Transcript Physics - Scituate Science Department

Physics:
It Explains Everything
What is physics?
• Physics is the study of matter and energy
• It deals with how the two interact and
interact with each other
• Chemistry is the study of matter but physics
deals with the energy involved
You do physics every day
• Whenever you do anything, you’re doing
physics
• Whenever you, as a body composed of
matter, interact with another body of matter,
you’re doing physics.
Here are some examples
We Will Keep it Simple
• Physics has many sub-divisions
• Such as..
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Kinematics
cosmology
quantum physics
theoretical physics
Condensed Matter Physics
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Atomic, molecular, and optical (AMO) Physics
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High Energy Physics
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Astrophysics
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Applied Physics
We Will stick to Kinematics
• This deals with motion on the large scale
• So we’ll be discussing things such as force,
mass, weight, acceleration, Newton’s Laws,
simple machines, speed, velocity, energy
and energy transfer
• All of these topics should be familiar to you
• Let’s start with an easy one
What is a force?
• It is a push or a pull on an object (f =ma)
• there are only four forces in the known
universe
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gravity
electromagnetic force
strong nuclear force
weak nuclear force
• all forces that we see in nature are one of
these four
Gravity
• This is the force of attraction between two objects
• It is determined by the masses of the two objects and the
distance between them
• gravity explains the motion between large objects very
well
• Large objects are considered things that are larger than the
microscopic scale
– so we are large objects, insects, dust and the sun are as
well
It is tricky though
• We really don’t know why or how gravity
works
• We know how much gravitational force an
object has
• we can predict its effect on another object
• we just don’t really know how it is
generated or where it comes from
• But make no mistake, as it stands today,
gravity IS NOT electromagnetic
Gravity at work
Strong Nuclear Force
• Lets talk atoms first
• Atoms are composed of smaller
subatomic particles called…
• Correct, protons, neutrons and
electrons
• They usually contain the same
number of electrons as they do
protons
neutrons
• Neutrons are neutral, this
means they have no electric
charge
• But, neutrons do have mass
• Protons and neutrons are packed
together tightly in the atom’s…
• Correct, the nucleus
• See…
The strong force weakens…
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…as the particles get farther apart
Soooooo……..huh?
Its like this
When protons are close together,
they are attracted to each other.
• This short-range force of attraction
is much stronger than the long-range
force of repulsion
• So if they are far away from each
other they will repel
SOMETHING LIKE THIS
STRONG
FORCE
ELECTRIC FORCE
TOTAL
FORCE
NO STRONG
FORCE
ELECTRIC FORCE
TOTAL
FORCE
SIZE MATTERS
• Atoms with small nuclei, such as hydrogen,
are held together more tightly because the
particles are close together
• This allows for the strong force to act on
most if not all the particles
• If a nucleus is larger, such as in
uranium, the strong force between
particles can only act on other particles
that are close to it
• It won’t act on those that are far away
• But, the repulsive, electric force acts on
all of them
• In many nuclei, the strong force
is able to permanently keep the
nucleus together
• These nuclei are stable
• However if the strong force is not
enough to hold the nucleus
together, the nucleus decays
• This means it gives off matter and
energy
• This “nuclear decay” is called
radioactivity
LARGE NUCLEI WILL DECAY
• Large nuclei tend to be unstable
• In fact all nuclei that contain more
than 83 protons are radioactive
• Awesomely enough, many smaller ones
are radioactive also
• Even some with only one or a few
protons
• Thanks universe!
FUN FACT!
• Almost all elements with more than 92 protons
DO NOT exist naturally on Earth.
• They have been produced in a lab
• They are so unstable that they decay almost
immediately after they are made
That’s the most
fun fact ever!
SCIENCE IS
AWESOME!
Remember this?
STRONG
FORCE
ELECTRIC FORCE
TOTAL
FORCE
NO STRONG
FORCE
ELECTRIC FORCE
TOTAL
FORCE
The weak nuclear force
• Is the one responsible for the decay
• If the protons get too far away from each
other the weak nuclear force pushes them
apart and the atom decays
Let’s Talk Energy
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What is it?
There are several definitions.
This one is found in several textbooks:
“The ability to do work”
Okay, but what is the definition of work?
If you don’t know, then this definition is
useless
Here’s one that is better but
pretty vague
• “Anything in the universe that is NOT
matter.”
• Okay so this gives you half of infinity, not
very helpful
Here is a good one...I mean it this
time...well almost.
• But, to first understand energy we must
understand matter. They go hand-in-hand
ya’know.
• In fact, by Einstein’s definition, they’re the same
thing E = mc2
• energy = mass times the speed of light squared
• Plus, we find in nuclear reactions that matter and
energy are converted into each other...so they’re
the same...I guess.
• But I digress
Matter
• Sometimes defined as “anything with mass that
takes up space”
• Okay..but what is mass?
• Mass is defined as “How much matter is in
something”
• So by this definition, matter is “anything that is
matter and takes up space”
• not very good. It is also defined as anything that
is NOT energy. Or, the other half of infinity.
Again, not very good.
Matter and energy
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These are the good definitions
really
for real this time
really
Matter: anything that can be detected with
one or all of the five senses
• Energy: causes the movement and change of
matter. CANNOT be detected with the five
senses.
Energy comes in many forms...
• ...well kind of
• its actually all just...energy but since it does
so many things and effects matter in many
different ways it is easy to think of it as
different types
• What are some of them?
Energy forms
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Light
heat
kinetic (1/2mv2)
sound
potential
Lets talk about potential energy
• It alone comes in many forms
• chemical potential
– stored in the chemical bonds of molecules
• gravitational potential
– energy stored due to an object’s position above
the surface of Earth (GPE = mgh)
• electric potential
– energy of a stored electric charge
Law of Conservation of Energy
• Energy cannot be created or destroyed
• The amount of energy is constant in the
universe
• it can change “forms” very easily but the
amount never changes
So this...
...is this
...is this
...is this
...is this
...is this
...is this.
The Force/Energy Connection
• When a force is applied from one object to
another, energy is transferred and converted
from one form to another.
• All of the energy you use in a day came
from the sun...ALL OF IT.
• How can that be true?
• Easy it just simply gets converted from
solar energy to all the other forms
photosynthesis
Nuclear
fusion
reaction
Energy back into
the universe as
light, heat, sound,
chemical potential
Cellular
respiration
Force applied to
log
Work
• The scientific definition of work is different
than the everyday definition
• W=Fxd
• or…work = force x distance
• This means that to actually do work one has
to apply a force to something and move it in
the direction of the force
Applying a force but
NOT doing any work
Doing work
The force/energy/work
connection
• Let’s tie it all together
• To move something, one must apply a force
to it
• In doing so, energy is transferred from one
object to another
• If the object moves, work was done on that
object
• But it is possible to apply a force to
something but NOT do work on it
Fun with math
• If you apply a 100 Newton force to a box,
and you move the box 22 meters, how much
work was done on the box?
• 2200
• 2200 what?
• The unit for work is the same as it is for
energy
• so…2200 Joules or 2200 J
More About Force
• Of course the amount of force needed is
directly proportional to the mass of the
object
• What is a direct proportion?
• It means as the mass of the object increases
the amount of force necessary to move it
increases