Friction & Applying Newton’s 2nd Law

Chapter 6.2

System

What is Friction?

• Friction is a that is parallel to the surfaces of two objects that are in contact with one another that the relative of the two objects.

Ff

What causes friction?

• Friction is caused by the temporary created between two objects in contact with one another.

Friction

• How does friction affect the motion of objects?– It can an object down

like the friction betweenthe tires and the road.

– It is responsible for the speed of an

object like a car.– It is also responsible

for objects being able to .

Friction ( Objects)

System

FAppliedFfriction

Fground-on-crate

Fgravity

Fnet =

Since the crate is not accelerating, Fnet = __

=

Note: As long as the crate does not move, _____ = _____

• Static Friction:– The that keeps an object from moving.– Since v = ___, a = ___.

Friction (Objects in )

System

FappliedFfriction

Fground-on-crate

Fgravity

• Kinetic Friction:– The that opposes the relative

motion of two contacting surfaces that are past one another.

– Fnet may or may be ___.

Fnet =

Fnet

Note: If the crate moves at a , then _____ =

_____ and Fnet = ___.

Determining the Frictional Force (The of Friction, __)

• The force of friction (Ff) is proportional to the normal force (FN) and a proportionality constant (___ - pronounced _____) called the of friction.

• For static friction:– 0 < Ff, static < sFN

• For kinetic friction:– Ff, kinetic = kFN

• As per the formula, the greater ___, the greater the .

• Note: FN = the ( ) to the on the object.

• __ is dimensionless ( )• _____ > _____

Ff

FN

fF

The Force

• The force is a force that most often opposes the Earth’s gravitational attraction and is to the

that an object rests or is moving on.– For a surface, ____ = ____ = ____.– For a surface that is as

seen in the figure below, _____ = ________.

FN

The Normal Force

FN

Fg

FN

Fg

____ = ____ = ____

cos = adj/hyp

____ = ________ = ________

Example 2: Determining Friction (Balanced Forces)

• Assume that the man in the figure is pushing a 25 kg wooden crate across a wooden floor at a constant speed of 1 m/s.

– How much force is exerted on the crate?

System

FappliedFf

FN

Fg

Diagram the Problem

System

y-direction: ____ = ____ x-direction: Fnet =

Since the crate is moving with speed, a = ___, Fnet = ___, and ______ = ______

+y

+x

State the Known and Unknowns

• What is known?o Mass (m) = o Speed = o Acceleration (a) = k =

• What is not known?o Fapplied = ?

Perform Calculations• y-direction:

o ___ = ___ = ___ (Note: surface)

• x-direction: Since a = ___, Fnet = ___

o Fnet = –

Example 3: Determining Friction ( Forces)

• Assume that the man in the figure is pushing a 25 kg wooden crate across a wooden floor at a speed of 1 m/s with a force of 73.5 N.

– If he doubled the force on the crate, what would the acceleration be?

System

FappliedFf

FN

Fg

Diagram the Problem

System

y-direction: ____ = ____ x-direction: Since , Fnet =

+y

+x

State the Known and Unknowns

• What is known?o Force = o Mass (m) = o Speed = k =

• What is not known?o a ?

Perform Calculations

• y-direction:

• x-direction:o Fnet = o ma =o a =

Key Ideas

• Friction is an that exists between bodies.

• Friction is to the and the

of ; static or kinetic.• The force required to overcome

static friction is than that required to overcome kinetic friction.