hold it together.
3. According to Newton’s law of universal gravitation,
every object in the universe attracts every other object.
4. Friction with the atmosphere slows artificial
satellites in low orbit.
5. Earth’s gravitational attraction acts as a centripetal
force on the moon.
Chapter 12 Universal Forces
 6. It would gradually be pulled closer to Earth and
eventually fall out of orbit.
 7. According to Newton’s third law, whenever one
object exerts a force on a second object, the second
object exerts an equal and opposite force on the first
object. Newton’s law of gravitation is a special case of
this in which the force is gravity.
Chapter 12 Forces and Motion
 Time for a test on Forces and Motion!