Hi. It's Mr. Andersen and this AP physics essentials video 48. It is on contact forces
which is kind of a misleading term. Because if you know anything about physics there are
four fundamental forces in nature. We have gravity. We have electromagnetic force. We
have strong and weak nuclear forces. And in all of these nothing is really touching. So
when we talk about a contact force we are really dealing with electric charge. Or electromagnetism
at the close level. And so in AP Physics I you should understand a number of contact
forces, like the normal force. If this object sits on a table, the table is exerting a normal
force which is perpendicular to the table. Or if we have this object hanging from a string,
there is a tensional force. Or by a spring we have spring force. That is applying a force
both up and down. Or we could apply a force, we call that an applied force to the object
and we move it across the table. But there is going to be a force that opposes that motion
and we call that a frictional force. And in AP Physics II you should understand the importance
of a buoyant force. If we put the object inside water there is a buoyant force. In other words
the water is actually pushing on that object. But in all of these situations the objects
are not really touching. And so if objects quote-unquote touch, we call that a contact
force. And what is causing that is interatomic interactions or forces between the atoms themselves.
And I have a model that will make this a little bit more clear. And so the number of contact
forces you should understand are these six. And with each of them you should understand
what is the force and then what is the interatomic cause or dependency of that force. And then
what direction is it going to act in. So this is a model that make sense to me. Each of
these blue spheres represents an atom and then these dotted lines represent bonds, or
chemical bonds between the atoms. But think of them almost like springs that are connecting
all of these atoms together. And then we have an object. And so what happens if I were to
just push on it like this? So if I push on that atom right here, it is going to change
its shape. And so we are going to start to get tension right here, tension in these bonds
here. And so there is going to be a resisting force that is going to be in the opposite
direction like that. What would happen if I were to pull on that atom right here? It
is going to stretch it out here. So we are going to have tension all along this way.
And so what is going to be the responding force? It would be in the opposite direction.
But let's say we were to push at an angle, if we push at an angle like that, so we are
going to go like that, we are now going to have tension right here. Where is going to
be the responding force? It is going to be in the opposite direction. And so in all the
forces that I am about to describe, it is the same interatomic cause. There is no difference
between a normal and a tension force and a buoyant force at the atomic level. It is all
interactions between atoms. And so let's start with a normal force. If we have this red object
on top of the blue, what it is going to look like if we look at the interatomic causes
is we are going to have all of these red atoms resting on top of all of these blue atoms.
And there are going to be electrons around the outside. And so it is literally hovering
on top of the other one. But the direction, since we have the weight of the red object
down, we are going to have a normal force that goes in the opposite direction like that.
It is always going to be perpendicular. That is what normal means to the platform on which
it is sitting. So let's say we turn it like this. What is the interatomic cause? Well
we have just simply rotated the atoms. And so it is also going to be perpendicular, that
normal force. What happens if we push on the object? So now we are pushing on it in this
direction. What is going to happen at the atomic level? Now we have the atoms of the
hand pushing on the atoms of the object itself. And so we are going to have an applied force
in that direction. Now what is happening if we look at it as it slides across the table?
So this is a little bit different. Now we have the atoms, almost kind of a shearing
effect. And so all these atoms are pushing in this direction on these atoms. And so what
is going to be the responding force? We are going to have a frictional force that is going
to oppose that movement. It is still interactions between the objects. What happens if we have
an object hanging, like this? If we look at it at the atomic level we are applying a force,
so the weight down and we are adding tension all along these bonds within the rope itself.
And so what is going to oppose that is a tensional force up. Let's say we look at a spring, like
this. The spring looks just like a rope. And so what we are going to do is we pull down
on it, there is going to be a spring force up. But what is interesting about a spring
force is if it moves and bounces up too high then there will be a spring force, a restoring
force that brings it back towards equilibrium. But let's say we take an object and drop it
in water like this. Again we are going to have a buoyant force. Now how is this different?
Well the water is able to move. There is still going to be bonds between the water molecules,
but it is able to kind of wrap around that object right here. It does not mean that it
is still is not applying a force. It is applying what we call a buoyant force back on that
object like that. And so it is going to be in the vertical. Did you learn to make claims
about these contact forces? And can you explain it at the atomic level? Again we are putting
stress on these bonds and the objects are simply responding. So I hope that was helpful.