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# Normal force and contact force

## 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 five kilograms they are both five kilograms let me write this down so they're both they are both five kilograms or they're both of their masses I should say are five 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 this in the rightwards direction and let's say that it's constant velocity is at five meters per second five meters per second and the whole reason why I made blocks of ice on top of ice is 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 zero or something that has a constant velocity well Newton's first law says well look they're going to be at they're going to they're going to 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 let's 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 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's going to be the 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 of near the surface of the earth times which is a vector times the mass of the object so times five kilograms five kilograms this right over here is 9.8 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 Newton's first law says an object at rest will stay at rest or an object in motion will stay in motion unless there are some unbalanced or unless there's some net force but basically what we've drawn right here it looks like it looks like there's some type of a net force it looks like I have 49 Newton's of force pulling this thing downwards but you say no no no Sal it you know obviously obviously this thing won't start accelerating downwards because there's ice here it's resting its resting on a big on a big pool of frozen water and so my answer to you is well if that's your answer then what is the what is the resulting force that cancels out 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 falling from essentially get going into freefall or accelerating towards the center of the earth and you say well I guess if these things are if 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 the if the force of gravity on each of these blocks of ice are 49 Newtons downward 49 Newtons downward is completely netted off by the force of the ice on the block upwards and that will be a force of 49 Newton's 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 they have a constant velocity this guy has a zero 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 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 Newton's 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 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 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 this top block of ice from falling down is that in order for it to go through these the it's it's 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 let me draw one of this guys let me draw one of this guy's molecules so you have you have an oxygen with two hydrogen's you have an oxygen with two hydrogen's and it forms big lattice structure and we can talk about more of that in the chemistry in the chemistry playlist and let's talk about this ice is one of these molecules so maybe it's maybe it's down maybe it looks something like this and it has its two hydrogen's and so what's keeping these guys from getting compressed what's keeping this 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 on an atomic level it's really just electromagnetic repulsion at work