If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:9:09

Video transcript

I want to make a quick clarification to the last video and then think about think about what for what's friction up to when the block is actually moving so in the last video we started off with the block being stationary we knew that the parallel component of the force of gravity on that block was 49 Newton's downwards down the slope and when so the block was stationary we said there must be an offsetting force and we said that's the force of friction and it must be 49 Newton's upwards and so they completely net out in that direction now what we said is we're going to keep applying a little bit more a little bit more force until we can budge until we can budge this block to start accelerating downwards and I said I applied I kept I kept applying a little bit more for us a little bit more force until I get to 1 Newton and then the block started to budge so at that point when it started to budge I'm applying this 1 Newton over here right over here there is already 49 Newton's of force or the component of gravity in this direction so combined we're providing 50 Newtons to just start budging it to just overcome the force of friction the one thing I want to clarify here is this this whole time the force of friction was not constant at 49 Newtons when it was when it was when I wasn't messing with this block and the parallel component of the force was 49 Newtons then the force of friction was 49 Newtons when I started to press on it a little bit apply a little bit force maybe I fried a tenth of a Newton on top of that then the force of friction was forty ninth and one tenth Newton because it was still providing enough force so that this block was not moving then maybe I applied half a Newton and so the total force in the downward direction would have been 49 and 1/2 Newtons but if it still was not moving then the force of friction was still completely overcoming it so the force of friction at that point must have been 49 and 1/2 Newtons all the way up to the combined force in the downward Direction being 49 point nine nine nine nine nine nine Newtons and then the force of friction was still forty-nine point nine nine nine nine nine Newtons all the way until I hit 50 Newtons and then the block started to budge which tells us that the force of friction now all of a sudden or at least the force of static friction all of a sudden now couldn't keep up and it started to accelerate downward so I want to show in that static scenario the four of friction changed as I applied as I applied more or less force in this downward direction now with that out of the way let's take a different scenario let's let me just redraw that same block just since all of this since all of this is getting messy so we have the same block we have the same block and as we said in the last video we're now assuming that this is wood on wood so this is the wedge this is the block right over here we know that the component of gravity that is parallel to the plane right there is 49 Newtons we know that this is 49 Newtons we know the component of gravity that is perpendicular to the plane we figured out this two videos ago is 49 square roots of 3 Newtons 49 square roots of 3 Newtons we know that this block is not accelerating in this normal direction so there must be some force counteracting gravity in that direction and that's the normal force of the wedge on the block so that is going in that direction at 49 square roots of 3 Newtons and now instead of assuming that this block is stationary let's assume that it's moving with a constant velocity so now we're dealing with we do that in a different color so now we're dealing with the scenario where the block has a constant velocity constant velocity and for the sake of this for the sake of this video we'll assume that that constant velocity is will assume that constant velocity is downward and so the constant velocity V is equal to I don't know let's say it is 5 meters per second down down the wedge or down the ramp or I guess we could say in the direction that is parallel to the surface of the ramp so it's in this direction right over here so that's the constant velocity so what are all the forces at play and be very careful here there might be a temptation says ok you know there's a net force here we're moving so maybe that's the net force is causing the move but remember remember this is super important if you had this is Newton's first law if you have a net force if you have an unbalanced force it will cause it to accelerate and we are not accelerating here we have a constant velocity we are not accelerating not accelerating here so if you're not accelerating in the in that direction that that means that the force in that direction must be balanced so there must be some force acting acting in the exactly opposite direction that keeps this thing from accelerating downwards and so it must be exactly 49 Newtons in the opposite direction and as you can imagine this is the force of friction this right over here is the force of friction and the difference between this video in the last video is last time friction was static even at 49 Newtons the box was stationary you have to keep nudging it until you get to 50 notes then it started moving here we're just jumping into this picture where we just see a box that's moving down the slope at five meters per second so we don't know how much force it took to overcome static friction but we do know that there is some force of friction that is keeping this box from accelerating that's keeping it at a constant velocity that is completely negating the parallel component of the force of gravity parallel to parallel to the surface of this plane so given this let's calculate another coefficient of friction but this is going to be the coefficient of kinetic friction because now we are moving down the block and I'll do a video on why sometimes the coefficient of static friction can be different than the coefficient of kinetic friction so the coefficient of kinetic friction will write it so this is the Greek letter mu Greek letter mu and we put this little lower kit we put this K here for kinetic or it's kind of say moving friction is going to be equal to the force of friction so it's going to be or I should say the magnitude of the force of friction the magnitude of the force of friction over over the normal force over the normal force overall is to say the magnitude the magnitude of the normal force and you can derive this experimentally one if you just observed this whole thing going on and you the mass of the block so you knew this the component of gravity that's going in this direction if you knew this angle was 30 degrees from the last situation you could figure out this coefficient of kinetic friction and what's cool about this is this is in general going to be true for any two materials that are like this so you know maybe this is a certain type of wood on a certain type of wood or a certain type of sandpaper on a certain type of sandpaper whatever you're talking about and then you can use that to make predictions if the incline was different or if the mass was different or even if you are on a different planet or if someone was pressing down on this block that would change the normal force so given this right here let's figure out for the sake of doing it the coefficient of kinetic friction here coefficient of kinetic friction the force of friction here completely offsetting the parallel force of gravity parallel to the surface is 49 Newtons and the normal force here the force of contact between these two these two things this block in this wedge is 49 square roots of 3 Newtons 49 square roots of 3 Newtons Newtons so we get 1 over the square root of 3 and let me get the calculator out to get an actual number here so we have 1 divided by the square root of 3 which gives us point 5 I'll just round point 5 8 is equal to 0.58 and there's no units here because the units cancel out is the unit list measurement now the interesting thing here is that the coefficient the way I've set up this problem the coefficient of kinetic friction is lower if we assume the same materials then the coefficient of static friction was and for some materials they're usually they might not be that different but for other materials the kinetic friction can be lower than static friction you never see a situation where static friction is lower the coefficient of static friction at least that I know of is lower than kinetic friction but you do see situations where the coefficient of kinetic friction is lower is lower than the coefficient of static friction that once once something is moving once something is moving for some reason and we'll theorize why that might be for some reason friction is a little less potent than when something is stationary so we can say this generally that the coefficient of kinetic friction is less than or equal to the coefficient the coefficient of static-friction it's a little bit easier or friction provides a little less or or less than or equal to the force once something's moving then when something is stationary so I'll think about that a little bit deeper in the next video