Friction is the force that opposes the relative motion between two objects in contact. It arises due to the roughness of surfaces at a microscopic level. There are two main types of friction - static friction and kinetic friction. Static friction acts when an object is at rest and opposes any applied force seeking to initiate motion, while kinetic friction opposes motion between objects already in motion relative to one another. Friction follows several laws - it is directly proportional to the normal force pressing the surfaces together and independent of contact area. Friction has both advantages like enabling motion and braking, as well as disadvantages like wear and reducing efficiency. Methods to reduce friction include lubrication and use of ball bearings.

1. Topic: FRICTION
© Adenle, H.A.

2. WHAT IS FRICTION?
• Friction is the force which opposes the motion
between two bodies in contact.
• It acts at the surface of separation between the two
bodies. The surface of any body is actually rough.
• It does not appear unless there is a relative motion.
• The rougher the surface, the more the friction.

3. TYPES OF FRICTION
• Static (limiting) friction: This is the maximum force which
must be overcome before a body can just start to move over
another.
• Kinetic (Dynamic) friction: This is the force that must be
overcome so that a body can move with uniform speed over
another body. Examples of dynamic friction are sliding
friction, pivot friction or rolling friction.

4. LAWS OF FRICTION
The laws are stated as follows:
1. Friction varies directly with the normal force R (reaction) pressing
the surface together.
2. Frictional force is independent of the area of the surfaces in
contact.
3. Frictional force depends on the nature of the two surfaces in
contact i.e. whether it is rough or smooth.
If the body moving over the other is on an horizontal plane, the 1st
law will result in the formula:
where μ is the coefficient of friction
R
W
FFr

5. There are two coefficient of friction:
(a) Coefficient of static or limiting friction (μs): This is the
maximum ratio of applied force to normal force with no
motion.
(b) Coefficient of dynamic or kinetic friction (μk): This is the
ratio of applied force to normal force when the body is
moving.
For a body moving on an inclined plane at angle θ, the
coefficient of static friction is μs = tan θ.
How?
LAWS OF FRICTION (contd)

6. ADVANTAGES OF FRICTION

7. (1) Friction helps one to walk or run without slipping.
(2) Friction between pen and paper helps us to write
on the paper.
(3) Friction enables the automobile tires to make a
firm grip with the roadway.
(4) Friction helps the fan belts used over wheels or
pulleys to turn.
(5) Friction enables brakes to stop the car and the
locomotive to pull the train.
(6) Friction helps nail to hold woods together.
ADVANTAGES OF FRICTION (contd)

8. • Friction produces heat.
• It causes wear and tear.
• Friction reduces the efficiency of a machine.
• Friction produces noise.
DISADVANTAGES OF FRICTION

9. • The use of lubricants like oil, grease, air, graphite, wax e.t.c.
• The use of ball or roller bearings
• Streamlining of body shapes of moving objects.
WAYS OF REDUCING FRICTION

10. Example1: A body of mass 5kg is placed on a
horizontal plane with frictional force 30N acting on
the body. What is the coefficient of static friction
(g = 10 ms-2)
Solution: m = 5kg, g = 10ms-2, Fr = 30N
Weight = mg = 5x10 = 50N
Reaction R = W
From the law of friction,

11. QUIZ
1. A metal block of mass 5 kg lies on a rough
horizontal platform. If a horizontal force of 8 N
applied to the block through its centre of mass just
slides the block on the platform, what is the
coefficient of limiting friction between the block
and the platform? (g=10ms-2) (Ans: 1.6)
2. A wooden block whose weight is 50 N rests on a
rough horizontal plane surface. If the limiting
friction is 20 N, calculate the coefficient of static
friction. (Ans: 0.4)