Transcript Motion 102: WHY things move

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Motion 101: HOW things move
Welcome to
Motion 102: WHY things move
Aristotle (384-322 BC)
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Ancient Greek
philosopher
Divided motion into two
types: natural and
violent
Natural motion: straight
up or straight down;
objects have natural
resting places
Violent motion:
imposed motion; result of
forces that pushed or
pulled
No force = no motion
Nicolaus Copernicus (1473-1543)
Renaissance
astronomer and
mathematician
 Poland, Italy
 Earth and other
planets move
around the Sun.
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Galileo Galilei (1564-1642)
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Italian physicist,
astronomer, and
philosopher
Force is NOT necessary
to keep an object
moving.
Force is any push or pull.
Friction is a force that
acts between materials.
“Inertia” – Every material
object resists change to
its state of motion.
Sir Isaac Newton (
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1643-1727)
English physicist,
mathematician,
astronomer, and
philosopher
Invented calculus
Law of gravity
Binomial Theorem
Light from prism
Reflecting Telescope
Sir Isaac Newton (
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Philosophiae
Naturalis Principia
Mathematica
Newton’s
laws of motion
Kepler’s laws
Theory of tides
Wave motion
Fluid dynamics
continued)
Newton’s First Law
An object at rest remains at rest,
and an object in motion
continues in motion with
constant velocity (that is
constant speed in a straight line)
unless it experiences a net
external force.
“Law of Inertia”
Two types of forces
A contact force arises when one object
touches another.
 A field force can exist between objects,
even in the absence of physical contact
between two objects. For example,
gravity or the electromagnetic force.
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What is a net external force?
Inertia
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Inertia is the tendency of an
object to maintain its state of
motion unless acted on by a net
force.
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An object that is either at rest or
moving with constant velocity is
said to be in equilibrium (no net
force is acting on it).
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Mass is a measurement of inertia.
Newton’s Second Law
The acceleration of an object is
directly proportional to the net
external force acting on the
object and inversely
proportional to the mass of the
object.
ΣF = ma
Applying a larger net force to an
object results in a larger
acceleration.
Applying equal net forces to two
different objects will accelerate
the smaller mass more.
If you want to cause two different objects
to have the same acceleration, the object
with the larger mass will require a larger
force.
Special Forces
weight – the magnitude of the force
of gravity on an object
 normal force – a contact force
exerted by one object on another in
a direction perpendicular to the
surface of contact
 tension – a force generated when a
string is attached to a body and
pulled taut; the direction of the
force is away from the body along
the string at the point of attachment
 friction and air resistance (later!)
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Newton’s Third Law
in his words
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From Principia:
Lex III: Actioni contrariam semper
et æqualem esse reactionem: sive
corporum duorum actiones in se
mutuo semper esse æquales et in
partes contrarias dirigi.
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“To every action there is always opposed an equal reaction; or, the
mutual actions of two bodies upon each other are always equal and
directed to contrary parts.”
Newton’s Third Law
If two bodies interact, the magnitude
of the force exerted on object 1
by object 2 is equal to the
magnitude of the force
simultaneously exerted on object
2 by object 1, and these two
forces are opposite in direction.
“Law of action – reaction”
“Forces come in pairs”
Tug-of-war
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Work with a partner.
Hook the two spring scales to either end
of the string.
With each of you pulling on each spring
scale, try to get the largest difference in
the readings on the scales. If both scales
read the same, the difference is 0. Try to
get one scale to read a large value for the
force and the other scale to read a small
value.
Record the results of your efforts in your
notebook.
Walking
Get up and walk normally.
Pay attention to what your body is doing.
Pay special attention to how you go about
starting from a standstill. In going from a
standstill to walking, you’re accelerating,
right? What force is causing that
acceleration?
 Remember that when you accelerate, that’s
caused by something other than you exerting
a force on you – NET EXTERNAL FORCE!
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Rifle
Horse and Cart