KINGDOM OF SAUDI ARABIA MINISTRY OF EDUCATION JAZAN UNIVERSITY COLLEGE OF ENGINEERING MECHANICAL DEPARTMENT Friction Section: 593 NO. Name ID 1 Ahmed Yahia Ahmadi 202100312 2 Mohammed Hassan Shykhin 201804055 3 Majed Ahmed Humaidi 201910764 Table of Contents Introduction ........................................................................................................... 3 Static and Kinetic Friction .................................................................................... 3 Static Friction Prevents Sliding ............................................................................ 5 Kinetic Friction Acts Opposite the Object’s Sliding Direction ............................ 6 How to Determine Magnitude of Friction Force .................................................. 7 Static Friction ..................................................................................................... 8 Kinetic Friction .................................................................................................. 8 Reference............................................................................................................... 9 Page 2 of 9 Introduction When a body is in motion, it has resistance because the body interacts with its surroundings. This resistance is a force of friction. Friction opposes relative motion between systems in contact but also allows us to move, a concept that becomes obvious if you try to walk on ice. Friction is a common yet complex force, and its behavior is still not completely understood. Still, it is possible to understand the circumstances in which it behaves. Static and Kinetic Friction The basic definition of friction is relatively simple to state. Friction is a force that opposes relative motion between systems in contact. There are several forms of friction. One of the simpler characteristics of sliding friction is that it is parallel to the contact surfaces between systems and is always in a direction that opposes motion or attempted motion of the systems relative to each other. If two systems are in contact and moving relative to one another, then the friction between them is called kinetic friction. For example, friction slows a hockey puck sliding on ice. When objects are stationary, static friction can act between them; the static friction is usually greater than the kinetic friction between two objects. If two systems are in contact and stationary relative to one another, then the friction between them is called static friction. If two systems are in contact and moving relative to one another, then the friction between them is called kinetic friction. Page 3 of 9 Imagine, for example, trying to slide a heavy crate across a concrete floor—you might push very hard on the crate and not move it at all. This means that the static friction responds to what you do—it increases to be equal to and in the opposite direction of your push. If you finally push hard enough, the crate seems to slip suddenly and starts to move. Now static friction gives way to kinetic friction. Once in motion, it is easier to keep it in motion than it was to get it started, indicating that the kinetic frictional force is less than the static frictional force. If you add mass to the crate, say by placing a box on top of it, you need to push even harder to get it started and also to keep it moving. Furthermore, if you oiled the concrete you would find it easier to get the crate started and keep it going (as you might expect). Figure 1 is a crude pictorial representation of how friction occurs at the interface between two objects. Close-up inspection of these surfaces shows them to be rough. Thus, when you push to get an object moving (in this case, a crate), you must raise the object until it can skip along with just the tips of the surface hitting, breaking off the points, or both. A considerable force can be resisted by friction with no apparent motion. The harder the surfaces are pushed together (such as if another box is placed on the crate), the more force is needed to move them. Part of the friction is due to adhesive forces between the surface molecules of the two objects, which explains the dependence of friction on the nature of the substances. For example, rubber-soled shoes slip less than those with leather Page 4 of 9 soles. Adhesion varies with substances in contact and is a complicated aspect of surface physics. Once an object is moving, there are fewer points of contact (fewer molecules adhering), so less force is required to keep the object moving. At small but nonzero speeds, friction is nearly independent of speed. Figure 1: Frictional forces, such as πβ , always oppose motion or attempted motion between objects in contact. Static Friction Prevents Sliding Static friction is the force holding an object in place on an incline, such as the cheese in Figure 2. The friction force points against the direction that the object would slide without friction. Static friction keeps gravity from pulling the cheese down the incline. Page 5 of 9 Figure 2: Cheese resting on an incline because static friction holds it in place. Kinetic Friction Acts Opposite the Object’s Sliding Direction Kinetic friction always opposes the object’s sliding direction. As seen in Figure 3 below, if an object is moving up an incline, the force of friction πβπ points down the incline. If the object is moving down an incline, the πβπ points up the incline. Figure 3. Kinetic friction πβπ direction for a block of cheese sliding on an incline with velocity v. Page 6 of 9 How to Determine Magnitude of Friction Force Friction is determined by the two surfaces in contact, and how tightly the two surfaces are pushed together (ππππππ πβππππ πβπ ) • Coefficient of friction (π): this describes the roughness between two surfaces. A high coefficient of friction produces more friction. • Normal forceππ΅ ): squeezing surfaces together more tightly increases the friction. This is one reason why heavy objects are harder to slide across the ground. These factors of friction are reflected in its generalized equation: Friction for a given object isn’t always just one value though, it can change. Let’s learn how by imagining a person pushing a refrigerator as shown in Figure 4 below. When we push an initially resting refrigerator with an external applied πΉπππ start it is moving, both static and kinetic friction push back on the object at different times. Figure 4: Horizontal forces acting on a refrigerator being pushed with increasing applied force Page 7 of 9 Static Friction Initially static friction πΉπ,π prevents the refrigerator in Figure 4 from moving. But as we continue applying more and more force πΉπππ , eventually the refrigerator begins sliding. This is because static friction has a maximum value that it can reach before it lets an object begin sliding. As long as |πΉπππ | ≤ |πΉπ,π πππ₯ | then the refrigerator remains at rest. This is described by the equation below: Kinetic Friction Once an object begins to slide, kinetic friction πΉπ,π acts with a constant amount to resist the sliding motion: Page 8 of 9 Reference Friction. bartleby. (2021, August 23). Retrieved from https://www.bartleby.com/subject/science/physics/concepts/static-and-kineticfriction Khan Academy. (n.d.). Friction Review (article) | friction. Khan Academy. Retrieved from https://www.khanacademy.org/science/high-school-physics/two-dimensionalmotion-2/friction/a/friction-ap-physics1#:~:text=Static%20friction%20magnitude%20is%20directly,roughness%20betwee n%20the%20sliding%20surfaces.&text=The%20coefficient%20of%20friction %20is,by%20the%20normal%20force%20magnitude. 6.2 friction - university physics volume 1. OpenStax. (n.d.). Retrieved from https://openstax.org/books/university-physics-volume-1/pages/6-2-friction Page 9 of 9
