1. Friction
Md. Sirajul Islam Sagor
Dept. of Textile Engineering
30th Batch
Daffodil International University
2. • Friction is the resistance to motion of one object moving relative to another. It is not
a fundamental force, like gravity or electromagnetism. Instead, scientists believe it
is the result of the electromagnetic attraction between charged particles in two
touching surfaces.
3. Friction is a “catch-all” term that collectively refers to all forces which act to
reduce motion between objects and the matter they contact.
Friction often transforms the energy of motion into thermal energy or the
wearing away of moving surfaces.
4. EQUATIONS
Models for friction
The friction force is approximately equal to the normal force multiplied by
a coefficient of friction.
kinetic friction
static friction
rolling friction
5. Symbol
• Ff
• Units
• Newtons (it’s a force!)
• Depends on
• Weight of object (normal force)
• Nature of the surfaces between the moving object and the supporting
surface
6.
7. TYPES OF FRICTION
• There are two main types of friction
• static friction and
• kinetic friction.
• Static friction operates between two surfaces that aren't moving relative to
each other
• while kinetic friction acts between objects in motion.
8.
9. If a crate is dragged to the right, friction points left. Friction acts in the
opposite direction of the (relative) motion that produced it.
Kinetic friction is sliding friction. It is a force that resists sliding or skidding
motion between two surfaces.
KINETIC FRICTION
10. Which takes more force to push over a rough
floor?
KINETIC FRICTION
11. The board with the bricks, of course!
The simplest model of friction states that frictional force is proportional to the
normal force between two surfaces.
If this weight triples, then the normal force also triples—and
the force of friction triples too.
FRICTION AND THE NORMAL FORCE
12. The force of kinetic friction Ff between two surfaces equals the coefficient of kinetic friction μk
times the normal force FN.
rection of
motion
A MODEL FOR KINETIC FRICTION
13. LAWS OF KINETIC OR DYNAMIC FRICTION
• The force of friction always acts in a direction, opposite to that in which the body
is moving.
• The magnitude of the kinetic friction bears a constant ratio to the normal reaction
between the two surfaces. But this ratio is slightly less than that in case of
limiting friction.
• For moderate speeds, the force of friction remains constant. But it decreases
slightly with the increase of speed
14. STATIC FRICTION
Static friction is gripping friction. It is a force that prevents
relative motion between surfaces in contact with each other.
• Without static friction between your feet and the
floor, you could not walk or run. Your feet would
slip.
• Without static friction between your tires and the
road, you could not start or stop a car.
15. The maximum static friction force Ff between two surfaces is the coefficient of static friction μs
times the normal force FN.
direction of
applied force
A MODEL FOR STATIC FRICTION
• When μs = 0 there is no friction.
• When μs = 0.5 the maximum friction force equals half the normal force.
• When μs = 1.0 the maximum friction force equals the normal force.
16. LAWS OF STATIC FRICTION :
• The force of friction always acts in a direction, opposite to that in which the
body tends to move.
• The magnitude of the force of friction is exactly equal to the force, which tends
the body to move.
• The magnitude of the limiting friction (F ) bears a constant ratio to the normal
reaction (RN) between the two surfaces. Mathematically, F/RN = constant
• The force of friction is independent of the area of contact, between the two
surfaces.
• The force of friction depends upon the roughness of the surfaces.
17. Static friction prevents this crate from
sliding when pushed . . .
. . . until the pushing force is greater
than the maximum static friction force
available.
STATIC FRICTION
18. The coefficient of friction is a constant that depends on both
materials. Pairs of materials with more friction have a higher μk.
direction of
motion
THE COEFFICIENT OF FRICTION
19. COEFFICIENT OF FRICTION
• Two solid objects moving against each other
experience kinetic friction.
• In this case, the friction is some fraction of the
perpendicular force acting between two objects
(the fraction is determined by a number called
the coefficient of friction, which is determined
through experiments).
• In general, the force is independent of the
contact area and doesn't depend on how fast the
two objects are moving.
