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• Let's say that I have a huge, maybe frozen over lake,

• or maybe it's a big pond.

• So I have a huge surface of ice over here-- my best attempt

• to draw a flat surface of ice-- and I'm

• going to put two blocks of ice here.

• So I'm going to put one block of ice

• just like this, one block of ice right over here.

• And then I'm going to put another block of ice right

• over here.

• And then another block of ice right over here.

• And these blocks of ice are identical.

• They're both 5 kilograms.

• They are both 5 kilograms-- let me write this down.

• So they are both 5 kilograms.

• Or both of their masses, I should say, are 5 kilograms.

• And the only difference between the two

• is that relative to the pond, this one

• is stationary-- this one is stationary--

• and this one is moving with a constant velocity--

• constant velocity.

• Constant velocity in the right-wards direction.

• And let's say that its constant velocity

• is at 5 meters per second-- 5 meters per second.

• And the whole reason why I made blocks of ice on top of ice

• is that we're going to assume, at least for the sake

• of this video, that friction is negligible.

• Now what does Newton's First Law of Motion

• tell us about something that is either not in motion--

• or you could view this as a constant velocity of 0--

• or something that has a constant velocity?

• Well Newton's First Law says, well

• look, they're going to keep their constant velocity

• or stay stationary, which is the constant velocity of 0,

• unless there is some unbalance, unless there

• is some net force acting on an object.

• So let's just think about it here.

• In either of these situations, there

• must not be any unbalanced force acting on them.

• Or their must not be any net force.

• But if you think about it, if we're

• assuming that these things are on Earth,

• there is a net force acting on both of them.

• Both of them are at the surface of the Earth,

• and they both have mass, so there

• will be the force of gravity acting downwards

• on both of them.

• There is going to be the downward force of gravity

• on both of these blocks of ice.

• And that downward force of gravity, the force of gravity,

• is going to be equal to the gravitational field

• near the surface of the Earth, times-- which

• is a vector-- times the mass of the object.

• So times 5 kilograms.

• This right over here is 9.8 meters per second squared.

• So you multiply that times 5.

• You get 49 kilogram meter per second squared, which

• is the same thing as 49 newtons.

• So this is a little bit of a conundrum here.

• Newton's First Law says, an object at rest

• will stay at rest, or an object in motion

• will stay in motion, unless there is some unbalanced,

• or unless there is some net force.

• But based on what we've drawn right here,

• it looks like there's some type of a net force.

• It looks like I have 49 newtons of force pulling this thing

• downwards.

• But you say, no, no no, Sal.

• Obviously this thing won't start accelerating downwards

• because there's ice here.

• Its resting on a big pool of frozen water.

• then what is the resulting force that cancels out

• with gravity to keep these blocks of ice,

• either one of them, from plummeting down

• to the core of the Earth?

• From essentially going into free fall,

• or accelerating towards the center of the Earth?

• And you say, well, I guess if these things would be falling,

• if not for the ice, the ice must be

• providing the counteracting force.

• And you are absolutely correct.

• The ice is providing the counteracting force

• in the opposite direction.

• So the exact magnitude of force, and it

• is in the opposite direction.

• And so if the force of gravity on each of these blocks of ice

• are 49 newtons downwards it is completely

• netted off by the force of the ice on the block upwards.

• And that will be a force 49 newtons upwards in either case.

• And now, hopefully, it makes sense

• that Newton's First Law still holds.

• We have no net force on this in the vertical direction,

• actually no net force on this in either direction.

• That's why this guy has a 0 velocity

• in the horizontal direction.

• This guy has a constant velocity in the horizontal direction.

• And neither of them are accelerating

• in the vertical direction.

• Because you have the force of the ice on the block,

• the ice is supporting the block, that's

• completely counteracting gravity.

• And this force, in this example, is called the normal force.

• This is the normal force-- it's 49 newtons upwards.

• This right here is the normal force.

• And we'll talk more about the normal force in future videos.

• The normal force is the force, when

• anything is resting on any surface that's

• perpendicular to that surface.

• And it's going to start to matter a lot when

• we start thinking about friction and all the rest.

• So what we'll see in future videos, when you have something

• on an incline, and let's say I have a block on an incline

• like this.

• The normal force from the, I guess

• you could say, this wedge on the block,

• is going to be perpendicular to the surface.

• And if you really think about what's happening here,

• it's fundamentally an electromagnetic force.

• Because if you really zoomed in on the molecules of the ice

• right over here, even better the atoms of the ice here.

• And you really zoomed in on the atoms or the molecules

• of the ice up here, what's keeping this top block of ice

• from falling down is that in order

• for it to go through its molecules would have to kind

• of compress against, or I guess it would have to get closer

• to, the water molecules or the individual atoms

• in this ice down here.

• And the atoms, let me draw it on an atomic level

• right over here.

• So maybe, let me draw one of this guy's molecules.

• So you have an oxygen with 2 hydrogens

• and it forms this big lattice structure.

• And we can talk about more of that in the chemistry playlist.

• So maybe it looks something like this.

• And it has its 2 hydrogens

• And so what's keeping these guys from getting compressed, what's

• keeping this block of ice from going down further,

• is the repulsion between the electrons in this molecule

• and the electrons in that molecule.

• So on a macro level we view this is kind of a contact force.

• But on a microscopic level, on an atomic level,

• it's really just electromagnetic repulsion at work.

Let's say that I have a huge, maybe frozen over lake,

B1 中級 美國腔 物體 牛頓 重力 分子 速度 冰塊

# 法向力與接觸力｜力與牛頓運動定律｜物理學｜可汗學院 (Normal force and contact force | Forces and Newton's laws of motion | Physics | Khan Academy)

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Sophia Chung 發佈於 2021 年 01 月 14 日