Where do forces come from?
All forces arise from the interaction
of two objects with each other.
In this example, we have two objects, One and Two,
interacting with each other in this case
because their surfaces are in contact with each other-- one
sitting on top of the other.
And the interaction between the two objects
gives rise to a pair of forces.
I have one force that's acting from object two down on object
one, and the other force that's acting from object one
back up onto object two.
And these forces, following Newton's third law,
are equal in magnitude and opposite in direction.
What if we now push these blocks horizontally
in opposite directions?
At their interface, there is a frictional interaction.
This interaction gives rise to a pair of frictional forces
that are parallel to the interface.
The friction force on the top block from the bottom block
is opposite in direction but equal in magnitude
to the friction from the bottom block due to the top block.
Let's look at some of the common misconceptions about forces.
First of all, motion is not a force.
If I have a block that's moving at a constant velocity
along a frictionless plane, then there's
no horizontal force that's acting on this block.
Because there is no interaction that would give
rise to a horizontal force.
The fact that it has a velocity does not necessarily
mean that there is a force acting on it.
Another common mistake is to think that forces
transfer through objects.
For example, if I'm pushing a book against a wall,
it might seem like the force from my hand
is being transferred to the wall.
But what is really happening is that there are
two independent interactions.
The interaction of my hand with the book.
And the interaction of the book with the wall.
And these are separate interactions.
The forces might happen to have the same magnitude
in some cases.
But it's not because the force from my hand
is being transferred.
Forces arise between pairs of objects.
And those forces can only act on those two objects.
Let's look more closely at Newton's third law.
So Newton's third law says that when there are two objects that
are interacting, there will be two forces that
arise from that interaction.
And these two forces are always equal in magnitude and opposite
in direction.
These two forces, this Newton's third law pair of forces,
never act on the same object.
But instead one force acts on the first object and one
on the second.
Because these forces arise out of the same interaction,
they have to be the same type of force.
So they could be gravitational forces, frictional forces,
normal forces, et cetera.
Let's look at an example of a book sitting on a table.
It's being pulled down by the gravitational force.
And it's being held up by the normal force.
Are these two forces a Newton's third law pair?
The book is not accelerating.
So these forces are equal in magnitude and opposite
in direction.
But they cannot be a Newton's third law for two different
reasons.
First of all, they both act on the same object-- the book.
And second of all, they arise from different interactions.
The normal force on the book is part of a Newton's third law
pair that comes from the interaction
between the book and the table.
What's the Newton's third law pair of the gravitational force
on the book?
Well, because the gravitational force
arises from the interaction between the book and the earth,
the other force in the Newton's third law pair
is actually the force that the book exerts on the earth.
You might not think about the book acting on the earth.
But the force is actually equal and opposite to the earth's
force on the book.
It's just that the earth is so much more massive,
the force has no detectable influence
on the Earth's behavior at all.