Transcript Force

The Four Force(men) of
the Apocalypse
Carl Wozniak
Northern Michigan University
What is a force?
 Any influence (PUSH OR PULL) that
gives energy to an object,
sometimes causing a change in the
motion of the object.
 We recognize the forces that occur
when one object touches another,
contact forces, but there are forces
that we can’t see, forces that work
at a distance.
Contact forces
Contact forces include:
frictional forces,
tensional forces (pull a string tight),
normal forces (support force: book resting on
table),
air resistance forces (special type of frictional
force),
and applied forces (pushing a desk).
Forces at a distance
Gravitational force
Electrical force
Magnetic force
Mass vs. weight
Many students of physics confuse weight
with mass.
The mass of an object refers to the amount of
matter that is contained by the object.
The weight of an object is the force of gravity
acting upon that object.
Mass is related to how much stuff is there and
weight is related to the pull of the Earth.
Four forces of nature
Electromagnetic force
Electromagnetism causes like-charged
objects to repel each other and oppositely
charged objects to attract each other.
The electromagnetic force binds negative
electrons to the positive nuclei in atoms
and underlies the interactions between
atoms.
Its force carrier particle is a photon.
Electromagnetism in
history
Ben Franklin didn’t invent
electricity (1752)
Lightning has been known
for all of history
Electromagnetic force
James Clerk Maxwell did
pioneering work in the mid1800s that expressed the
basic principles of electricity
and magnetism.
Maxwell’s work predicted
that the energy of
electromagnetism moved in
waves.
Electromagnetic force
E-M force is an infinite-range attractive or
repulsive force which acts between
charged particles.
Coulomb's Law: Like charges repel, unlike
charges attract.
The amount of force between two charges is
directly proportional to amount of charge and
inversely proportional to the square of the
distance between the two.
It is observed as light or magnetism.
Gravity is the
brainchild of Isaac
Newton, although it
was also conceived
much earlier by
Hildegard von Bingen
(1099-1179)
Newton upset
centuries of thought
espoused by
Aristotle.
Gravity
Aristotle’s world
Aristotle taught that the
everything was made of a
combination of 4 elements
and the “quintessence.”
Air
Earth
Fire
Water
Aristotle’s world
In Aristotle’s view, heavier
objects have a tendency to
be at rest in their natural
environment
Thus, earth and water
were constantly striving to
reach their “center,” the
Earth’s center.
Aristotle’s world
A stone falls to the ground
because the stone and the
ground are similar in
substance.
The natural place for water
is to be just above earth and
air’s natural place is just
above that.
The natural place for the
element fire is somewhere
above us (but well below the
Moon).
Aristotle’s world
Aristotle devised laws
governing the motion of
objects as they had outside
forces imparted on them,
just as Newton did.
Aristotle’s world
Aristotle’s Laws of Motion
An object’s velocity is
proportional to the force
applied to it.
The speed of a falling object
is proportional to the weight
of the object.
The speed at which an
object falls is inversely
proportional to the density
of the medium through
which it falls.
Galileo’s contribution
 Galileo (1564-1642)
studied the speed of
falling bodies to see if
they, as Aristotle said,
fell at different rates
depending on their
mass.
 Let’s examine some
hypotheticals.
What do you think?
Setting the stage: You are holding a bullet
in your hand, a foot above the ground, and
your lab partner has an identical bullet in a
gun, pointed exactly horizontally and also a
foot above the ground.
The question: If you drop the bullet at the
instant the gun is fired, which bullet hits
the ground first?
Galileo’s thought
experiment
Imagine two objects, one heavy and one
light, falling together and tied together
by a thin string.
Are the objects falling as one object, hence
falling faster (because of their increased
mass) than if the objects were untethered?
If you were falling alongside and you
snipped the string, would the objects start
to fall at differing rates?
Back to Newton
Newton probably really
did get inspiration for
his theory of gravity by
watching an apple fall.
But it most likely didn’t
fall on his head.
Newton’s gravity
 Newton realized the apple was accelerating,
starting at zero as it hung on the tree, picking
up speed until it reached the ground.
 Newton’ imagined a very tall tree, and what
would happen when the apple fell from it.
 Newton’s genius came from the next
postulation: If the force of gravity reaches the
top of the tree, might it not continue far
beyond that, all the way to the moon?
Newton’s gravity
If so, the orbit of the
Earth and moon could
be due to the gravity
of the two bodies
countered by the
acceleration of the
revolving system.
• The cannonball falls,
but continually
“misses” the Earth.
Newton’s gravity
Newton came to
the conclusion
that any two
objects in the
universe exert a
gravitational
attraction on
each other.
Newton’s gravity
What the formula
essentially says
is that the
force of gravity
is dependent on
the masses of
the two bodies
and the
distance
between them.
Newton’s gravity
• The greater the
masses, the
greater the
attraction
• The farther
away the two
masses are, the
less the
attraction