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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.

  • And so my answer to you is, well, if that's your answer,

  • 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.

  • And let's talk about this ice as one of these molecules.

  • 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,

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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
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