Untitled Lesson

Bellringer: Which
type of tires will
give you a better
grip when driving
down a dry street?

Is friction a good thing, or a bad thing?

What would happen to you if all friction
was turned off for the rest of the day?

How do you walk? What are the forces?

Friction - The force that resists one
object from sliding against another.

You are trying to pull a couch, the only thing
resisting you is the force of friction. What are
some things we can do to make it harder to pull?

There are only two factors that
affect the force of friction:

1. How “sticky” or “how much

grip” the two surfaces have.

2. The normal force on the

object.

Called the
“Coefficient of
Friction”

The force of friction is equal to the coefficient of
friction “μ” multiplied by the normal force

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If a 10 kg object is sliding on a table, and the
coefficient of friction is 0.54, what is the force of
friction on the object?

What are the
units for
coefficient of
friction???

Time

So there are two different kinds of
friction:

Static Friction

Static friction is
the friction
between two
surfaces that are
not sliding.

Kinetic friction is
the friction
between two
surfaces that
ARE sliding.

Kinetic Friction

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An object a rest on a
ramp.

Is this STATIC or
KINETIC Friction?

An object sliding on
a ramp.

Is this STATIC or
KINETIC Friction?

“Static” friction does
not mean stationary.
It means not sliding.

A person walking?

A car driving on the road?

STATIC!!!!

Coefficient of Friction Depends on Materials

What do you notice that
is different between
Kinetic and Static
Coefficients?

What are the units for
coefficient of friction?

A waxed ski is sliding across a snowy path. What
is the coefficient of friction between the ski and
the snow?

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If the person has weight of 550 N then what is the
force of kinetic friction?

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1. Draw a free body diagram for

all the forces on mass m.

Masses on an Inclined Plane

x

y

x

y

Masses on an Inclined Plane

2. If the mass of the block was
25kg and the angle of the
incline plane was 15 degrees,
then determine the magnitude
of the Normal force

3. If the mass is still 25 kg and
the angle is still 15 degrees.
Determine the component of the
block’s weight that goes down
the ramp.

Masses on an Inclined Plane

4. If the mass is still 25 kg and
the angle is still 15 degrees.
Determine the acceleration of
mass m down the ramp.

Masses on an Inclined Plane

A car is parked on an icy hill (no friction)
Calculate the normal force and acceleration down
the incline.

The mass on this
incline is 40 kg
and the incline
has an angle of 25
degrees.

Bellringer:
A crate is sliding down a frictionless incline. The crate has a mass of
30 kg, and the incline has an angle of 23 degrees with the surface,
then draw a Free Body Diagram for the crate and calculate:

1.

The Normal Force

2.

The Component of Gravity Pulling Down the Incline

3.

The Acceleration of the Crate Down the Incline

4. What if there was friction?

Masses on an Inclined Plane

1.

The Normal Force

2.

The Component of Gravity
Pulling Down the Incline

3.

The Acceleration of the
Crate Down the Incline

Bellringer:
A crate is sliding down a frictionless incline. Draw a Free Body
Diagram for the crate and derive an equation for:

1.

The Normal Force

2.

The Component of Gravity Pulling Down the Incline

3.

The Acceleration of the Crate Down the Incline