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Chapter 12: Forces & Motion
12.1 (pg’s 356-362) Forces
You need force to
change motion

Every object continues in a state of rest, or of motion, unless
force is applied to change things. This is a fancy way of saying
that things tend to keep doing what they are already doing.

Force: can cause a resting object to move, or it can accelerate a
moving object by changing the object’s speed or direction.

A force is what we call a push or a pull, or any action that has
the ability to change motion. This definition does not, however,
mean that forces always change motion! If you push down on a
table, it probably will not move.
What is force?
Force is an action that has the ability to change
motion.
Pounds and
newtons

Force is measured in newtons (N). 1 N is the force that cause
a 1-kg mass to accelerate at a rate of 1 meter per second each
second (1N = 1kg x m/s2 )

There are two units of force that are commonly
used: pounds and newtons. Scientists prefer to use
newtons. The newton is a smaller unit than the
pound. There are 4.48 newtons in one pound. A
person weighing 100 pounds would weigh 448
newtons.
Units of Force
Unit
1 newton
1 pound


Equivalents
0.228 pounds
4.48 newtons
A spring scale is a
The metric unit of force, the newton, relates force
tool for measuring
and motion. One newton equals 1 kilogram
force. A force of 1
multiplied by 1 meter per second squared. This
pound is the same
means that a force of one newton causes a 1as a force of 4.48
kilogram mass to have an acceleration of 1
newtons.
m/sec2.
In talking about force, “newton” is easier to say than “1
kilogram · m/sec2.”
Newton
A newton is the metric
unit of force.
A force of one newton
acting on a mass of 1
kilogram produces an
acceleration of 1 m/sec2.

Combining Forces:
o Net Force: the overall force acting on
an object after all the forces are
combined.

Forces in the same direction
add together

Forces in opposite directions
subtract from one another.
Balanced and unbalanced forces

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.

Unbalanced Forces: an unbalanced force acts on an object, the
object accelerates.
Friction and motion
What is
friction?

Friction: a force that opposes the motion of objects that touch
as they move past each other.
Friction is a term that is used to describe forces that result from
relative motion between objects (like the wheel and axle of a car).
Frictional forces always work against the motion that produces them.
What causes
friction?
Friction comes from two surfaces moving against each other. Surfaces
of objects that appear smooth and shiny actually have microscopic
hills and valleys. As the surfaces slide across each other the hills and
valleys interfere causing friction.
We use the word friction to describe any force that is caused by
motion and that acts to slow motion down. Some examples of friction
include:
o Static Friction: force that acts on objects that are not
moving.
Kinds of
friction
o Sliding Friction: force that opposes the direction of
motion of an object as it slides over a surface.
o Rolling Friction: force that act on rolling objects.
o Fluid Friction: force that opposes the motion of an
object through a fluid.
How does
friction affect
acceleration?
Friction is a force that . That means the force of friction is opposite
whatever force is causing motion. For a car rolling downhill, gravity
supplies a force pulling down the hill. Friction opposes motion, so it
pushes the car up the hill while gravity is pulling the car down the hill.
The difference between force and mass
Use the correct
units in formulas
Defining force
and mass
Force and mass have different units. Force units are pounds or
newtons. Mass units are grams or kilograms. To get the right answer
when using formulas that include force or mass, you need to use the
correct units!
Force is a push or pulling action that can change motion. Mass is the
amount of “stuff” or matter in an object. Mass is a basic property of
objects. Mass resists the action of forces by making objects harder to
accelerate.
Weight is
different
from mass
Your mass is the
same everywhere
in the universe,
but your weight
is different
Units of force and
mass can describe a
quantity
Different units can
describe the same
quantity
The weight of a person can be described in pounds or newtons. On
Earth, a child weighs 30 pounds or about 134 newtons. In other words,
the force acting on the child, due to the influence of Earth’s gravity, is
134 kilograms · m/sec2.
A child that weighs 30 pounds on Earth has a mass of about 14
kilograms because on Earth 2.2 pounds equals 1 kilogram. Because
mass is an amount of matter, mass is independent of the force of
gravity. Therefore, the mass of a person is the same everywhere in the
universe. However, the weight of a person on Earth is different from
what it would be on the moon or another planet because the force of
gravity is different at these other places.
Mass and weight are commonly used to describe the quantity of
something. For example, a kilogram of bananas weighs 2.2 pounds.
You can describe the quantity of bananas as having a mass of 1
kilogram, or a weight of 2.2 pounds. Using two different kinds of
measurement to describe the same quantity of bananas does not mean
pounds and kilograms are the same thing.
We often use different units to describe a quantity. For bananas, you
can use a unit of mass (kilograms) or a unit of force (pounds).
Likewise, buying one gallon of milk is the same as buying 8.4 pounds
of milk. Pounds and gallons both describe the same quantity but one
unit is a measure of volume (gallons) and one is a measure of force
(pounds).
Weight, Gravity, and Friction
Gravity
What is gravity?
What is the force that causes an object like a car to accelerate down a
ramp? You probably know gravity is involved. Gravity is a force that
pulls every mass toward every other mass. Since Earth is the biggest
mass around, gravity pulls everything toward the center of Earth.


Gravity depends
on mass
Ask someone the meaning of the word down and they point
toward the center of Earth.
Down is the direction of the force of gravity.
The force of gravity depends on how much mass you have. If you have
more mass, gravity pulls on you with more force. That is why we can
use force to measure mass. When you use a digital balance to measure
the mass of a sample, you are really measuring the force of gravity
acting on your sample.
 If you are on the surface of Earth, every kilogram of mass
creates a gravitational force of 9.8 newtons.
 You may recognize this number—9.8 newtons is the same as
9.8 m/sec2, the acceleration of gravity.
Mars’ gravity is
weaker than
Earth’s
If you were on Mars, your force/mass balance would have to be
adjusted. The planet is much smaller than Earth and therefore Mars’s
gravity is weaker. Every kilogram of mass on Mars results in a gravity
force of only 3.8 newtons (figure 3.8). The larger the planet you are
on, the greater the force of
gravity.

On Jupiter, the largest
planet, gravity has a
force 2.6 times
stronger than on the
surface of Earth. If
you weighed 110
pounds on Earth, you
would weigh 286
pound on Jupiter!

Gravity: is a force
that acts between any
2 masses.
o Gravity is an
attractive force
that pulls
object together.
o Earth’s
gravitational
force exerts a
force of attraction on every other object that is near
Earth.
o The force of gravity does not require objects to be in
contact for it to act on them.
o Earth’s gravity act downward toward the center of
Earth.
o An upward force or supporting force balances the
forward force of gravity.
Mass and weight
What is weight?
How to calculate
weight
Weight is what we call the force created by gravity on objects. The
weight of an object depends on its mass. Your mass is constant
throughout the universe, but your weight changes depending on what
planet you happen to be on. For example, because the gravitational
force on Mars is less than that on Earth, you weigh less on Mars than
on Earth, but your mass is the same at both locations!
If you know the mass of an object, you can calculate its weight using
Newton’s second law. When you drop something on Earth, gravity
causes it to accelerate at 9.8 m/sec2. Because there is acceleration, you
know there must be a force. You also know the force is exactly equal
to mass times acceleration. The force we call weight is equal to an
object’s mass times the acceleration of gravity (9.8 m/sec2).
 Since weight is a force, we use the letter F to represent it.
DON’T use
kilograms for
weight
Because we live and work on the surface of Earth, we tend to use
weight and mass interchangeably. Heavy objects have lots of mass
and light objects have little mass. Boxes and people are “weighed”
in both kilograms and pounds. This is OK for everyday use, but
you must remember the difference when doing physics.
Physics is about the true nature of how the universe works and
mass is truly a fundamental property of an object. Force often
depends on outside influences, like gravity. You cannot
interchange force and mass in a formula; doing so would be like
substituting a fork for a spoon when you are trying to eat soup. In
physics, force and mass are different quantities with different units.

Falling Objects: Gravity causes objects to accelerate
downward, where as air resistance acts in the direction opposite
to the motion and reduces acceleration.
12.2 (pg’s 363-371) Newton’s First & Second Laws of Motion:
Mass and inertia
Newton’s first
law
Inertia is a
property of
mass
The kilogram
Newton’s first law is also called the law of inertia. Inertia is defined as
the property of an object to resist changing its state of motion. An
object with a lot of inertia takes a lot of force to start or stop. Big
trucks have more inertia than small cars, and bicycles have even less
inertia.
The amount of inertia an object has depends on
its mass. Mass is a measure of the inertia of an
object. Mass is what we usually think of when
we use words like “heavy” or “light.” A heavy
object has a large mass while an object
described as “light as a feather” has a small
mass. We can also define mass as the amount
of matter an object has.
Mass is measured in kilograms. The kilogram is one of the primary
units of the metric system, like the meter and second. For reference, 1
kilogram has a weight of about 2.2 pounds on the Earth’s surface. That
means gravity pulls on a mass of 1 kilogram with a force of 2.2
pounds.
The mass of an object does not change, no matter where the object is,
what planet it is on, or how it is moving.

1st Law: the state of motion of an object does not change as
long as the net force acting on the object is zero.
o Sometimes called the Law of Inertia: the tendency of
an object to resist a change in its motion.
o 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.
Newton’s second law of motion
Newton’s second law relates the applied force on an object, the mass
of the object, and acceleration.

2nd Law: The acceleration of an object is equal to the force
acting on it divided by the object’s mass.
Acceleration =
Mass =
Force
Mass
Force
Acceleration
Force = Mass x Acceleration

Doubling the mass of an object cuts its acceleration in half.

The acceleraion of an object is always in the same direction as
the net force.

Mass & Weight:
o Mass is a measure of the inertia of an object, how much
“stuff” something is made up of.
o Weight is a measure of the force of gravity acting on an
object.
Weight = Mass x Acceleration due to gravity
12.3 (pgs. 372-377) Newton’s Third Law of Motion &
Momentum

Newton’s 3rd Law: whenever on object exerts a force on a
second object, the second object exerts an equal and opposite
force on the first object. (action and reaction forces)

Momentum: is the product of an object’s mass and its
velocity.

An object has a large momentum if the product of its mass and
velocity is large.
Momentum = Mass x Velocity

Conservation of Momentum: if not net force acts on a
system, then the total momentum of the system does not
change.

Closed system: other objects and forces cannot enter or leave a
system.

In a close system, the loss of momentum of one object equals
the gain in momentum of another object, momentum is
conserved.
12.4 (pgs. 378-382) Universal Forces:

Electromagnetic Forces: are associated with charged particles

Nuclear Forces: the strong nuclear force and the weak nuclear
force, act within the nucleus to hold it together

Gravitational Force: every object in the universe attracts
every other object.
o It takes a huge mass such as Earth’s to exert a large
gravitational force.
o Gravity is the weakest universal force, but it is the most
effective over long distances.