Objectives: The student will be able to: Distinguish between the different types of

friction. Use free body diagrams and Newton's laws

of motion to solve word problems.

Calculate and compare the coefficients of friction of various surfaces.

Phone Book and Friction

Separate the phone books http://www.youtube.com/watch?v=hOt-D_ee-

JE

TWO types of Friction

Static – Friction that keeps an object at rest and prevents it from moving

Kinetic – Friction that acts during motion

• Kinetic friction occurs when force is applied to an object and the object moves.

• Examples:

Sliding Friction: pushing an object across a surface

Rolling Friction: between wheels and a surface

Fluid Friction: opposes the motion of objects traveling through a fluid (air or water)

Types of FrictionTypes of Friction

• To reduce the amount of friction, apply a lubricant between two surfaces.

• Motor oil, wax, and grease are examples.

• Friction can also be reduced by rolling, rather than pushing, an object.

Affecting FrictionAffecting Friction

• Friction increases as surfaces are made rougher.

• Friction increases when the force between two objects is increased.

Affecting FrictionAffecting Friction

Friction ForcesFriction ForcesWhen two surfaces are in contact, friction When two surfaces are in contact, friction forces oppose relative motion or forces oppose relative motion or impending motion.impending motion.

PPFriction forcesFriction forces are are parallel parallel to the surfaces in to the surfaces in contact and contact and opposeoppose motion or impending motion or impending motion.motion.

Static Friction:Static Friction: No No relative motion.relative motion.

Kinetic Friction:Kinetic Friction: Relative motionRelative motion.

22 N N

Friction and the Normal Friction and the Normal ForceForce

4 N4 N

The force required to overcome The force required to overcome staticstatic or or kinetic kinetic

friction is proportional to the normal force, friction is proportional to the normal force, nn.

fk = knfk = knfs = snfs = sn

nn

12 N12 N

6 N6 N

nn8 N8 N

4 N4 N

nn

Friction forces are independent of Friction forces are independent of area.area.

44 NN 44 NN

If the total mass pulled is constant, the same If the total mass pulled is constant, the same force (4 N) is required to overcome friction force (4 N) is required to overcome friction even with twice the area of contact.even with twice the area of contact.

For this to be true, it is essential that ALL For this to be true, it is essential that ALL other variables be rigidly controlled.other variables be rigidly controlled.

Friction forces are independent of Friction forces are independent of temperature, provided no temperature, provided no

chemical or structural variations chemical or structural variations occur.occur.

44 NN 4 N4 N

Heat can sometimes cause surfaces to Heat can sometimes cause surfaces to become deformed or sticky. In such cases, become deformed or sticky. In such cases, temperature can be a factor.temperature can be a factor.

Friction forces are independent of Friction forces are independent of speed.speed.

2 2 NN2 2 NN

The force of kinetic friction is the same at The force of kinetic friction is the same at 5 m/s5 m/s as it is for as it is for 20 m/s20 m/s. Again, we must . Again, we must assume that there are no chemical or assume that there are no chemical or mechanical changes due to speed.mechanical changes due to speed.

5 m/s5 m/s 20 m/s20 m/s

The Static Friction ForceThe Static Friction Force

When we use the following equation, we When we use the following equation, we refer only to the refer only to the maximummaximum value of static value of static friction and simply writefriction and simply write::

fs = snfs = sn

When an attempt is made to move an object When an attempt is made to move an object on a surface, static friction slowly on a surface, static friction slowly increases to a increases to a MAXIMUM MAXIMUM valuevalue.

s sf nn

fs

P

W

Constant or Impending Constant or Impending MotionMotion

For motion that is For motion that is impendingimpending and for and for motion at motion at constant constant speed, the resultant speed, the resultant force is zero and force is zero and F = 0F = 0. (Equilibrium). (Equilibrium)

Pfs

P – fs = 0

Rest

Pfk

P – fk = 0

Constant Speed

Here the Here the weightweight and and normal forcesnormal forces are are balanced and do not affect motion.balanced and do not affect motion.

Friction and AccelerationFriction and Acceleration

When P is greater than the maximum fs the resultant force produces acceleration.

Note that the kinetic friction force remains Note that the kinetic friction force remains constant even as the velocity increases.constant even as the velocity increases.

Pfk

Constant Speed

fk = kn

a

4-8 Applications Involving Friction, InclinesOn a microscopic scale, most surfaces are rough. The exact details are not yet known, but the force can be modeled in a simple way.

For kinetic – sliding – friction, we write:

is the coefficient of kinetic friction, and is different for every pair of surfaces.

4-8 Applications Involving Friction, Inclines

FrictionIf the coefficient of kinetic friction between a 35-kg crate and the floor is

0.30, what horizontal force is required to move the crate to the right at a constant speed across the floor?

mg

Fn

Fa

Ff

(0.30)(35)(9.8)

a f f k N

a k N

N

a k

a

a

F F F F

F F

F mg

F mg

F

F

102.9 N

FrictionSuppose the same 35 kg crate was not moving at a constant speed, but

rather accelerating at 0.70 m/s/s. Calculate the applied force. The coefficient of kinetic friction is still 0.30.

mg

Fn

Fa

Ff

(35)(0.70) (0.30)(35)(9.8)

NET

a f

a k N

a k

a k

a

a

F ma

F F ma

F F ma

F mg ma

F ma mg

F

F

127.4 N

4-8 Applications Involving Friction, Inclines

Static friction is the frictional force between two surfaces that are not moving along each other. Static friction keeps objects on inclines from sliding, and keeps objects from moving when a force is first applied.

The static frictional force increases as the applied force increases, until it reaches its maximum.

Then the object starts to move, and the kinetic frictional force takes over.

4-8 Applications Involving Friction, Inclines

Friction Lab

4-9 Problem Solving – A General Approach

1. Read the problem carefully; then read it again.

2. Draw a sketch, and then a free-body diagram.

3. Choose a convenient coordinate system.

4. List the known and unknown quantities; find relationships between the knowns and the unknowns.

5. Estimate the answer.

6. Solve the problem without putting in any numbers (algebraically); once you are satisfied, put the numbers in.

7. Keep track of dimensions.

8. Make sure your answer is reasonable.

Homework Chapter 4

Friction activity due in 2 days

Questions#8, 12, 21

Problems

#s 36, 37

Homework Chapter 4 Problems

Friction activity due next class.#s 38, 39, 44, 47