ENGINEERING MECHANICS SESSION – 14 FRICTION

SESSION OUTCOMES At the end of this session, Students will be able to Understand the cause of friction. Explain laws of coulomb friction. Solve relevant problems.

SESSION PLAN Time in Minutes Topic Blooms Taxonom y Level-1 Blooms Taxonom y Level-2 Blooms Taxonom y Level-3 05 Introduction: Recap of forces in space. 25 Sub-topic-1 (Lecture) Introduction to friction, cause of friction, types of friction, lawsof coulomb friction.(Physical model, animation assisted).  15 Participative session Quiz  25 Sub-topic-2 (Lecture) Angle of friction, angle of repose & cone of friction (Physical model, presentation & animation assisted).  20 Participative session Assignment problems will be practiced by students.  05 Conclusion Session will be concluded by summarizing friction. 95 Minutes Total Contact Session + 5 Minutes for Attendance and Transition activities = 100 minutes

FRICTION Suppose you decide to ride a skateboard. You push off the ground and start moving. According to Newton’s First Law of Motion, if no other forces are acting on your skateboard, you will continue to move….BUT What happens?

 Your skateboard slows down because of friction.  Friction is the force that opposes motion between two surfaces that are touching each other.  The amount of friction depends on two things: 1. Kind of surface. 2. Forces pressing the surfaces together.

W HAT C AUSES F RICTION ?  Even though a surface may look smooth, if you magnify the surface it isn’t smooth.  Everything is made of molecules. The molecules come together to make bumps. Some surfaces have molecules that leave large bumps and some leave smaller bumps, but all surfaces have bumps.

D EFINITION OF F RICTIONAL F ORCE If two surfaces are in contact with each other, when one of the surface tends to move in one direction, then the force experienced by another surface is called Friction

D EFINITION OF F RICTIONAL F ORCE (C ONTD.)

T YPES OF F RICTION

STATIC FRICTION  Suppose your little brother gets into a box and wants you to move him. He is too heavy to lift.  You try to push the box and it doesn’t move.  What type of acceleration does it have? Answer is Zero Acceleration

STATIC FRICTION (C ONTD.)  Remember Newton’s Second Law of Motion  If the acceleration is zero, the net force is zero.  Therefore, because you can’t move it, another force is being applied to cancel out your force.  That opposing force is friction caused by the microwelds between the bottom of the box and the floor.  This type of friction is called static friction.

STATIC FRICTION (C ONTD.)  Static Friction is the friction between two surfaces that are not moving past each other.  In this case, your push is not large enough to break the microwelds and the box remains stuck to the floor.

STATIC FRICTION (C ONTD.)  To help you move your little brother, you decide to ask a friend.  You both push and slowly, but not easily, the box starts to slide across the floor.  When you stop pushing, it quickly stops moving.  Sliding the box is difficult and there is still resistance.  Although you have overcome the microwelds, there is still a sliding friction occurring.

STATIC FRICTION (C ONTD.)  Sliding Friction is the force that opposes the motion of two surfaces sliding past each other.  Sliding friction is caused by microwelds constantly breaking and then forming again as the box slides along the floor.  To keep the box moving, you must continually apply a force to overcome sliding friction.

ROLLING FRICTION  Have you ever seen a car stuck in the mud. Every time the driver steps on the accelarator, the tires just spin and the car doesn’t go anywhere?  This is because there isn’t enough friction between the slippery ground and the tires.  How could you create more friction to get the tires to move the car?  Drop sand, gravel, put a board under each of the front tires.

ROLLING FRICTION (C ONTD.)  The friction between the rolling tires and the ground is called rolling friction.  Rolling friction works partly because of microwelds.  Rolling friction is much less friction than static or sliding friction.  That is why it is easier to use a trolly than pushing your little brother in the box.

A PPLIED FORCE VS F RICTIONAL FORCE

A PPLIED FORCE VS F RICTIONAL FORCE G RAPH

L IMITING F RICTION Limiting friction is defined as the maximum value of frictional force experienced by the body.

C OEFFICIENT OF F RICTION Coefficient of friction is defined as the ratio of Limiting frictional force to Normal reaction

C OEFFICIENT OF F RICTION VALUES

A NGLE OF F RICTION Angle of friction is defined as the angle made by the resultant of frictional force and the normal reaction with the normal reaction.

A NGLE OF R EPOSE Angle of repose is defined as the angle made by inclined plane with the horizontal plane, when the body on the inclined plane slides down without any external forces acting on it.

C ONE OF F RICTION Cone of friction is defined as the right circular cone with vertex at the point of contact of the two bodies (or surfaces), axis in the direction of normal reaction and semi vertical angle equal to angle of friction.

L AWS OF C OULOMB F RICTION  The force of friction acts in the opposite direction in which surface is having tendency to move.  The force of friction is equal to the force applied to the surface, so long as the surface is at rest.  When the surface is on the point of motion, the force of friction is maximum and the maximum frictional force is called as limiting frictional force.

L AWS OF C OULOMB F RICTION (C ONTD.)  The limiting frictional force bears a constant ratio to the normal reaction between two surfaces  The limiting frictional force does not depend upon the shape and areas of the surfaces in contact.  The force of friction is independent of velocity of slliding.

P ARTICIPATIVE S ESSION 1. Whenever the surfaces of two bodies are in contact there will be a limited amount of resistance to sliding between them, which is called (a) Attraction (b) Internal forces (c) Lubrication (d) Friction Ans : d

P ARTICIPATIVE S ESSION (C ONTD.) 2. The force of friction developed between two surfaces in contact is independent of (a)Roughness of surface (b) Area of contact between the surfaces (c) Reaction of surface (d) Force tending cause motion Ans : b

P ARTICIPATIVE S ESSION (C ONTD.) 3. The coefficient of friction between two surfaces is the constant of proportionality between the tangential force and normal reaction at the instant of (a) Application of force (b) Impending motion (c) Body at rest (d) Motion of the body Ans : b

P ARTICIPATIVE S ESSION (C ONTD.) 4. Angle of friction is the angle between the (a) Horizontal and the inclined plane on which the body tends to move under force. (b) Frictional force and the resultant force and normal reaction (c) Horizontal and the resultant of normal reaction and frictional force (d) Normal reaction and the resultant of normal reaction and frictional force. Ans : b

P ARTICIPATIVE S ESSION (C ONTD.) 5. A body of weight W is placed on an inclined plane. The angle made by the inclined plane with the horizontal, when the body is on the point of moving down is called (a) Angle of inclination (b) Angle of repose (c) Angle of friction (d) Angle of limiting friction Ans : b

P ARTICIPATIVE S ESSION (C ONTD.) 6. A body of weight Q is placed on an inclined rough plane. The inclination of the plane with the horizontal is less than angle of friction. The body will (a) Be in motion (b) Move downwards (c) Be in equilibrium (d) Move upwards Ans : c

P ARTICIPATIVE S ESSION (C ONTD.) 7. Units of coefficient of friction is ______________ Ans : No units

PROBLEM SET 2.7