Frictional Forces Static and Kinetic

It's professor date, let's learn about friction. In examining Newton's laws of motion, we have to understand that the kinds of motion we observe on earth don't always appear to obey these laws, because there are extraneous variables acting upon earthbound objects, and most of these involve some kind of frictional force. Friction is an important concept to understand so let's go over it in some detail. Whenever an object is in motion along surface the surface exerts force upon the object. One component of this force is the normal force, which is perpendicular to the surface. There is also component of this force that is parallel to the surface, and this is called the frictional force, or simply friction. This is the force that will resist the motion of the object along the surface. Every surface has some frictional coefficient that will vary depending on its composition. To see this demonstrated, try to push small block across some ice and then try to push it across some sandpaper. These materials differ in their resistance to motion for reasons that relate to their composition. The smoother surface is the less friction it will provide, but even surfaces that appear perfectly smooth will have imperfections on the microscopic level that provide some friction. As the object moves across the surface there are select points of contact where atoms in the objects interact with atoms in the surface, and this attractive interaction hinders motion to some measurable degree no matter how small. Let's define two main types of friction: static and kinetic. Static friction is the friction that resists the initiation of motion. If you place block on table and try to very lightly push it into motion it will first resist that motion because of the frictional force operating in the direction opposite the applied force of your push. You can push harder and it will still remain still because the frictional force will always precisely oppose the applied force. Static friction will increase until the magnitude of the applied force exceeds the maximum static frictional force the table can exert, then the force of the push can no longer be cancelled out and the block will begin to accelerate. This frictional force is proportional to the normal force so the heavier the object, the greater the normal force, and the greater the frictional force. This is because as the weight of the object increases, the harder it presses down on the surface which will increase the number of contact points between the object and the surface. The static frictional force will be anywhere from zero to the maximum possible value, depending on the forces operating on the object, since the static frictional force will be equal to the applied force until the maximum is reached. The magnitude of this maximum can be calculated this way: max is equal to the coefficient of static friction times the magnitude of the normal force. This coefficient, represented by the Greek letter mu, is unitless and unique to the surface in question, and we have tabulated these coefficients for variety of common surfaces like glass, steel, wood, and rubber, and the various combinations thereof. As we said, once the applied force exceeds the maximum static friction, the object will begin to move. Bear in mind that this equation involves scalar quantities, not vectors, and therefore implies nothing about direction. As we said, static friction opposes the initiation of motion, but once an object is in motion it is now moving against kinetic friction. This is the force that opposes relative sliding motion. Kinetic friction is always lesser than static friction, which you will notice if you try to push any object across the surface, like heavy box across the floor. It will be more difficult to get the box going than it is to keep it moving once you've started. There are coefficients of kinetic friction as well, and these will be different from the coefficient of static friction for the same materials. These values allow us to calculate the magnitude of the kinetic frictional force acting on sliding object. Friction isn't always nuisance, it can also be used to our advantage. When we walk, the static friction between our feet and the ground allows us to propel ourselves forward, rather than our feet simply sliding back. Car tires take advantage of friction to move the car forward, and they are designed with grooves to divert water away so that it does not interfere with the contact between the tire and the ground. This allows it to maintain traction rather than skidding. We should note that air resistance is another type of fluid friction. When car or plane moves through the atmosphere, the particles in the air hinder its motion, offering some kinetic friction. This is true of motion through any fluid in way that depends on the viscosity of the fluid, which represents the fluid's resistance to flow. So by now we are familiar with few of the vectors we will commonly use in physics. An object at rest on flat surface on earth will experience downward force due to its weight, as well as an upward normal force that is equal in magnitude. If some horizontal force is applied there will also be an opposing frictional force. If the applied force is less than the maximum static frictional force of that surface, the horizontal vectors will cancel each other out, just like the vertical ones and the object will remain at rest. If the applied force exceeds the maximum friction, the object will accelerate in the direction of the push and the kinetic frictional force will oppose its forward motion. So we can expect to see these four vectors in lots of the free body diagrams from this point forward. common example is the inclined plane. In this scenario, we can examine block sliding down ramp. Gravity, represented by mg, will pull straight down, and this vector can be divided into components that are perpendicular and parallel to the incline. Those will be mg cosine theta and mg sine theta. The force opposite the perpendicular component will be the normal force, equal in magnitude and opposite in direction. We can then include vector for the force of friction, which opposes the other component of gravity. If we calculate the net force acting on the block this will allow us to predict the acceleration on the block as it slides down the incline, and since the two perpendicular forces cancel each other out, we just add the parallel ones together to find the net force. To try this more quantitatively, let's check comprehension. Thanks for watching, guys. Subscribe to my channel for more tutorials, support me on patreon so can keep making content, and as always feel free to email me: