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

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


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.

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61 thoughts on “Normal force and contact force | Forces and Newton’s laws of motion | Physics | Khan Academy

  1. @DJAMATURE @DJAMATURE not sure if this is much good… but an easy or obvious choice…
    h t t p : / / en. wikipedia. org / wiki / German _ reunification
    remove the spaces or course …

  2. How could anyone rate this video poorly? In judging the significant figures of a number, this 1 person is certainly not a significant one. Unless they made a mistake and were instead trying to rate this video positively?

    Thanks for another great vid Sal!

  3. That even one person can dislike shows that we are not all on the same page here.
    Some come to learn and grow, others COUNTERACTS;)!
    Sal, you are a noble prince of online tuition and I, SolVatar, hereby knight you in "The Noble Order of Higher Learning" and dub you "The Online Guru" – thanks, heaps, on behalf of all who wish to evolve!

  4. @HabeKeinMitleid I'm using an analogy, to explain further, if the person who voted negatively were a digit in a number they would not be classified as a significant figure. Meaning, the positive votes are significant figures and the negative one's are not. The negative votes (1 so far) are simply redundant, just as non-significant figures are, the majority of people agree with the idea that Sal makes great video's.

    Anyway, what I said seems to have lost its flavor (no longer amusing)…

  5. @DJAMATURE I see. Well I won't use up Khanacademy space for this, but of course if you were to quote sentences verbatim you might get into trouble. I thought you were just looking for some basic revision material to read to increase your general knowledge on the subject per se as you said "I don't get it". Anyway, good luck all the same.

  6. You forgot to add the force of friction on the right block. In your situation it doesn't have a constant velocity, but a constant increasing velocity. a= 5 m*s^-2
    Also… please do some more complex stuff. You're on university right? This is highschool stuff

  7. @Psalm789: “I think I can safely say that nobody understands quantum mechanics. ” – Richard Feynman, 1965.
    I think that nobody truly understands them. Because they are utterly counter-intuitive. I may take a couple more generations for humanity to be able to intuitively “get” it. 🙂

  8. @SOLVATAR: Uuum, have you thought about that *maybe* he disliked it, *because* it was too simplistic? I, for one, certainly missed him mentioning whether it is the Lorentz force or the Fermi force pushing.

  9. Electro magnetic repulsion! Interesting! But you never mentioned anything about the coefficient of static friction or the coefficient of kinetic friction which is illustrated by both ice cube examples.

  10. Please
    some one tel me what is the Name of this program software ??
    i wanna install it to use it in my lessons too

  11. I have an idiotic question but someone please refresh my mind

    If you apply a constant force of 10N to a mass of 1kg, it will accelerate 10m/s^2 until the application of the force ceases, in which case the velocity of the mass will be whatever it was when the force stopped being applied. 

    Now to the question. If you bench press for instance (a small weight that you can do pretty much whatever you want to) you're applying a force to the weighted bar, right? However, the weight is not necessarily accelerating. Say you intentionally press the bar from your chest all the way up with a constant velocity (ofc t(0) won't be the same as after 2 seconds but anyway) of 0,2 m/s, what is the catch in that situation? A force must be applied constantly or the weight would drop on your chest, why isn't it accelerating?

    thanks a lot for answers!

  12. You guys used ice to negate the effects of friction, but forgot the effects of air resistance that's on Earth 🙂

  13. This video contains an incorrect statement that the normal force is caused by the electrostatic repulsion between electron clouds of the atoms in the opposing surfaces. If that were true then no molecular substance could hold together because the electron clouds around the molecules would push them apart. Life would be impossible, among other problems. What is actually causing the normal force is the quantum electron degeneracy pressure (also known as the Pauli exclusion principle). Electrons from the opposing surfaces cannot enter the territory already claimed by the electron orbitals of the opposing molecules. (Remember chemistry, in which you learned no more than two electrons can occupy each orbital.)

  14. If things have different downward force, does that mean they will fall at the different speeds? I remember watching a video of a feather and a weight being dropped in a vacuum at the same time and height, and they both reached the ground at the same time. Why is that?

  15. My new PHYSICS SOLVING APP.More then 150+ formulas,Solves for any variable you want,Covers up all physics.download now.https://play.google.com/store/apps/details?id=com.physics.lenovo.myapplication

  16. If the forces are balanced why is the object not mouving towards the center of the earth with constant velocity.

  17. Does this mean that if the gravity was shut down instantly then there would be no normal force on these blocks of ice?
    Or in simple terms if there was no gravity there would be no normal force?

  18. From my 12 year old : gravitational force is the force that attracts me to Earth and is also the force that attracts the Earth to me. So if we have a population explosion will the Earth expand?

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