This document discusses friction, including its causes and types. Friction is the force that resists motion between two objects in contact. It is caused by uneven surfaces on the objects, with tiny particles from one surface embedding in the other when rubbed together. There are two main types of friction: dry friction between solid surfaces, and fluid friction between lubricated surfaces. Static friction is higher than kinetic friction and acts when surfaces are not moving relative to each other, while dynamic or kinetic friction acts when surfaces are moving relative to each other. The amount of friction depends on factors like the contact materials and surface smoothness, and is proportional to the force pressing the surfaces together.

1. FRICTION
BY
Usman Sajid Sohrani

2. Friction
• Any force that resists motion
• It involves objects that are in contact with
each other.
• This is the force that keeps an object from
sliding down and incline plane.

3. • Some scientists believe that friction is
caused by uneven surfaces of the touching
objects – when rubbed together resistance is
offered.
• Experiments have shown that tiny particles
are actually torn from one surface and
imbedded in the other.
• If two surfaces were carefully polished,
there is a limit to the amount by which
friction may be reduced. If made too
smooth, the friction between them actually
increases.

4. Examples of FrictionExamples of Friction
• Tires on the road
• Walking/Running
• Nails into wood

5. Principles of Friction
• Friction acts parallel to the surface that are
in contact.
• The direction that friction acts is
OPPOSITE the direction of the motion (or
intended motion).
Fweight
Fnormal
FappliedFfriction(k)

6. Types of Friction
Dry Friction
Occurs between the non-lubricated
surfaces of solid objects
Fluid Friction
Occurs with fluids,or
lubricated surfaces
Dynamic Friction
When dry friction acts
between two surfaces that are
moving relative to each other
>
Static Friction
When dry friction acts between
two surfaces that are not
moving relative to each other

7. • Friction depends on the nature of the
material in contact and the smoothness of
their surfaces.
• Static Friction is usually higher than Kinetic
Friction because it is harder to get
something started than it is to keep it going.

8. The Coefficient of Friction
Frictional Force, both static and kinetic, is directly
proportional to the force pressing the two surfaces
together. The more weight, the more friction.
normal
(kinetic)friction
kinetic
normal
(static)friction
static
normalfriction
F
F
F
F
FF
==
=
µµ
µ
normalfriction FF ∝
The missing link to the above equation is the
Coefficient of Friction, or μ: