welcome to lecture line here's our first video of a series on friction in mechanical engineering now we want to start out with some basic concepts and we have four situations here we have a block sitting on a horizontal surface in all four cases but here there's no horizontal force acting on the block there's a horizontal force acting on the block but it's not sufficient to overcome the friction force between the surface and the block and therefore it's not moving here the force has increased and I guess I should put an F there for force the force has increased but it's exactly equal to the friction force and notice here that the friction force is caused by the static friction between the block and the surface and when they're equal the block will not be moving it will be on the verge of moving any additional force will cause the block to overcome the force of friction and here in the final case we have a situation where the force is less than it is here but the block is already moving at a constant speed which means that the force applied on the block horizontally here equals the maximum friction force that can be mustered up of the friction between the block and the surface because in this case it's kinetic friction and here there's still static friction notice that here we have the weight of the object due to gravity pushing down on the surface the surface pushing back so the normal force of the surface pushing back equals the reaction force so we'll be talking about both the normal and the reaction force notice if there's a horizontal force here smaller than the maximum force you can have between the two surfaces maximum friction force notice that the force Hill will be counterbalanced by the friction force pushing back on the other side therefore the block is not moving we still have the normal force which is equal to the weight of the block this way but not notice that the reaction force have now turned at an angle so we have a small angle here between the vertical and the reaction force notice that the two components of the reaction force are the friction force here by the way this friction force is equal to the force here pushing against the block and the normal force which is equal to the weight of the block over there notice we are now able to draw a triangular setup of a forces involved or acting on the block we have the weight of the block we have the force pushing to the right and then with the reaction force pushing back this way everything is static therefore the sum of the forces add up to zero on the third case we increase the force here so that it now matches the maximum force friction force that we can have here so again the block is not moving notice that the drawing of the forces now looks similar to this but the angle is now larger it's the maximum angle we can have and we can actually find the value of this angle by realizing that the tangent of the angle is equal to the opposite side divided by the adjacent side the opposite side is the friction force which is equal to the force here pushing against the block in the horizontal direction and the adjacent side is simply equal to the weight or the normal force pushing back so they're going to be equal to one another noticing that the friction force the definition is the normal force times the coefficient of friction in this case a static coefficient of friction the normal forces cancel out and so we can say that the tangent of this angle in this case where the block is on the verge of moving is going to be equal to the coefficient of static friction which allows us to calculate the angle by taking the inverse tangent of the coefficient of static friction the last case here we still have a static situation there will not be an acceleration acceleration is zero because the force applied to the Block in the horizontal Direction equals the maximum friction force between the block and the surface in this case the maximum friction force is caused by the kinetic coefficient of friction between the two surfaces because the block is indeed moving notice the angle has now become smaller we can still say that the tangent of that angle is equal to the opposite side divided by the adjacent side which is equal to the friction force divided by the normal force notice that this component here the horizontal component of the reaction force is the friction force which again is going to be equal to the force applied on the block and we divide that by the adjacent side which is the weight which is equal to the normal force pushing back against the surface the normal forces cancel out and now we can say that the tangent of theta equals the kinetic coefficient of friction because the block is moving and so we can solve for the angle in the same way that we did over here so those are the basic concept this concept here is very important when we're dealing with friction in static and non-static situations and we'll see some examples of that later also notice that from the verge of moving or if we're moving at a steady constant velocity when these two forces recal we can actually calculate those angles that the reaction force makes with the vertical and that's the beginning of a section on friction in mechanical engineering