1. اصطکاک
جناب آقای دکتر جوادیان

2. What is friction?

3. 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

4. Friction Friction causes objects to slow down. Friction creates heat.
Friction degrades an object’s energy

5. 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

6. Places where friction is good
The soles of shoes
Car tires
The feet of table legs
Rock climbing holds
Brakes
etc

7. 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

8. What causes friction?

9. 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

10. 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

11. 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.

12. Friction is a Force That Resists Motionfs 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.

13. 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

14. 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

15. 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

16. Coefficient of Static FrictionM1
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

17. 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 = ?

18. 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

19. 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

20. 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

21. 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

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

23. Record the Weight (W ) of the Battery

24. 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

25. 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

26. 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

27. What is the normal force?

28. Block at Rest on a Table

29. 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

30. 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

31. .

32. Questions?

33. Solution

34. 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?

35. 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

36. 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.

37. 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

38. 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/(cos *sin300)
T = 180N/( )
T = 180 N/(1.166)
T = 154 N

39. 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?

40. What is rolling friction?

41. Rolling friction:
-nature
-normal force
-diameter

42. Rotational friction

44. 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