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Presentation on theme: "BHORANIYA AHEMAD ABBAS A."— Presentation transcript:


2 What is Friction Friction is a force
A frictional force can exist when two substances contact each other. The molecules of each surface interact according to Newton’s Laws of Motion. Friction always opposes motion, i.e., it is opposite to the direction of velocity. If there is no motion, then friction opposes the sum of all the other forces which are parallel to the surfaces in contact.

3 Friction Force When an object is in contact with a surface there is a force acting on that object. The component of this force that is parallel to the surface is called the friction force. This resistive force is exerted on a moving object due to viscosity or other types of frictional property of the medium in or surface on which the object moves. Always opposite to the movement!! 3

4 Contact Force Force that occurs between objects that are in contact with each other. Contact forces can be resolved into components that are perpendicular and parallel to the surfaces in contact. The perpendicular component is called the normal force. The parallel component is called friction.

5 Contact Force in Running
Resultant force on runner runner’s push Normal Force Friction Force During the push off phase in running, the normal force acts upward on the runner, while the friction force acts forward on the runner. The friction force is the only force capable of moving the runner horizontally down the track. The normal force can only accelerate the runner upwards.

6 Friction and the Normal Force
The maximum frictional force is proportional to the normal contact force. An increase in the normal force results in an increase in the maximum friction. This is because the molecules on the two surfaces are pushed together more, thus increasing their interactions.

7 “Normal” Forces and Frictional Forces
4/18/2008 “Normal” Forces and Frictional Forces “Normal” means perpendicular Reaction Force From Ramp Normal Force Friction Decompose Vector Weight of block Weight of block Friction Force = Normal Force  (coefficient of friction) Ffriction = Fnormal Lecture 8

8 Friction and Surface Area
Friction is not affected by the size of the surface area in contact. If the normal force remains constant, but the contacting surface area is increased, then the normal force is spread out over more molecules, thus the force on each molecule is reduced. Amontons (1699) What about race car tires?

9 Calculating Friction Ff_max = FN
Ff_max is the maximum force of friction  (Mu) is the coefficient of friction FN is the normal force Friction can range in value from -Ff_max to +Ff_max  depends on the types of surfaces that are interacting. It would be low for rubber on ice, but high for rubber on asphalt. It also depends on whether the surfaces are moving relative to each other ( static or dynamic ) 4/15/2017 Dr. Sasho MacKenzie - HK 376

10 > Types of Friction Dry Friction Fluid Friction Static Friction
Occurs with fluids,or lubricated surfaces Dry Friction Occurs between the non-lubricated surfaces of solid objects Static Friction When dry friction acts between two surfaces that are not moving relative to each other Dynamic Friction When dry friction acts between two surfaces that are moving relative to each other > 4/15/2017 Dr. Sasho MacKenzie - HK 376

11 Static Friction the force exerted on a stationary object by a surface that prevents the object from starting to move (Fs) the object remains at rest because the static friction is equal in magnitude and opposite in direction to the applied force for the object to move, the applied force must overcome the maximum amount of static friction (Fsmax)

12 The Force of Static Friction keeps a stationary object at rest!
FN F fs Fg

13 Types of Kinetic Friction
sliding friction : an object is scraping or sliding across a surface rolling friction : an object rolls across a surface fluid friction or air resistance : an object travels through water or air and experiences drag

14 Kinetic Friction Once the Force of Static Friction is overcome, the Force of Kinetic Friction is what slows down a moving object! Motion FN F fk Fg

15 Kinetic or Dynamic Friction
the force exerted on a moving object by a surface (Fk) acts in the opposite direction of the motion if applied force is equal in magnitude to the force of friction, the object will move with a constant velocity if the applied force increases, the object will accelerate

16 “Fluid” Friction This type of friction is what happens with liquids and gases (In Physics, liquids and gases are both called "fluids". They behave in similar ways.) Fluid friction is also known as "drag". On aircraft it's also called "air resistance". It depends on:- how thick the fluid is      (its "viscosity") the shape of the object the speed of the object

17 Coefficient of friction
Normal force (N) Ff = m Fn Friction force (N) Coefficient of friction



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