The document discusses friction as the opposing force to movement between surfaces, detailing static and kinetic friction along with their coefficients for various material pairs. It emphasizes factors affecting friction such as surface quality and normal force, and includes problems to calculate friction in different scenarios. Overall, it highlights the importance of friction in physical interactions and applications.

7. FRICTION

8.  An opposing force to an outside force that tries
to move a body surface past another body
surface.
 The direction of the frictional force is opposite
the direction of motion.
Friction

10. Why?

13. 1. Static Friction (ƒs)
– Friction present in bodies at rest
ƒs = μsN
2. Kinetic or Sliding Friction (ƒk)
– Friction present in moving bodies
ƒk = μkN
Types of Friction

14. Static & Kinetic Friction

15. 1. Quality or Nature of the Surfaces in Contact
Rough surfaces mean greater friction, smooth
surfaces mean lesser friction
Basis of the value of coefficients of static and
kinetic friction
Independent of the magnitude of the area in
contact
Factors Affecting Friction (1)

16. Quality of surfaces in contact affect friction.

17. MATERIALS STATIC
FRICTION
KINETIC
FRICTION
Rubber on dry ice 1.0 0.7
Rubber on wet concrete 0.7 0.5
Wood on wood 0.5 0.3
Waxed wood on wet snow 0.14 0.1
Metal on wood 0.5 0.3
Steel on steel (dry) 0.6 0.3
Steel on steel (oiled) 0.05 0.03
Teflon on steel 0.04 0.04
Bone lubricated with synovial fluid 0.016 0.015
Shoes on wood 0.9 0.7
Shoes on ice 0.1 0.05
Ice on ice 0.1 0.03
Steel on ice 0.4 0.02
*Values are approximate.
Source: Physics With Health Science Applications by Paul Peter Urone
Coefficients of Friction*

18. 2. Load or Normal Force
 Force with which the two surfaces are pressed
together
 Acts perpendicular to the surfaces in contact
Factors Affecting Friction (2)

19. F
θ θ
Normal Force
θ
W = mg
F
W = mg
N = W
N = W + F sin θ N = W - F sin θ
N = W cos θ

20.  Friction is proportional to the normal force.
 Static friction acts to keep the object from
moving.
 The force of kinetic friction acts when
the object is in motion.
 The force of static friction is generally
greater than the force of kinetic friction.
More about Friction

21. If the shoe fits…

22. Why rubber?

23. Push back or threaded?

24. . 1. A) Find the normal force on a block of wood of
mass 1 kg placed on a horizontal wooden surface.
Determine friction.
B) If the block of wood is pressed against a
vertical wooden wall with a force of 5 N,
what is the normal force?
Problem 1

25. .
2. A mass of 50 kg is kept in uniform motion
along a horizontal surface by a horizontal force.
If the coefficient of friction between the surface
and the mass is 0.5, how great is the friction force?
3. How much friction will there be in a knee joint
of a standing person of weight 500 N? How about
in your knee joint?
Problems 2 & 3

26. 4. An object of mass 10 kg is on a rough floor and
being pulled by force F = 15 N to the right. If the
coefficient of static friction between the object
and the floor is 0.2 and kinetic coefficient is 0.1,
a. is friction static or kinetic? Give the
magnitude.
b. Should the force cease to pull, how much
friction is present?
c. If the pull is doubled, determine friction.
d. If the floor is inclined at 10°, will the object
slide down?
Problem 4

27. A gem cannot be
polished without friction,
nor man perfected
without trials.
Lucius Annaeus Seneca