Lecture 4 Friction Part B PDF

Static Friction The Force of Static Friction keeps a stationary object at FN rest! F fs f s  FN   s Fg  s  coefficien t of static frict...

Static Friction The Force of Static Friction keeps a stationary object at FN rest! F fs f s  FN   s Fg  s  coefficien t of static friction  Kinetic Friction Once the Force of Static Friction is overcome, the Force of Kinetic FN Motion Friction is what slows down a F moving object! fk Fg f k  FN   k  k  coefficien t of kinetic friction  Types of Friction To initiate motion of the box the man must overcome the Force of Static Friction Upon sliding, the baseball player will come to a complete stop due to the Force of I better be safe Kinetic Friction Ump!!  Static & Kinetic Friction Coefficients Material Coefficient of Static Coefficient of Kinetic Friction S Friction k Rubber on Glass 2.0+ 2.0 Rubber on Concrete 1.0 0.8 Steel on Steel 0.74 0.57 Wood on Wood 0.25 – 0.5 0.2 Metal on Metal 0.15 0.06 Ice on Ice 0.1 0.03 Synovial Joints in Humans 0.01 0.003 Static VS. Kinetic Friction  f s   s FN static frictional force “Mu” coefficient of Normal force static friction f s (max)  s FN maximum force of fs max is the force you must exceed to static friction move the object.  f k   k FN Kinetic frictional “Mu” coefficient of Normal force force Kinetic friction Once object moves.  Use k  fk. becomes constant  There is no “maximum fk.”  The coefficient of static friction is f s  FN f N k   where us= coefficient FN N FN= normal force f = friction force The coefficient of friction is dimensionless – no units. The greater the us the more force needed to move the object. The frictional force increases until it reaches a maximum (in this case, 2.5N) If we push harder still, (say, 2.6N), the block will move.  Forces Link Friction On the verge of slipping F f Sliding Maximum Static Friction Sliding (Kinetic) Friction Friction, f Applied Force, FA  Static friction, fs  Kinetic friction, fk The object begin to move if the applied force is larger than the max static friction. Here the kinetic friction remains constant no matter how large is the applied force. Here, fl stands for the max static friction and fk is the kinetic friction. It is slightly less than the max static friction. Approximate Coefficients of Friction  SUMMARY Friction force, f, is proportional to normal force, FN. fs  sn f k  k n us and uk are coefficients of friction Frictional force is opposite to the motion Values of us and uk depend on surfaces us and uk don’t depend on surface area us and uk don’t depend on the velocity Friction is due to the surfaces interacting with each other on the microscopic level (sliding over bumps, chemical bonds) You push a giant barrel of monkeys placed on a table with a force of 63 N. If k = 0.35 and s = 0.58, when will the barrel have moved 15 m? 14.7 kg 1. A boy slides over a wet surface rapidly after missing his steps and pulls both his elbow and hip joints accidentally during the slide. If the frictional force opposing the slide is 1N, find the weight of the boy (mass). 2. If an aluminium rod is pulled over a flat iron surface and a normal force of 50 N acts on it. Find the frictional force that will act in the opposite direction A crate of mass 20 kg is sliding across a wooden floor. k between the crate and the floor is 0.3. Determine the strength of the net force acting on the crate. If the crate is being pulled by a force of 90 N (parallel to the floor), find the acceleration of the crate. FN.. fk F Fg  Fluid Friction The force that tries to slow objects down when they move through a liquid or a gas. It's also known as "drag", or "air resistance". All gases and liquids are fluids. Fluid Friction increases as the speed of the object increases. An airplane and a swimmer both experience fluid friction.  Rolling Friction Rolling friction is friction that occurs between surfaces in motion that are rolling in which one of the surfaces is a wheel, roller, or ball. Examples: – Riding a bike – tires and ground – Bowling – ball and lane Rolling friction is a force resisting the motion which takes place when a ball, tire or wheel surface rolls on a counter surface. Unlike the case of sliding friction, a formula for the force of rolling friction is not easily derivable. This is due to the fact that there are many physical mechanisms leading to rolling friction, including plastic deformation, elastic hysteresis and adhesion hysteresis. However, according to the illustration in Fig. 3.8, the rolling friction formula can be empirically expressed as  Ways to Reduce Friction Smooth the surface – less hills and valleys and there are not as deep. Replace sliding with rolling (Use Wheels) Add a lubricant like oil, wax, or grease – Fills in the hills and valleys. Less force pushing together – The hills and valleys are not pushed together as hard so they have less contact.  Ways to Increase Friction Rough the surface – more hills and valleys and there are not are deeper. Replace rolling with sliding More force pushing together – The hills and valleys are pushed together harder so the hills and valleys are in closer contact.  Forces: Friction Friction is both harmful and helpful. some examples of friction being harmful and friction being helpful. Forces: Friction Forces: Friction Is it beneficial to reduce or increase friction? Why or Why Not?  Measurement of Friction Measurement techniques involve either the direct measurement of forces that resist relative motion between two or more bodies of matter or the indirect measurement of the effects of those forces. Some tribometers are designed to measure the force required to initiate relative motion (static friction force), and some are intended to measure the resisting force on objects that are already in motion (kinetic friction force). Many friction measurement systems are designed to simulate specific applications, however, other measurement systems are designed to isolate and study frictional effects under highly controlled laboratory conditions that do not simulate any particular engineering application. As a result, there are literally hundreds, if not thousands of devices designed to quantify frictional resistance. A. Sliding Friction Measurement 1. The inclined plane method, ASTM D4521. This simple method to measure friction force (Ff ) is based on a static balance of forces that requires only the angle (θ) of tilt to be measured at the instant when relative motion begins. According to Fig. 3.18, the static friction coefficient (μs) is given by: μs = tan θ  2. The horizontal plane method, ASTM D4521. This method is directly utilized to measure the force required to begin motion and to continue relative motion, i.e., to measure static and kinetic friction forces between a specimen and counterface, as shown in Fig. 3.19. : Some of the earliest measurements of the coefficient of friction were done by an arrangement of pulleys and weights as shown in Figure. Weight W is applied until sliding begins and one obtains the static coefficient of friction with μs = Ws/N. If the kinetic coefficient of friction μk is desired, a weight is applied to the string, and the slider is moved manually and released. If sliding ceases, more weight is applied to the string for a new trial until sustained sliding of uniform velocity is observed. In this case, the final weight Wk is used to obtain μk = Wk/N. 3. Indirect friction measurements. Friction, both static and kinetic, can be measured indirectly by monitoring parameters, such as the current drawn by a motor that moves one of the sliding components against another 4. Friction measurement using torque data According to Fig. 3.20, the friction force (Ff) that produces a torque (T) acts at a fixed radius from the center of rotation. Torque is the product of friction force times the radius from the center of rotation, therefore: T = Ff r (3.17) and the friction coefficient (μ), in terms of the measured torque, is μ = T/(r P) (3.18) where (P) is the normal force on the block. B. Rolling Friction Measurement Rolling friction, or rolling resistance, is the positive force resisting the motion of a rolling body (ball, tire or wheel) on a surface, as illustrated in Fig. 3.23. Rolling resistance is often expressed as a coefficient times the normal force. This coefficient of rolling resistance is, generally, much smaller than the coefficient of sliding friction. The coefficient of rolling friction (CRF) is defined by the following equation Fr = Cr N (3.19) where, Fr is the rolling friction force. N is the normal force, the force perpendicular to the surface on which the wheel is rolling. Cr is the dimensionless CRF. The coefficient of rolling resistance for a slow rigid wheel on a perfectly elastic surface, not adjusted for velocity, can be calculated by: Cr= √z /d (3.20) where, z is the sinkage depth, as shown in Fig. 3.23. d is the diameter of the rigid wheel. Angle made by the resultant of normal reaction and limiting friction with the normal reaction is called angle of friction Worked Example 1. A force of 18 N acts on a particle, of mass 7.5 kg, at an angle of 30◦ above the horizontal. The particle is on a rough horizontal plane. Given that the particle is on the point of slipping, what is the coefficient of friction, between the particle and the plane?