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REGENTS PHYSICS
STUDENT NOTES-MECHANICS
FRICTION

FRICTION - the force that opposes _____________________ .
-
Occurs when two surfaces make contact.

This force is always in the __________________ direction of the motion.

It always tries to __________ down the moving object.

Coefficient of Friction – a number that represents the nature of the surfaces. A
___________________ indicates a higher amount of friction between the
surfaces.
Example: Tire on a dry road = 0.70
Tire on a wet road = _________

How much Friction? Depends on…
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The type of surfaces that are in contact.
Examples of Surfaces with little friction…
Examples of Surfaces with a lot of friction…
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The amount of force pressing the two surfaces together.
- Normal Force – the force that always acts ___________________
to the surface.

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Normal Force on a horizontal surface.
-
Normal Force on an inclined surface.
The Frictional Force Equation
Ff =  FN
(on your reference table)
Where Ff is the Frictional Force, FN is the Normal Force and  is the
coefficient of friction.
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STATIC VS KINETIC FRICTION

Static Friction – sometimes called __________________ friction.

Kinetic Friction – Sometimes called __________________ friction.
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Static Friction is always ___________________ than kinetic friction.
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Example: Static Friction of wood on wood = 0.50
Kinetic Friction of wood on wood = ____________
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SUMMARY

Friction acts parallel to the surfaces in contact and in the ________________
direction to the motion of the object.
Ff
Fa
-These forces are balanced. No ____________ Force.
-The object could be at ___________ or moving at a ____________ speed.
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
Friction depends on the nature of the materials in contact.
 =
Ff
FN
This is reflected by a high
.
A high  means that there is a lot of
friction between the two surfaces.

The greater the weight of the two objects, the greater the frictional force. The
objects press themselves together more.
 =
Ff
FN
FN gets larger as the weight gets higher.  stays the same as the
objects don’t change composition. Ff gets larger.

Friction is independent of the area of contact.
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EXAMPLE PROBLEMS:
1. The coefficient of sliding friction between a crate and a horizontal floor is 0.27.
If the crate has a mass of 250 kg, what force is needed to slide it across the floor
at a constant velocity?
2. A wood block weighing 130 N rests on an inclined plane. The coefficient of
sliding friction between the block and the plane is 0.620. Find the angle of the
inclined plane at which the block will slide down the plane at a constant speed
once it has started to move.
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3. A book weighing 20 N slides at a constant velocity down a ramp inclined at 30
degrees to the horizontal.
(a) What is the force of friction between the book and the ramp?
(b) What is the coefficient of friction between the book and the ramp?