Normal Force and Contact Force The force that keeps a block of ice from falling towards the center of the earth
Normal Force and Contact Force
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- Let's say that I have a huge frozen over lake, or maybe it is a big pond.
- So I have a huge surface of ice over here.
- This is my best attempt at drawing a flat surface of ice.
- And I am 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 here.
- And these blocks of ice are identical.
- They are both 5 kilograms. Let me write this down
- or both of their masses, I should say, are are 5 kilograms.
- And the only difference between the two
- is that relative to the pond
- this one is stationary, and this one is moving with a constant velocity.
- A constant velocity in the rightwards direction.
- And lets say that its constant velocity is at 5 meters per second.
- And the whole reason that I made blocks of ice on top of ice,
- is because we are going to assume, at least for the sake of this video,
- that friction is negligible.
- Now what does Newtons first law of motion tell us
- about something that is either not in motion,
- or you can view this as having a constant velocity of zero,
- or something that has a constant velocity.
- Well, Newton's first law says,
- they are going to either keep their constant velocity or stay stationary,
- which is a constant velocity of zero
- unless there is some unbalance,
- unless there is some net force acting on an object.
- So lets just think about it here.
- In either of these situations there must not be any unbalanced force acting on them
- or there must not be any net force.
- But think about it, if we are assuming that these things are on earth,
- there is a net force acting on both of them.
- Both of them are on the surface of the Earth,
- and they both have mass. So there will be the force of gravity acting down on them
- There is going to be the downward force of gravity on both of these blocks of ice.
- And that downward 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 time 5 kilograms.
- This right over here is 9.8 meters per second squared
- So you multiply that times 5, you get 49 kilogram meters 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 net force
- but based on what we have drawn right here, it looks like there is some sort of net force.
- It looks like I have 49 Newtons of force pulling this thing downwards,
- but you say, "No no Sal, obviously this thing won't start accelerating downwards
- because there is ice here. It is resting on a big pool of frozen water."
- 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 from plummeting down to the core of the Earth?
- From essentially free falling or accelerating down to the center of the Earth?
- And you say, "Well, I guess these things would be falling if not for the ice,
- the ice must be providing the counter acting force."
- And you are absolutely correct.
- The ice is providing the counter acting 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 are 49 Newtons downward,
- it is completely netted off by the force of the ice on the block upwards,
- and that will be a force of 49 Newtons upwards, in either case.
- And now hopefully it make sense that Newtons 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 is why they have a constant velocity.
- This guy has a zero velocity in the horizontal direction,
- this guy has a contant velocity in the horizontal direction.
- and neither of them are accelerating in the vertical direction,
- because you have force of ice on the block.
- the ice supporting the block, that is completely contracting gravity.
- And this force, in this example is called the normal force.
- It is 49 Newtons upwards.
- This right here is the normal force
- and we will talk more about normal force in future videos
- the normal force is the force when anything is resting on any surface, that is perpendicular to the surface.
- and it will start to matter a lot when we start talking about friction.
- We will see in future videos that when you have something on an incline,
- lets say I have a block on an incline like this.
- The normal force from the wedge on this block is going to be perpendicular to the surface.
- And if you really think about what is happening here it is fundamentally in electromagnetic force
- because if you really zoom in on the molecules of the ice, or even better, the atoms of the ice.
- What is keeping this top block of ice from falling down,
- is that in order for it to go through, it's molecule would have to compress against
- or get closer to the water molecules or individual atoms in this ice down here.
- And the atoms, let me draw it on an atomic level right here.
- Let me draw one of this guys molecules
- So you have an oxygen with two hydrogens,
- and it forms this big lattice structure. And we can talk more of that in the chemistry play list
- and lets talk about one of this ice's molecules
- So maybe it looks something like this and it has it's two hydrogens
- and so what is keeping these two guys from getting compressed,
- what is 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 as kind of a contact force
- but on a microscopic level or an atomic level, it is really just electromagnetic repulsion at work.
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