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Hi. It's Mr. Andersen and this AP physics essentials video 48. It is on contact forces

hi it's mr. Andersen and this is ap physics essentials video 48 it's on contact forces which is kind of a

which is kind of a misleading term. Because if you know anything about physics there are

misleading term because if you know

four fundamental forces in nature. We have gravity. We have electromagnetic force. We

anything about physics there are four fundamental forces in nature we've got gravity we've got electromagnetic force we've

have strong and weak nuclear forces. And in all of these nothing is really touching. So

got strong and weak nuclear forces and in all of these nothing is really

when we talk about a contact force we are really dealing with electric charge. Or electromagnetism

touching so when we talk about a contact force we're really dealing with electric charge or electromagnetism at a close

at the close level. And so in AP Physics I you should understand a number of contact

level and so ap physics one you should understand a number of contact forces

forces, like the normal force. If this object sits on a table, the table is exerting a normal

like the normal force if this object sits on a table the table is exerting a

force which is perpendicular to the table. Or if we have this object hanging from a string,

normal force which is perpendicular to the table or if we have this object

there is a tensional force. Or by a spring we have spring force. That is applying a force

hanging from a string there's a tensional force or by a spring we have a spring force that's a plane of force

both up and down. Or we could apply a force, we call that an applied force to the object

both up and down or we could apply a force we call that an applied force to

and we move it across the table. But there is going to be a force that opposes that motion

the object and we move it across the table but there's going to be a force

and we call that a frictional force. And in AP Physics II you should understand the importance

that opposes that motion and we call that a frictional force and in AP Physics 2 you should understand the

of a buoyant force. If we put the object inside water there is a buoyant force. In other words

importance of a buoyant force if we put the object inside water there's a

the water is actually pushing on that object. But in all of these situations the objects

buoyant force in other words the water is actually pushing on all on that object but in all of these situations

are not really touching. And so if objects quote-unquote touch, we call that a contact

the objects aren't really touching and so if objects quote-unquote touch we call that a contact force and what's

force. And what is causing that is interatomic interactions or forces between the atoms themselves.

causing that is inner atomic interactions or forces between the atoms

And I have a model that will make this a little bit more clear. And so the number of contact

themselves and I got a model that'll make this a little bit more clear and so

forces you should understand are these six. And with each of them you should understand

the number of contact forces you should understand are these six and with each

what is the force and then what is the interatomic cause or dependency of that force. And then

of them you should understand what's the force and then what's the inner atomic cause or dependency of that force and

what direction is it going to act in. So this is a model that make sense to me. Each of

then what direction is it going to act in so this is a model that makes sense

these blue spheres represents an atom and then these dotted lines represent bonds, or

to me each of these blue spheres represents an atom and then these dotted lines represent bonds or chemical bonds

chemical bonds between the atoms. But think of them almost like springs that are connecting

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

all these atoms together and then we have an object and so what happens if I

just push on it like this? So if I push on that atom right here, it is going to change

were to just push on it like this so if I push on that atom right here it's

its shape. And so we are going to start to get tension right here, tension in these bonds

going to change its shape and so we're going to start to get tension right here tension in these bonds here and so

here. And so there is going to be a resisting force that is going to be in the opposite

there's going to be a Steen force that's going to be in the

direction like that. What would happen if I were to pull on that atom right here? It

opposite direction like that what would happen if I were to pull on that atom

is going to stretch it out here. So we are going to have tension all along this way.

right here it's going to stretch it out here so we're going to have tension all

And so what is going to be the responding force? It would be in the opposite direction.

along this way and so what's going to be the responding force it be in the

But let's say we were to push at an angle, if we push at an angle like that, so we are

opposite direction but let's say we were to push out an angle if we push out an angle like that so we're going to go

going to go like that, we are now going to have tension right here. Where is going to

like that we're now going to have tension right here where is going to be

be the responding force? It is going to be in the opposite direction. And so in all the

the responding force it's 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

forces that I'm about to describe it's the same inner atomic cause there's no

between a normal and a tension force and a buoyant force at the atomic level. It is all

difference between a normal force and a tension force and a buoyant force at the

interactions between atoms. And so let's start with a normal force. If we have this red object

atomic level it's 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

on top of the blue what it's going to look like if we look at the inner atomic

is we are going to have all of these red atoms resting on top of all of these blue atoms.

causes it we're going to is we're going to have all of these red atoms resting on top of all of these blue atoms and

And there are going to be electrons around the outside. And so it is literally hovering

they're going to be electrons on around the outside and so there it's literally

on top of the other one. But the direction, since we have the weight of the red object

hovering on top of the other one but the direction since we have the weight of

down, we are going to have a normal force that goes in the opposite direction like that.

the red object down we're going to have a normal force that goes in the opposite

It is always going to be perpendicular. That is what normal means to the platform on which

direction like that it's always going to be perpendicular that's what normal means to the platform on which it's

it is sitting. So let's say we turn it like this. What is the interatomic cause? Well

sitting so let's say we turn it like this what's the inner atomic cause what

we have just simply rotated the atoms. And so it is also going to be perpendicular, that

we've just simply rotated the atoms and so it's also going to be perpendicular

normal force. What happens if we push on the object? So now we are pushing on it in this

that normal force what happens if we push on the object so now we're pushing

direction. What is going to happen at the atomic level? Now we have the atoms of the

on it in this direction what's going to happen at the atomic level now we've got

hand pushing on the atoms of the object itself. And so we are going to have an applied force

the atoms of the hand pushing on the atoms of the object itself and so we're

in that direction. Now what is happening if we look at it as it slides across the table?

going to have an applied force in that direction now what's 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

so this is a little bit different now we've got the atoms almost kind of a

effect. And so all these atoms are pushing in this direction on these atoms. And so what

shearing effect and so all these our atoms are pushing in this direction on these atoms and so what's going to be

is going to be the responding force? We are going to have a frictional force that is going

the responding force we're going to have

to oppose that movement. It is still interactions between the objects. What happens if we have

a frictional force that's going to oppose that movement it's still interactions between the objects what

an object hanging, like this? If we look at it at the atomic level we are applying a force,

happens if we have an object hanging like this if we look at it at the atomic level we're applying a force o the

so the weight down and we are adding tension all along these bonds within the rope itself.

weight down and we're 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

and so what's going to oppose that is a tensional force up let's say we look at

this. The spring looks just like a rope. And so what we are going to do is we pull down

a spring like this spring looks just like a rope and so what we're going to do is

on it, there is going to be a spring force up. But what is interesting about a spring

we pull down on it there's going to be a spring force up but what's interesting

force is if it moves and bounces up too high then there will be a spring force, a restoring

about a spring force is if it moves and bounces up too high then there will be a

force that brings it back towards equilibrium. But let's say we take an object and drop it

spring force a restoring force that brings it back towards equilibrium let's

in water like this. Again we are going to have a buoyant force. Now how is this different?

say we take an object and drop it in water like this again we're going to have a buoyant force now how is this

Well the water is able to move. There is still going to be bonds between the water molecules,

different well the water is able to move they're still going to be bonds between the water molecules but it's able to

but it is able to kind of wrap around that object right here. It does not mean that it

kind of wrap around that object right

is still is not applying a force. It is applying what we call a buoyant force back on that

here it doesn't mean that it still isn't applying a force it's applying what we call a buoyant force back on that object

object like that. And so it is going to be in the vertical. Did you learn to make claims

like that and so it's going to be in the vertical so did you learn to make claims

about these contact forces? And can you explain it at the atomic level? Again we are putting

about these contact forces and can you explain it at the atomic level again

stress on these bonds and the objects are simply responding. So I hope that was helpful.

we're putting stress on these bonds and the objects are simply responding so I hope that was helpful

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