اصطکاک جناب آقای دکتر جوادیان

What is friction?

Hard to Live With It, Can’t Live Without It Coefficient of Friction What’s Stopping You? Fn Mu Friction Hard to Live With It, Can’t Live Without It Friction is a double edge sword. It is the same force that allows a wheeled mechanism to start, and stop. Good designers maximise or minimize frictional forces where needed. This requires a thorough understanding and experiential data base of frictional information. Information that comes through reading, watching and doing. 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 Demonstration: Slide Box Across Table and Watch It Stop

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 Force of Static Friction This value represents the relative force necessary to make an object move Force of Kinetic Friction There are other types of friction including fluid (air and liquids) and deformation friction. These are the only This value represents the relative force necessary to keep an object moving at a constant rate

Types of Friction Kinetic friction Static Friction -Rolling 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 Demonstration: Show that it's Hard to Start a Box Sliding. Demonstration: Show that You Must Push a Box to Keep it Sliding.

Friction is a Force That Resists Motion fs M1 Surfaces in Contact Spring Scale W Which Way is the pink block being pulled? What direction is the force of Static Friction acting in? The Pink Block M1 Will not Move Until the Force F (Pull on the scale ) Exceeds the Force of Static Friction fs.

Static Friction Increases linearly For a given pair of surfaces, the ratio of Frictional force to Applied force is a constant. Frictional Force Applied Force, N

Frictional Force Resisting Motion Max Frictional Force Resisting Motion Static Region Kinetic Region The Force of Static Friction is greater than the Force of Kinetic friction. The implication is clear: It is harder to start an object moving, than it is to keep it moving. Put another way, it takes more force to start an object moving than to keep that same object moving given that all other variables (materials in contact) remain the same. 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

Coefficient of Static Friction M1 Surfaces in Contact Spring Scale W Experimenting with different materials and contact forces (weights) will yield different values for the coefficient of friction. The implication of this is clear, different materials grip or slide better or worse with respect to each other, and to increase traction on hard surfaces it is best to add weight directly over the driving wheels of a vehicle. = 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 Static Friction S Coefficient of 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 Humans 0.01 0.003

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

Using the Gears-IDS Battery to Calculate The Static This is an experiment that is easy to do with the GEARS-IDS battery and a 1 kg spring scale. Remember the static coefficient of friction is found using the maximum force recorded before the battery begins to move. Coefficient of Friction

Record the Maximum Force (F) (Before the Battery Begins to Move) Maximum Force F = 110 g

Record the Weight (W ) of the Battery

The Coefficient of Static Friction Between the Wood Desktop and the Plastic Battery is Described Algebraically: = 110 g = 580 g This experiment yields a value for the coefficient of friction between the hard plastic of the battery and wooden desktop. Try this experiment with a variety of other materials. = .190

Record the Force Required to Move the Battery at a Constant Rate The Coefficient of Kinetic Friction Can be Found Using the Same Technique Record the Force Required to Move the Battery at a Constant Rate

The frictional force depends only on 1-the type of surfaces how hard the surfaces are pressed together the type of surfaces The frictional force depends only on:- 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

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Questions?

Solution

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 Lift from person,T Normal,N N + T = W, so N = W – T. If T pulls at some angle, then just decompose into components. 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 -بيومكانيك فنون ورزشي دكتر نمازي زاده -بيومكانيك فنون ورزشي دكتر نمازي زاده -بيومكانيك كاربردي ورزش كماسي -دانش حركات انسان در ورزش فرزام معلميان -Mechanical analysis of human motion