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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 5: Normal force and contact force

# Normal force and contact force

Explore Newton's First Law of Motion with two blocks of ice, one stationary and one moving. Understand how the force of gravity and the normal force from the ice keep the blocks from accelerating downwards. Dive into the atomic level to see how electromagnetic repulsion between molecules creates the normal force. Created by Sal Khan.

## Want to join the conversation?

• If you stripped all of the electrons from an atom, does that mean it could pass through matter?
(87 votes)
• Yes!
You just re-invented famous Rutherford experiment!
http://www.youtube.com/watch?v=5pZj0u_XMbc
(92 votes)
• I'm confused. It sounds like 'normal' force and 'contact' force describe the same thing. If I understand correctly, 'contact' force is happening on an atomic level between the object and the surface, which results both in friction and not allowing the object to pass through the surface. If the contact force is responsible for not allowing the object to pass through the surface, then isn't the normal force part of the contact force? Or is it something completely separate?
(12 votes)
• At around , Sal begins to explain that there needs to be a force to oppose the weight of the ice cube. At , he calls that the normal force. The normal force is a force perpendicular to the ground that opposes the downward force of the weight of the object. That's all there is to it and you don't have to think of it in terms of individual atoms in most problems you come across.

The 'normal' force is a type of 'contact' force. What Sal doesn't clarify in this video is that the contact force is ANY force that results when two things (and their atoms and molecules) touch each other. It's a general term that can refer to normal force, friction, collision force, tension, etc. For example, if you throw a ball at a wall and it hits the wall at an acute angle, the force at the angle of contact on the ball is a contact force. Don't confuse this force with normal force. Remember the term "normal force" specifically refers to a force that's perpendicular to a surface. So if a contact force is, for example, at a 45 degree angle, it can't be considered a normal force.
(64 votes)
• what is a situation where no normal force is acting upon a person.
(15 votes)
• well, it depends:
1. If you are on earth´s surface you´ll just pass through earth till you´ll get fused into earth mantel and PAM PAM PAM you are one with the earth and you the earth is one with you.
2. If you are in deep space on a platform and suddently there is no normal force then if we supposely tell that there are no other forces applying on you then you´ll just float around because there is no gravity net force that is strong enough to make you go through the platform so yeah in both case it wouldn´t end so well for you...
(3 votes)
• Does Newtons 3rd Law apply in deep space vaccum?
(7 votes)
• Yes. That is why it is unsafe to spacewalk without a tether as a simple push will send you floating away forever (because there is no friction or air resistance to counteract equal/opposite reactionary force).
(15 votes)
• So, normal force would be the "reaction" of the force of gravity? I don't get it
(11 votes)
• The word "Normal" really just means "perpendicular to a surface". The "normal force", as you are using the term, is referring to the force that a surface, like a table, would exert on an object, like a box, to support its weight (the "weight" is the force an object exerts due to the acceleration of gravity). If the table is level, that force is equal and opposite to the weight of the box, and so is the only "reaction" to gravity.
If the table is not level, the "normal force" will no longer be directly opposite gravity. In a simple static problem, the normal force will be equal to the component of the weight which is perpendicular to the table. The remaining weight will need to be supported by the addition of friction or an applied force.
(2 votes)
• what happens when the constant moving block gets tilted to the left and hits the other block?
will the stationary block go as fast as the other block goes on impact?
(7 votes)
• Let's just take out the tilt and say the block is moving at 5 m/s to the left, on a collision course with the stationary block. Now think of Newton's 3rd law of motion: Every action has an equal and opposite reaction. The force will be divided between the two blocks and they will both move away from each other at 2.5 m/s assuming no energy lost in the collision.
(7 votes)
• The electrostatic force between to molecules makes them repel each other and this force gets stronger the closer they are to each other so i guess this means they never actually come into contact. Does this mean we never actually 'touch' anything or do the particles eventually come into contact?
(6 votes)
• wow; cool question.

My answer is this; 'it depends on how you define touch'. and I encourage you to have a go at producing your own definition
This also raises the question of how BIG the atom is. ie how do you define 'size'?

Remember, you are talking about the repulsion between electrons here right? The outer shells of the molecules fixed in the two materials.
To develop your idea:
if we push two protons together....at what point would they 'touch' in your definition?

Now if we keep pushing the protons until they get very close (about a proton width apart) then another force comes into action; the strong nuclear force and this is a) attractive and b) much stronger than the electrostatic force. So the protons can 'stick' together, forming a larger atom. However, this is unstable and neutrons will also be required to help it all stick together.
(6 votes)
• At , if it's electrostatic repulsion that keeps MOLECULES (not atoms) of matter to disallow compression, then why does every individual molecule not repel all molecules? Also, if we keep two absolutely identical blocks of matter in contact, then why do the molecules in contact not form a bond with each other ? I am completely aware that this question is not directly related to the topic but a little help will do me good.
(2 votes)
• Every molecule is made of combinations of atoms and the atoms are made of combinations of both positive and negative charges. The interaction of these charges is very complex, in part because the electric force varies with the square of distance. So you have to take into account where the charges are located when you try to predict how they interact. There are other factors involved as well but this is a good basic understanding for now.
(3 votes)
• If we call Gravity and Normal force Action-Reaction pair then it would be wrong right? Since gravity is not a force exerted by the earth but the object!
Am I correct?
(2 votes)
• That is wrong. Gravity and normal are not a third law pair because they act on the same object. Third law pairs act on different objects. They also have to be the same "kind" of force: gravity pairs with gravity, electric pairs with electric, etc.
(3 votes)
• so normal force is the force that keeps us from falling through the ground
(2 votes)
• Correct, provided you are standing on a surface capable of providing an equal and opposite reaction force to your weight, without collapsing.
(2 votes)

## Video transcript

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.