Name: Normal force and contact force | Forces and Newton's laws of motion | Physics | Khan Academy
Uploaded: 2020-07-02T04:41:54.000Z
Duration: 7 min 18 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

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

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

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

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

And then another block of ice right over here.

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

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

and this one is moving with a constant velocity--

Constant velocity in the right-wards direction.

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?

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

must not be any unbalanced force acting on them.

there is a net force acting on both of them.

Both of them are at the surface of the Earth,

will be the force of gravity acting downwards

There is going to be the downward force of gravity

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.

This right over here is 9.8 meters per second squared.

You get 49 kilogram meter per second squared, which

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,

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

Obviously this thing won't start accelerating downwards

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,

or accelerating towards the center of the Earth?

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

The ice is providing the counteracting force

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.

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

actually no net force on this in either direction.

This guy has a constant velocity in the horizontal direction.

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

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Normal force and contact force | Forces and Newton's laws of motion | Physics | Khan Academy

essentially

assume

absolutely

individual