Here are the steps to solve this problem: a) To raise the block B: - Normal reaction force (N) = Weight of block B (W) - Maximum static friction force (Fs) = Coefficient of static friction (μs) x N = 0.35 x W - Force P must be greater than Fs to overcome static friction and raise the block - P ≥ Fs P ≥ 0.35W b) To lower the block B: - Normal reaction force (N) = Weight of block B (W) - Maximum static friction force (Fs) = Coefficient of static friction (μs) x N = 0.35 x W (acts

1. Friction

2. Outline
• What is Friction?
• Factors Affecting Friction
• What Causes Friction?
• Types of Friction
• Application of Friction
• Friction Problem with Solution

3. Friction
• Friction is a force between two surfaces that
are sliding, or trying to slide, across each
other. Friction always slows a moving object
down. The amount of friction depends on the
materials from which the two surfaces are
made. The rougher the surface, the more
friction is produced. Friction also produces
heat.

5. • Coefficient of friction, ratio of the frictional force
resisting the motion of two surfaces in contact to the
normal force pressing the two surfaces together. It is
usually symbolized by the Greek letter mu (μ).
Mathematically, μ = F/N, where F is the frictional force
and N is the normal force.
• Two types of friction coefficient are distinguished: the
static friction coefficient and the kinetic friction
coefficient. The former is sometimes called the starting
friction coefficient, and the latter is sometimes called
the dynamic or sliding friction coefficient.

6. • The coefficient of friction depends on the objects that are
causing friction. The value is usually between 0 and 1 but
can be greater than 1. A value of 0 means there is no
friction at all between the objects; such is possible with
Super fluidity. A value of 1 means the frictional force is
equal to the normal force.
• Coefficient of friction is the amount of friction
existing between two surfaces. A low value of
coefficient of friction means less force is required
for sliding to occur. A higher value means more
force is required.

7. Example
• As an example, ice on steel has a low coefficient
of friction – the two materials slide past each
other easily – while rubber on pavement has a
high coefficient of friction – the materials do not
slide past each other easily.
• Among all measured sites, the palm of the hand
has the highest coefficient of friction (0.62±0.22).
For all the materials tested, silicone has the
highest coefficient of friction (0.61 ±0.21), while
nylon has the lowest friction (0.37±0.09).

8. Frictional force causes a lot of losses in general upkeep and wear and
tear of machinery. ... But almost all crucial tasks cannot be carried out
without the presence of friction. Basic activities like walking and writing
on a surface are possible due to friction. Hence it is considered as a
necessary evil

9. Applications
• Driving of a vehicle on a surface.
• Applying brakes to stop a moving vehicle.
• Skating.
• Walking on the road.
• Flying of aero planes.
• Drilling a nail into wall.
• Sliding on a garden slide.

10. • Friction finds application when matchsticks
are ignited.
• Motion of pistons in a cylinder is an
application of friction.
• It is possible to write on books and board as
there is friction between pen and the board.

11. Increasing Friction
• There are two methods of
increasing friction: one is
by making the surfaces
rough and the other by
increasing the mass of the
object that is moving. For
example, the tyres of
vehicles have treads
which increase the
friction between the tyre
and the road

12. Factors Affecting Friction
1. The nature of the two surfaces that are in
contact
Friction is dependent on the smoothness or roughness of
the two surfaces that are in contact with each other.
When the surface is smooth, the friction between the
two reduces as there is not much interlocking of
irregularities. While the surface is rough, friction
increases.
2. The force that is acting on these surfaces
Friction increases when the force is applied along with
the irregularities.

13. Friction
Advantages
• Friction enables us to walk
freely.
• It helps to support ladder
against wall.
• It becomes possible to
transfer one form of energy
to another.
• Objects can be piled up
without slipping.
• Breaks of vehicles work due
to friction.
Disadvantages
• Friction produces a lot of heat
in various parts of the
machinery and this leads to
wastage of energy as heat.
• Opposes motion, hence more
energy is needed to overcome
friction.
• Noise production in machines
is irritating as well as leads to
energy loss.

14. • Produce heat in moving parts of
machines/engines. Produce noise in machines
/engines. Makes engines to consume more
fuel, in other words it reduces the efficiency of
engines. Cause wear and tear in moving parts
of machines/engines.

15. Types of Friction
• Static Friction
• Kinetic Friction
• Rolling Friction
• Sliding Friction
• Fluid Friction

21. The laws of dry friction, coefficients of
friction

22. • The coefficient of static
friction is the ratio of the
maximum static friction force
(F) between the surfaces in
contact before movement
commences to the normal (N)
force.
• The coefficient of friction
depends on the materials
used; for example, ice on steel
has a low coefficient of
friction, while rubber on
pavement has a high
coefficient of friction.

23. Laws

24. Angles of friction
• It is sometimes convenient to replace normal force “N”, and
friction force “F” by their resultant “R”.

25. • Consider block of weight W resting on board
with variable inclination angle ϴ

26. Friction Angles

27. Numericals on friction

28. Sample Problem

29. Solution

30. Wedges

31. Assignment
• The position of the machine block B is adjusted by moving the
wedge A. Knowing that the coefficient of the static friction is
0.35 between all surfaces of contact, determine;
a) The force P required to raise the block B,
b) To lower block B.