mr.p: Good morning, today we're going
to review the dynamics topics covered
on the AP Physics I exam.
Dynamics is the study of the motion
of objects and the forces causing those motions.
Bo: Hey guys
Billy: Hey Bo.
Bobby: Hi Bo.
♫ flipping physics ♫
mr.p: Bo, what is the difference between
inertial and gravitational mass?
Bo: Inertial mass is a measure of
an object's inertia, or a measure
of it's resistance to acceleration.
In other words, the more inertial mass
something has, the more it resists acceleration.
Gravitational mass...
Bobby: Gravitational mass is what we use
to determine the force of gravity
or the weight of an object.
The force of gravity equals the gravitational mass
of an object times the acceleration due to gravity.
Bo: But gravitational mass and inertial mass
are experimentally identical, so we
don't normally identify the difference between the two.
mr.p: Now let's talk about Newton's Three Laws of Motion.
Billy, what is Newton's First Law of Motion?
Billy: Newton's First Law, an object at rest
will remain at rest, an object in motion
will maintain a constant velocity
unless acted upon by a Net External Force.
mr.p: Correct.
An object at rest will stay at rest
and an object in motion will maintain
a constant velocity unless acted upon
by a Net External Force.
Two common mistakes, one, students often say
an object in motion will stay in motion
which is not correct.
An object in motion will maintain a constant velocity.
And because velocity is a vector
it means it will maintain
a constant speed and a constant direction.
Two, students often leave out the word,
"Net", on "Net External Force."
It is the Net External Force,
the sum of the forces that needs
to be equal to zero for the object
to stay at rest, or for the object
to maintain a constant velocity.
Bobby, what is Newton's Second Law?
Bobby: Newton's Second Law, Net Force equals
mass times acceleration where both force
and acceleration are vectors.
mr.p: True.
However, it's rearranged on the AP Physics I equation sheet.
They have it set up that the acceleration
equals the Net Force divided by the mass
where both the acceleration and the force are vectors.
You can see that the acceleration
is directly proportional to the Net Force
acting on the object,
and inversely proportional to the mass of the object.
Now when working with Newton's Second Law,
it is important that you do a couple of things first.
You need to identify the object or
the objects you are summing the forces on
and you need to identify the direction
in which you are summing the forces.
And you need to draw free body diagrams
which are also called force diagrams.
Let's do one.
Assuming I'm pushing horizontally on the book,
Billy, what are all the forces acting on the book
and what are their directions?
Billy: The force of gravity is always straight down.
The force normal is normal to the surface,
or perpendicular to the wall and always a push,
so away from the wall.
You said the force applied is horizontal
so it is directly into the wall.
The force of friction is parallel
to the surface, independent of the direction
of the force applied, and opposes motion.
So the force of friction is up
to prevent the book from sliding down.
And it's static friction because the book isn't moving.
And never break forces into components
in your initial free body diagram
on the AP test because your free body diagrams are answers.
When you need to break a force into components
you redraw the free body diagram with those components.
mr.p: Very nice.
On the AP test do not break forces
into components on your first free body diagram.
When you need to break forces into components
redraw the free body diagram, please.
Ok, let's detail some of what you just said about forces.
The force of gravity acting on an object
or the weight of the object is always down.
And the equation is the force of gravity equals
the mass of the object, times the
acceleration due to gravity.
The force normal is the force normal to
or perpendicular to the surface causing that force normal.
And the force normal is always a push,
a surface can't pull, it can only push.
We also need the dimensions for force,
so Bobby, what are the dimensions for force?
Bobby: Force is in Newtons.
mr.p: Bobby, can you please give that
to me in base SI dimensions?
Bobby: In base metric units, okay.
A Newton is a kilogram times a meter per second squared.
mr.p: And you can remember that using Newton's Second Law.
The net force, which is in Newtons,
is equal to the mass, which is in kilograms,
times the acceleration in meters per second squared.
To help you find the direction of the "Force of Friction,"
remember that the force of friction
is always parallel to the surface.
It opposes motion, and the direction
of the force of friction is independent
of the direction of the force applied.
There are two types of friction.
Static friction, or non-moving friction,
more specifically where the two surfaces
do not slide relative to one another.
And kinetic friction or moving friction,
where the two surfaces do slide relative to one another.
The equation on the equation sheet
for the force of friction is this.
The absolute value of the force of friction
is less than or equal to the coefficient of friction,
Mu, times the absolute value of the force normal
where both forces are vectors.
Now, because we actually have two
different kinds of friction, this equation
actually works out to be three subtly different equations.
The force of kinetic friction is
always equal to the coefficient of kinetic friction
times the force normal.
However, because the force of static friction
will increase or decrease in an attempt
to prevent the object from moving.
The force of static friction is less than
or equal to the coefficient of
static friction times the force normal.
And the maximum force of static friction
is then equal to the coefficient
of static friction times the force normal.
Now Bo, what can you tell me about the relationship
between the coefficient of kinetic friction
and the coefficient of static friction
between the two surfaces.
Bo: The coefficient of friction is
experimentally determined and dimensionless.
But, I don't know what you mean by relationship.
Bobby: Remember, for two surfaces, the coefficient
of static friction is always greater
than the coefficient of kinetic friction?
Bo: That's right.
It's harder to put something into motion
than it is to keep it moving.
mr.p: We still have one of Newton's Three Laws of Motion
that we need to talk about.
For every force object one applies on object two,
there is an equal but opposite force
that object two applies on object one
where both forces are vectors.
Going back to our free body diagram
of the book against the wall and looking
just at the force normal, which is the force
from the wall on the book.
There exists then, an equal but opposite force
from the book on the wall.
Those two forces form a Newton's Third Law force pair,
also sometimes called an action reaction pair.
And notice that those two forces in
the Newton's Third Law force pair
act on two different objects, the book,
and the wall, and those two forces act simultaneously.
On an incline, we usually break
the force of gravity into its components
which are parallel to the incline
and perpendicular to the incline.
Therefore when we sum the forces
in the directions that are parallel
and perpendicular to the incline
because the acceleration perpendicular
to the incline is usually zero,
then the net force perpendicular
to the incline is also equal to zero
which makes it easier to solve.
The force of gravity parallel equals
the mass of the object times the acceleration
due to gravity times the sine of the incline angle.
And the force of gravity perpendicular
equals the mass of the object times the acceleration
due to gravity times the cosine of the incline angle.
And it's easy to remember which one is which
because if the incline angle is equal to zero degrees,
then the force of gravity parallel would be equal to zero,
and the sine of zero degrees equals zero.
And if you are freaking out and can't remember values
of trigonometric functions for common angles,
be aware that there is a table of those values
on the table of information which
you will be given on the AP exam.
Billy: I'd probably just use my calculator.
Yep. The sine of zero is zero.
Bo: Nice.
mr.p: Billy, what is translational equilibrium?
Billy: Translational equilibrium is when the net force
on an object equals zero, which means
the object isn't moving.
Bobby: Actually no, it means the acceleration of the object
is equal to zero, which means it might be at rest.
Billy: Yeah. Or it could be moving at a constant velocity.
Ok, because the net force equals mass times acceleration,
the acceleration is zero, which means
an object in translational equilibrium
is either at rest, or moving at a constant velocity.
mr.p: That is correct.
An object in translational equilibrium will have
a net force equal to zero, therefore,
the mass times acceleration is equal to zero.
Therefore, the acceleration of the object
is equal to zero.
The object is either at rest or moving
at a constant velocity.
That is the end of my review of the dynamics topics
covered on the AP Physics I exam.
My next review will be about work,
energy, and power.
You are more than welcome to enjoy
that review.
And you can also go to my website
where you'll find all of my lectures with lecture notes.
Thank you very much for learning with me today,
I enjoyed learning with you.