friction is a type of force between two touching surfaces. section 2: friction k what i know w what...

Friction is a type of force between two touching surfaces.

Section 2: Friction

KWhat I Know

WWhat I Want to Find Out

LWhat I Learned

Essential Questions• What is the friction force?• How do static and kinetic friction differ?

FrictionCopyright © McGraw-Hill Education

Review• force

New• kinetic friction• static friction• coefficient of kinetic friction• coefficient of static friction


Vocabulary


• Friction is a force that opposes the motion of two surfaces that are in contact with each other.

• There are two types of friction—kinetic friction and static friction.

• Static friction is the force exerted on one surface by another when there is no motion between the two surfaces.

• For example, the force applied to the couch below is balanced by the static friction force.

Kinetic and Static Friction

Animation

Animation


• If you push hard enough, the couch will begin to move and kinetic friction will act on it.

• Kinetic friction is exerted on one surface by another when the two surfaces rub against each other because one or both of them are moving.



• Frictional force depends on the materials that the surfaces are made of.

• The different lines correspond to dragging a block along different surfaces.



• The slope of this line is the coefficient of kinetic friction (μk) between the two surfaces and relates the frictional force to the normal force, as shown below.

• Notice that the normal force (FN) also plays a role in the size of the friction force.

• Similarly, μs is the coefficient of static friction between the two surfaces.

• μsFN is the maximum static friction force that must be overcome before motion can begin.


f, kinetic k NF μ FKinetic Friction Force

f, static s NF μ FStatic Friction Force



EVALUATE THE ANSWER• Force is in newtons, so the units are

correct.

Use with Example Problem 3.

Problem A child drags a heavy, rubber-soled shoe by its laces across a sidewalk at a constant speed of 0.35 m/s. The shoe has a mass of 1.56 kg and the coefficient of kinetic friction is 0.65. What is the force exerted by the child? (Assume the sole is in contact with the sidewalk, not bouncing around and that the child pulls horizontally.)ResponseSKETCH AND ANALYZE THE PROBLEM• Draw a vector diagram.• List the knowns and unknowns.

KNOWN

v = 0.35 m/sm = 1.56 kgμk = 0.65

UNKNOWN

Fchild = ?

Fchild

FN

Fg

Ff

vg

SOLVE FOR THE UNKNOWN• Because the shoe is not accelerating

in the y-direction, Fnet, y = 0. • Thus, the normal force and the shoe’s

weight are equal in magnitude.

• Because the shoe is not accelerating in the x-direction, Fnet, x = 0.

• Thus, the force the child exerts and the frictional force are equal in magnitude.

child f k N k

0.65 1.56 kg 9.8 N/kg 9.9 N

F F F mg

μ μ

N gF F mg



Use with Example Problem 4.

Problem If the child in the previous Additional In-Class Example pulls with an extra 2.0 N in the horizontal direction, what will be the acceleration of the shoe?

ResponseSKETCH AND ANALYZE THE PROBLEM• Draw a force diagram.• List the knowns and unknowns.

KNOWN UNKNOWN

Fchild = 11.9 N a = ?

m = 1.56 kg

μk = 0.65

SOLVE FOR THE UNKNOWN• Find the net force in each direction.

• Fnet, y = 0 and FN = mg, as it did in the previous example.

• Ff is same as it was in the previous example, so Fnet, x = Fchild − Ff = 2 N.

• Use Newton’s second law.net

2net, 2.0 N1.3 m/s

1.56 kgx

x

m

Fa

m

F a

Fchild

FN

Fg

Ff

a

EVALUATE THE ANSWER• 1.3 m/s2 is about (1/8)g, so it is a

reasonable acceleration.


Review

Essential Questions• What is the friction force?

• How do static and kinetic friction differ?

Vocabulary

• kinetic friction• static friction

• coefficient of kinetic friction

• coefficient of static friction

friction is a type of force between two touching surfaces. section 2: friction k what i know w what...

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