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Chapter 12
Mechanics: Physics of Motion
The Path to Modern Physics
Greek Philosophy:
Science began with the Greeks…
However…the Greek philosophers rarely
checked their ideas experimentally and so
had false conclusions…
The Path to Modern Physics
In the 1600’s Italian astronomer, Galileo
began to question the truthfulness of
Aristotle’s ideas about falling objects…
Galileo viewed things as special creations of
God and expected the universe to be
orderly…and to operate under specific
laws…
The Path to Modern Physics
Galileo greatly influenced an English
scientist named Sir Issac Newton…
Also guided by a desire to show God’s order
in the universe, Newton built on Galileo’s
ideas…
Newton discovered the laws of motion and
of gravity…
Modern Physics
We are now in the modern age of physics..
Scientists like Robert Boyle,
Daniel Bernoulli,
and Albert Einstein…
Mechanics: The Science of Motion
• Our world is in constant motion…
• Our earth moves, our moon moves… and
our galaxy moves…
• The same laws of physics that govern the
motion of the planets and stars controls
the motion of objects here on earth like the
motion of cars…
• Mechanics: the study of objects in motion
Mechanics: The Science of Motion
• All motion is relative to some point…called a
reference point.
• The term “position” refers to the location of the
object.
• If an object is found at one position and later at
another, we say the object has moved, or
changed position.
• When the object is undergoing a continuous
change in position, we say the object is in
motion.
Mechanics: The Science of Motion
• In order to measure motion, scientists
must be able to describe it.
• They use three terms to describe motion:
1. speed
2. velocity
3. acceleration
Speed: Rate of Motion
• Speed is the distance traveled in a given
time.
• The equation to calculate speed is:
speed = distance/time
s = d/t
• Usually you are calculating for average
speed…
Velocity: Speed and Direction
• In physics, speed and
velocity are NOT the
same thing…
• Speed refers to the
distance covered by an
object in a given time.
• Velocity takes into
account direction as
well…
• Velocity is a “vector”
quantity…which means it
includes magnitude and
direction…
Velocity: Speed and Direction
• Physical quantities with only magnitude
are called scalar quantities…
• A vector quantity is signified by an arrow…
Velocity: Speed and Direction
While on vacation, Lisa Carr traveled a total distance of 440
miles. Her trip took 8 hours. What was her average speed?
Velocity Vectors
Combining Velocities:
If an object is moving in more than one
direction we can find a resultant vector
velocity using Pythagorean theorem…
a2 + b2 = c2
Velocity Vectors
Velocity Vectors
Forces and Motion
• Objects do not move on their own…
• They need a push or a pull to change their
motion…
• That push or pull is called a FORCE…
• Without a force, their can be no change in
motion…
Acceleration: A Change in Motion
• A change in velocity is called acceleration.
• There are three ways to change
acceleration:
1. speed up
2. slow down
3. change direction
Acceleration: A Change in Motion
• Acceleration can only occur if a force acts
on it…
• Without a force…there can be no
acceleration!!
Measuring Acceleration
The unit for acceleration is m/s2
Newton’s First Law of Motion
Inertia causes an object
to resist change in its
state of motion…
Newton’s First Law
An object in motion
stays in motion, an
object at rest stays at
rest unless acted on
by an outside force.
Newton’s 1st Law
An example of this law is
the wearing of seat
belts…
When a car crashes, your
body’s inertia carries it
forward as the car
stops…an outside force
(a seatbelt) is the only
thing that will cause a
change in your bodies
motion!
Inertia
Newton’s 2nd Law
The value of an unbalanced force is equal to
its mass multiplied by its acceleration.
Or
F = ma
Force = mass x acceleration
The unit of force is the Newton
Newton’s 2nd Law
An object accelerates in the direction of the
force applied.
It takes more force to get a big car rolling
and likewise to stop it than a smaller car.
Newton’s 3rd Law
For every action there is an equal and
opposite reaction.
This is known as the
action-reaction law.
Newton’s 3rd Law
Force always comes in
pairs…
if there is a
push on an object the
object will push back.
Example: A rocket
Newton’s 3rd Law
Forces
A force is a push or a
pull…
Forces always come in
pairs…
Remember…equal and
opposite.
Momentum
Momentum (p) is defined as mass (m) times
velocity (v).
p = mv
Momentum is the quantity of motion.
The greater the mass, the greater the
momentum the moving object has.
Momentum
The unit for momentum is kg·m/s.
A small bullet can have a large momentum
because of its velocity…
A large truck could have a small amount of
momentum if moving slowly.
Conservation of Momentum
Momentum is conserved.
In a collision, the momentum of one object is
transferred to another so that the total
momentum of the two objects after a
collision is the same as before.
Gravity
Gravity is a distance force (or as your book
says “action-at-a-distance force”).
No one really knows how distance forces
work or why…
It is still hotly debated and
researched by scientists today.
Universal Law of Gravity
Gravitational forces are directly related to
the mass of the objects.
Gravity
Falling bodies:
All objects near the
earth fall at the
same rate…
Acceleration due
to gravity is 9.8 m/s2
Air Resistance
So…Why don’t the
elephant and feather fall
at the same rate?
Air Resistance!!
Distance of a Falling Object
For a falling object, the distance that it
travels can be calculated with the following
equation:
d = ½ a·t2
distance = ½ acceleration x time2
For a falling object acceleration is gravity so
it is 9.8 m/s2.
Drag and Terminal Velocity
When an object falls through the
atmosphere, gravity is pulling it down…
air is pushing up on it…
eventually, these two forces equal
out and the object no longer falls any
faster.
Drag and Terminal Velocity
When this happens, it is called…
Terminal Velocity
The object is still falling, it just is not gaining
speed.
It is falling at a constant speed.
Weight as a Force
Weight is a measurement of the pull of
gravity on a mass.
W = mg
Weight = mass x acceleration due to gravity
W = m(9.8m/s2)
Weight as a Force
Remember, mass and weight are two
different things!!
Mass is the amount of matter an object has
and weight is the pull of gravity on the
mass.
If you were placed on the moon, your mass
would not change! But, your weight would!
Forces
Two basic types of forces:
1. Contact Forces
Example: Friction
2. Distance Forces:
Example: Gravity and Nuclear
Forces
General Types of Forces
Gravity
Friction
Nuclear
Air Resistance
Magnetic
Electrical
Forces
Only an unbalanced
force (net force)
causes a change in
the motion of the
object.
A balanced force does
not cause change.