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# Friction & Applying Newton’s 2 nd Law - PowerPoint PPT Presentation

Friction & Applying Newton’s 2 nd Law. System. Chapter 6.2. F f. 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. What causes friction?.

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### Friction & Applying Newton’s 2nd Law

System

Chapter 6.2

Ff

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.

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

• How does friction affect the motion of objects?

• It can an object down

like the friction between

the 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

.

Fground-on-crate

FApplied

Ffriction

System

Fgravity

Friction ( Objects)

• Static Friction:

• The that keeps an object from moving.

• Since v = ___, a = ___.

Fnet =

Since the crate is not accelerating, Fnet = __

=

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

Fground-on-crate

Fapplied

Ffriction

System

Fgravity

Friction (Objects in )

• 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 = ___.

FN

Ff

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 ()

• _____ > _____

FN

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

Fg

The Normal Force

FN

cos = adj/hyp

Fg

____ = ____ = ____

____ = ________ = ________

FN

Fapplied

Ff

System

Fg

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?

+x

System

Diagram the Problem

• y-direction: ____ = ____

• x-direction: Fnet =

• Since the crate is moving with speed,

• a = ___, Fnet = ___, and ______ = ______

• What is known?

• Mass (m) =

• Speed =

• Acceleration (a) =

• k =

• What is not known?

• Fapplied = ?

• y-direction:

• ___ = ___ = ___ (Note: surface)

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

• Fnet = –

FN

Fapplied

Ff

System

Fg

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?

+x

System

Diagram the Problem

y-direction: ____ = ____

x-direction: Since , Fnet =

• What is known?

• Force =

• Mass (m) =

• Speed =

• k =

• What is not known?

• a ?

• y-direction:

• x-direction:

• Fnet =

• ma =

• a =

• 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.