Module 5 Lesson 2: Friction

Module 5 Lesson 2: Friction

Focus Question

What force causes a train to stop?

New Vocabulary

static friction

kinetic friction

coefficient of static friction

coefficient of kinetic friction

Review Vocabulary

force: a push or a pull exerted on an object; has
both direction and magnitude and may be a
contact or a field force

Kinetic and Static Friction

• Friction is a force that opposes the motion of two

surfaces that are in contact with each other.

• There are two types of friction—static friction and

kinetic friction.

Kinetic 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, in the image on the left, the force

applied to the couch is balanced by the static
friction force. The couch does not move.

Kinetic and Static Friction

• 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.

Kinetic and Static Friction

• Frictional force depends on the materials that the

surfaces are made of.

• The different lines

correspond to
dragging a block
along different
surfaces.

Kinetic and Static Friction

• The slope of this line is the coefficient of kinetic

friction (μk) between the two surfaces. It relates the
frictional force to the normal force, as shown below.

Kinetic Friction Force

Ff, kinetic = μkFN

• Notice that the normal force (FN) also plays a role in

the size of the friction force.

Kinetic and Static Friction

• Similarly, μs is the coefficient of static friction between

the two surfaces.

Static Friction Force

Ff, static = μsFN

• μsFN is the maximum static friction force that must be

overcome before motion can begin.

Kinetic and Static Friction

Balanced friction Forces: You push a25.0 kg
wooden box across a wooden floor at a
constant speed of 1.0 m/s. The coefficient of
kinetic friction is 0.20. How large is the force
that you exert on the box?

Gwen exerts a 36 N horizontal force as she
pulls a 52 N sled across a cement sidewalk at
constant speed. What is the coefficient of
kinetic friction between the sidewalk and the
metal sled runners? Ignore air resistance.

Mr. Ames is dragging a box full of books from
his office to his car. The box and books
together have a combined weight of 134 N. If
the coefficient of static friction between the
pavement and the box is 0.55, how hard must
Mr. Ames push horizontally on the box in order
to start it moving?