Survey

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Survey

Transcript

اصطکاک جناب آقای دکتر جوادیان What is friction? What’s Stopping You? Mu F n Friction Hard to Live With It, Can’t Live Without It Coefficient of Friction Friction Friction causes objects to slow down. Friction creates heat. Friction degrades an object’s energy Friction Opposes relative motion of two surfaces Consists of a matched pair of forces: Obj1 pushes Obj2 while Obj2 pushes Obj1 Equal magnitudes, opposite directions Comes in two types: static and kinetic Places where friction is good The soles of shoes Car tires The feet of table legs Rock climbing holds Brakes etc Places where friction is bad In engines In transmissions On the bottoms of skis and snowboards (To a point) When you are trying to move a box by pushing it along the floor etc What causes friction? Source of Friction At the microscopic level even the smoothest of surfaces is dotted with little “mountain peaks”. The tips of the peaks are the only parts that touch the other material. Only a very small portion of the apparent surface area is in contact with the other surface This causes extremely high pressures to form on the parts that touch. This causes the two surfaces to become “welded” almost at the points of contact Picture of extremely smooth mica with a scanned probe microscope Consider 2 Types of Friction Fs Force of Static Friction This value represents the relative force necessary to make an object move Fk Force of Kinetic Friction This value represents the relative force necessary to keep an object moving at a constant rate Types of Friction Static Friction Acts to prevent objects from starting to slide Forces can vary from zero to an upper limit Kinetic friction -Sliding Friction Acts to stop objects that are already sliding -Rolling friction Friction is a Force That N Resists Motion F fs M1 Surfaces in Contact Spring Scale W The Pink Block M1 Will not Move Until the Force F (Pull on the scale ) Exceeds the Force of Static Friction fs. Increases linearly For a given pair of surfaces, the ratio of Frictional force to Applied force is a constant. Frictional Force Static Friction Applied Force, N Fs Fs Fk Max Fk Fs Fk Static Region Kinetic Region Force Causing the Object to Move Why? -The slightly larger value for static friction results from irregularities and contaminants on the surfaces and is less accurate in general than the coefficient of kinetic friction -inertia F s W Coefficient of Static Friction N F fs M1 Surfaces in Contact Spring Scale W s F W = Coefficient of Friction = Force Required to Cause Motion = Weight of Object Example of Static Friction What is the coefficient of static friction between a tabletop and a 2 kg block of wood if a 2 N force is required to start the block moving? Identify knowns and unknown: m = 2 kg, applied force = 2 N, v = 0, μs = ? Appropriate equation: f = μ N. What is N? On a level surface the normal force upward is equal to the weight of the object downward, i.e. N = W = mg. So, f = μ mg or μ = f/mg = 2 N/(2 kg *9.8m/s2) μ = 0.102 Static & Kinetic Friction Coefficients Material Coefficient of Coefficient of Static Friction S Kinetic Friction S 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 0.01 0.003 Humans Application Analysis Sand is often placed on an icy road because the sand: Decreases the coefficient of friction between the tires of a car and the road Increases the coefficient of friction between the tires of a car and the road Decrease the gravitational force on a car Increases the normal force of a car on the road .1 .2 .3 .4 Using the Gears-IDS Battery to Calculate The s Static F s W Coefficient of Friction Record the Maximum Force (F) (Before the Battery Begins to Move) Maximum Force F = 110 g F s W Record the Weight (W ) of the Battery F s W 580 g The Coefficient of Static Friction Between the Wood Desktop and the Plastic Battery is Described Algebraically: F s W s = .190 = 110 g = 580 g The Coefficient of Kinetic Friction Can be Found Using the Same Technique F k W Record the Force Required to Move the Battery at a Constant Rate r r f f t The frictional force depends only on 1-the type of surfaces 2-how hard the surfaces are pressed together What is the normal force? Block at Rest on a Table Normal Force From Newton’s third law we know that if gravity or some other force pushes an object (like a block) into a second object (like a table) that second object will be exerting an equal force back on the first. Normal force is the force the table exerts back on the block Normal force is always exerted perpendicular to the surface Friction Force is always parallel to the surface So if the table is horizontal and gravity is the only force on the block Fn = -Fg Normal force on a hill Normal force is exerted perpendicular to the surface in accordance with Newton’s Third law No unbalanced force so the block is stationary or at least not accelerating . Questions? Solution Fn Fg cos( 20) Fn m g cos( 20) Ff k Fn Fg ( paralleltosurface) m g sin( 20) Ff Fg ( paralleltosurface) m g cos( 20) k m g sin( 20) cos( 20) k sin( 20) sin( 20) k cos( 20) k tan( 20) Frictional force and normal force. Frictional force is proportional to the normal force, f α N. On a level surface N = W, but what if someone is lifting up on the object? Won’t that reduce the normal force? Free Body Diagram N + T = W, so Normal, N N = W – T. If T pulls at some angle, then just decompose into components. Lift from person,T Weight, W Example: A person pulls on a 300 N crate with a rope that makes a 300 angle to the ground. If the coefficient of static friction is 0.6, how much tension must the person exert to get the crate moving? Knowns: W = 300 N, θ = 30o, μ = 0.6. Unknown: T = ? Equations: f = μN. All forces balance when at rest. Horizontal Tension = T cos θ Vertical Tension = T sin θ Net vertical force = N + Tsin θ – W = 0, so N = W - Tsin θ. Net horizontal force = Tcos θ – f = 0 Tcos θ = f = μN = μ(W - Tsin θ) , so Tcos θ = μ(W - Tsin θ) Tcos θ + μ Tsin θ = μW T(cos θ + μ sin θ) = μW T = μW/(cos θ + μ sin θ) T= 0.6*300N/(cos300 + .6*sin300) T = 180N/(0.866+.3) T = 180 N/(1.166) T = 154 N Experiment: Does the friction force depend on surface area? We stick a 1kg mass on a piece of on top of the clean surface and, using a Newton meter, pull the weight across the table with the string provided at constant speed. We put the same weight on a different piece of plexiglass with drastically different surface area and repeat What do you notice? What is rolling friction? Rolling friction: -nature -normal force -diameter Rotational friction منابع دكتر نمازي زاده بیومكانیك فنون ورزشيبیومكانیك كاربردي ورزش كماسيدانش حركات انسان در ورزش فرزام معلمیان-Mechanical analysis of human motion