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Chapter 12
Forces and Motion
12.1 Forces
Forces and Motion
12.1 Forces
What is a Force?
A force is a push or pull that acts on an
object.
 A force can cause a resting object to
move, or it can accelerate a moving object
by changing the object’s speed or
direction.

12.1 Forces
Forces and Motion
Units of Force
Force is measured in newtons (N).
 One newton is the force that causes a
1-kilogram mass to accelerate at a rate of
1 m/s/s.
 A newton is equal to 1 kg-m per second
per second (kg∙m/s2).

12.1 Forces
Forces and Motion
Representing Forces
The direction and strength of a force can
be represented by an arrow.
 The direction of the arrow represents the
direction of the force.
 The length of the arrow represents the
magnitude of the force.

12.1 Forces
Forces and Motion
Combining Forces
Force arrows can be combined to show
the result of how forces combine.
 Forces in the same direction add together
and forces in opposite directions subtract
from one another.
 The net force is the overall force acting on
an object after all the forces are combined.

12.1 Forces
Forces and Motion
Balanced Forces
Balanced forces are forces that combine
to produce a net force of zero.
 When forces on an object are balanced,
the net force is zero and there is no
change in the object’s motion.
 An unlimited number of individual forces
can act on an object and still produce a
net force of zero.

12.1 Forces
Forces and Motion
Unbalanced Forces
An unbalanced force is a force that results
when the net force acting on an object is
not equal to zero.
 When an unbalanced force acts on an
object, the object accelerates.

12.1 Forces
Forces and Motion
Friction
All moving objects are subject to friction, 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.

12.1 Forces

Forces and Motion
There are four main types of friction:
 static friction
 sliding (kinetic) friction
 rolling friction
 fluid friction (drag)
12.1 Forces
Forces and Motion
Static Friction
Static friction is the friction force that acts
on objects that are not moving.
 Static friction always acts in the direction
opposite to that of the applied force.

12.1 Forces
Forces and Motion
12.1 Forces
Forces and Motion
Sliding Friction
Once an object is moving, static friction no
longer acts on it.
 Instead, a smaller friction force called
sliding (kinetic) friction acts on the object.
 Sliding friction is a force that opposes the
direction of motion of an object as it slides
over a surface.

12.1 Forces

Forces and Motion
Because sliding friction is less than static
friction, less force is needed to keep an
object moving than to start it moving.
12.1 Forces
Forces and Motion
Rolling Friction
Rolling friction acts on rolling objects.
 For a given set of materials, the force of
rolling friction is about 100 to 1000 times
less than force of static or sliding friction.

12.1 Forces
Forces and Motion
Fluid Friction
The force of fluid friction opposes the
motion of an object through a fluid.
 Fluid friction increases as the speed of the
object moving through the fluid increases.
 Fluid friction acting on an object moving
through the air is known as air resistance.
 At higher speeds, air resistance can
become a significant force.

12.1 Forces
Forces and Motion
Gravity
Gravity is a force that acts between any
two masses.
 Gravity is an attractive force, that is, it
pulls objects together.
 Gravity is a field force in that it does not
require objects to be in contact for it to act
on them.
 Earth’s gravity acts downward toward the
center of Earth.

12.1 Forces
Forces and Motion
Falling Objects
Gravity causes objects to accelerate
downward, whereas air resistance acts in
the direction opposite to the motion and
reduces acceleration.
 As objects fall to the ground, they
accelerate.

12.1 Forces
Forces and Motion
If an object falls for a long time, the
upward force of air resistance becomes
equal to the downward force of gravity.
 At this point, the forces acting on the
object are balanced.
 Acceleration is zero and the object
continues falling at a constant velocity.

12.1 Forces

Forces and Motion
Terminal velocity is the constant velocity of
a falling object when the force of air
resistance equals the force of gravity.
12.1 Forces
Forces and Motion
12.1 Forces
Forces and Motion
Projectile Motion
Projectile motion is the motion of a falling
object after it is given an initial forward
velocity.
 Air resistance and gravity are the only
forces acting on a projectile.

12.1 Forces
Forces and Motion
Figure 9 shows the motion of two balls
released at the same time.
 Figure 9A shows that balls of different
masses fall at the same rate.
 Figure 9B shows a ball given an initial
horizontal velocity falls vertically at the
same rate as a ball with zero horizontal
velocity.

12.1 Forces
Forces and Motion
The combination of an initial forward
velocity and the downward force of gravity
causes the ball to follow a curved path.
 The two balls fall with the same vertical
acceleration and strike the ground at the
same time.

12.1 Forces
Forces and Motion
12.1 Forces
Forces and Motion
12.2 Newton's First and Second Laws of Motion
Forces and Motion
12.2 Newton’s First and
Second Laws of Motion
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Newton’s First Law of Motion
Newton summarized his study of force and
motion in several laws 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.

12.2 Newton's First and Second Laws of Motion
Forces and Motion
Unless an unbalanced force acts on it,
 an object at rest remains at rest
 an object in motion remains in motion
with the same speed and direction.
 Newton’s first law is sometimes called the
law of inertia.
 Inertia is the tendency of an object to
resist a change in its motion.

12.2 Newton's First and Second Laws of Motion
Forces and Motion
Newton’s Second Law of Motion

An unbalanced, or net force causes an
object’s velocity to change.
12.2 Newton's First and Second Laws of Motion
Forces and Motion
The acceleration is directly proportional to
the net force acting on the object.
 Acceleration also depends upon the mass
of the object (inversely proportional)
 Mass is a measure of inertia of an object
and depends on the amount of matter
the object contains.

12.2 Newton's First and Second Laws of Motion
Forces and 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.
 The equation is a=F/m or more familiarly
F=ma.
 The acceleration of an object is always in
the same direction as the net force.

12.2 Newton's First and Second Laws of Motion

Forces and Motion
Newton’s second law also applies when a
net force acts in the direction opposite to
the object’s motion.
12.2 Newton's First and Second Laws of Motion
Forces and Motion
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Weight and Mass
Although related to each other, mass and
weight are not the same.
 Weight is the force of gravity acting on an
object.
 An object’s weight is the product of the
object’s mass and acceleration due to
gravity acting on it.
 The equation is W=mg.

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
12.3 Newton’s Third Law of
Motion and Momentum
Newton’s Third Law
A force cannot exist alone.
 Forces always exist in pairs.
 According to Newton’s third law of motion,
whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first
object.

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
These two forces are called action and
reaction forces.
 The forces are equal in size and opposite
in direction.
 The forces do not cancel each other
because they act on different objects.

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Momentum
Momentum is the product of an object’s
mass and its velocity.
 The momentum for any object at rest is
zero.
 Momentum is calculated by multiplying an
object’s mass (in kg) and its velocity (in
m/s).

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
The formula is p=mv.
 Momentum is measured in kg∙m/s.

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum
Under certain conditions, collisions obey
the law of conservation of momentum.
 In physics, conservation means that
something has a constant value.
 Conservation of momentum means that
momentum does not increase or
decrease.

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
A closed system means other objects and
forces cannot enter or leave the system.
 Objects within the system can exert forces
on one another.

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
According to 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

12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum
12.4 Universal Forces
Forces and Motion
12.4 Universal Forces
Observations of planets, stars, and
galaxies strongly suggest four different
forces exist throughout the universe.
 The four universal forces are

electromagnetic
 strong nuclear
 weak nuclear
 gravitational

12.4 Universal Forces
Forces and Motion
Electromagnetic Forces
Electric and magnetic forces are two
different aspects of the electromagnetic
force.
 Electromagnetic force is associated with
charged particles.
 Electric force and magnetic force are the
only forces that both attract and repel.

12.4 Universal Forces
Forces and Motion
Electric Forces
Objects with opposite charges attract each
other.
 Objects with like charges repel each other.

12.4 Universal Forces
Forces and Motion
Magnetic Forces
Magnetic forces act on certain metals, on
the poles of magnets, and on moving
charges.
 Magnets have two poles that attract each
other.
 Like poles repel each other.

12.4 Universal Forces
Forces and Motion
Nuclear Forces

Two forces, the strong nuclear force and
the weak nuclear force, act within the
nucleus to hold it together.
12.4 Universal Forces
Forces and Motion
Strong Nuclear Force
The strong nuclear force is a powerful
force of attraction that acts only on the
neutrons and protons in the nucleus.
 Although this force acts over only
extremely short distances, it is 100 times
stronger than the electric force of repulsion
at these distances.

12.4 Universal Forces
Forces and Motion
Weak Nuclear Force
The weak nuclear force is an attractive
force that acts only over a range that is
shorter than the strong nuclear force.
 The existence of beta decay indicates that
there must a force much weaker than the
strong nuclear force.

12.4 Universal Forces
Forces and Motion
Gravitational Force
Gravity is the weakest universal force.
 Gravitational force 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.
 F=G(m1m2/d2)
 Value of G is 6.67×10-11N∙m2/kg2

12.4 Universal Forces
Forces and Motion
Gravity Acts Over Large
Distances
The gravitational force between two
objects is proportional to their masses.
 Gravitational force decreases with the
square of the distance between the
objects.
 Gravity is the weakest universal force but
it is the most effective force over long
distances.
