ObjectivesDefine the friction force.Distinguish between static and kinetic friction.

INTROFriction – force opposing relative motion

of 2 objects that are in contact. Examples tires on road, shoes on ground. The direction of the force is parallel to the surface and in a direction that opposes the slipping of the 2 surfaces.

STATIC AND KINETIC FRICTIONKinetic Friction – the force exerted on one surface by a second

surface when the 2 surfaces rub against one another because one or both of the surfaces are moving. Also called Sliding Friction.

From Old Book Sliding Friction – force between 2 surfaces in relative motion. The force

of sliding friction is less than that of static friction. Thus a car will stop faster if the wheels are not skidding.

To keep a book sliding at constant velocity on a desk you must exert a constant force that is just the same size as the frictional force but in the opposite direction.

  Experimentally it has been found that the force of friction depends

primarily on the force pushing the surfaces together, FN, and on the nature of the surfaces in contact. FN is the force pushing the surfaces together (Normal or Perpendicular Force)

STATIC AND KINETIC FRICTIONStatic Friction – the force exerted on one surface

by a second surface when there is no motion between the 2 surfaces.

 From Old Book 

Static Friction – force that opposes start of motion between 2 surfaces. Static friction forces have maximum values.

When the magnitude of your push on the box is greater than the max value of the static friction between the floor and the box, the box starts moving. When the box starts to move the force of friction decreases.

STATIC AND KINETIC FRICTIONOnce your force is greater than this maximum

STATIC Friction, the couch begins moving and KINETIC Friction begins to act instead of static friction.

 You have Static Friction acting when trying to

get an object moving and once it is moving you have Kinetic or Sliding Friction acting.

Go Over example from book dealing with figure 5.8 p. 126.

A MODEL FOR FRICTION FORCESOn what does a Frictional Force depend?

The materials that the surfaces are made of play a role.

The normal force between the 2 objects. The harder an object is pushed against the other, the greater the force of Friction.

 If you pull a block along a surface at a

constant velocity, according to Newton’s Laws, the Frictional Force must be equal and opposite to the force with which you pull.

 

A MODEL FOR FRICTION FORCESCoefficient of Kinetic Friction - ratio of

frictional force and the normal force between 2 surfaces. It is indicated by the symbol “”. It is a constant that depends on the 2 surfaces in contact. This is for an object that is already moving.

 Kinetic Friction Force – the force exerted on one

surface by a second surface when the 2 surfaces rub against one another because one or both of the surfaces are moving. It is equal to the product of the coefficient of kinetic friction and the normal force.

FFKinetic = μKFN FF = μFN

A MODEL FOR FRICTION FORCESCoefficient of Static Friction - ratio of frictional

force and the normal force between 2 surfaces. It is indicated by the symbol “”. It is a constant that depends on the 2 surfaces in contact. This is for an object you are trying to get to move.

Static Friction Force - the force exerted on one surface by a second surface when there is no motion between the 2 surfaces. It is less than or equal to the product of the coefficient of static friction and the normal force.

FFStatic ≤ μSFN FF = μFN

The static Friction Force will increase up to a maximum value before it is overcome and motion starts.

A MODEL FOR FRICTION FORCESDo Example Problem 3 p. 128

Fg = mg = 25(9.8) = 245 N and Fg = FN

FPush = μFN = .2(245) = 49 N (note .2 from table 5.1)

 Do Practice Problems p. 128 # 17-21

Show and Go Over Table 5.1 p. 129

A MODEL FOR FRICTION FORCES

A MODEL FOR FRICTION FORCESDo Example Problem 4 p. 129

FPush doubles to 98 N and FF = original FPush

FNet = FPush – FF

ma = 98 – 4925a = 49a = 1.96 m/s2

 Do Practice Problems p. 130 # 22-26

IMPORTANT THINGS ABOUT FRICTION Important Things to remember about Friction

Friction acts in the direction opposite to the motion.

The magnitude of the force of friction depends on the magnitude of the normal force between the 2 surfaces.

Multiplying the coefficient of static friction and the normal force gives you the maximum static friction force.

Do 5.2 Section Review p. 130 # 28-32 (SKIP 27 and 31)