2. KINESIOLOGY
M Farrukh Shahzad
BSPT, PPDPT
Lesson 5
Inertia
Friction
Lever
Faculty of Rehabilitation Sciences

3. Inertia
 The resistance of a body to any change in its
state of rest or motion
 Body at rest, can remain at rest indefinitely
 While a moving body tends to continue
moving at a constant speed and in a straight
line unless acted upon by a force
 A property of matter that resists changes in
motion

4. Example

5. Example
 Man standing in a train, holding the handle for
stability
 Train is moving at a constant speed the man is
in a state of rest
 When the train slows down a the force applied
brings the man into a state of motion
 Weak muscles may be unable to exert
sufficient force to over come inertia

6.  May be able to produce movement or control
with assistance at the right moment
 Once inertia is over come then it is more
economical to continue moving

7. Example
 A railway truck needs considerable force to
start it moving but once its gets going it
continues until another force such as a
collision with the buffers of another truck
impedes it

8. Friction
 The force which opposes motion when one
surface slides upon another
 Can be sufficient to prevent movement
altogether
 Rough substances such as rubber when they
are in contact with one another i.e tyres
moving on a road

9. Friction
 Frictional resistance obtained during
movement is said to be less compared to
limiting friction
 Limiting friction is the friction obtained just as
sliding is about to set in

10. Friction
 Friction plays an important role in safety
measures
 Non slip floors in gymnasiums
 Slopes, stairs, non slip foot wear, walking aids
etc

11. Levers
 Lever
 Lever types
 Applications

12. What is a lever
 A lever is a rigid bar capable of movement
from a fixed point called a fulcrum
 A lever can also be a simple machine
consisting of a relatively rigid bar-like body
that may be made to rotate about an axis

13. Components of a Lever System
 Fulcrum – The center or axis of rotation of the
system.
 Moment Arm – The distance from any force or
weight that produces torque to the fulcrum.
 Force Arm – The distance from an applied force
to the fulcrum. (The moment arm of the force.)
 Resistance Arm – The distance from the
resistance to the fulcrum. (The moment arm of the
resistance.)

14. Work done by lever
 Work is said to be done by any lever when a
‘’force or effort’’ (E) applied at one point on
the lever acts upon another ‘’force or weight’’
(w) acting at 2nd point on the lever.

15. Example

16. Classes of levers
 There are three classes or orders of levers
 Every class is characterized by the relative
positions of the fulcrum, effort and weight.
 1st order lever
 2nd order lever
 3rd order lever

17. 1st order lever
 Fulcrum is between the effort and the weight.
 Fulcrum may be situated centrally, or either
towards the effort or the weight.
 Consequently the effort and weight arm may
be equal or one may exceed the other in length

18. 2nd order lever
 In this class the weight is
between the fulcrum and the
effort.
 In this effort arm always exceed
the weight arm.

19. 3rd order lever
 The effort is between the fulcrum and the
weight.
 So the weight arm therefore always exceed
than the effort arm.

20. Mechanical advantage
 The efficiency of a force in relation to the
lever depends upon two factors
1. Force exerted in the form of (W) or (E)
2. (W) and (E) perpendicular distance from the
fulcrum i-e weight arm or effort arm.
 The product of these two factors is known as
moment of force (Torque)

21. Weight and arm equal length
 An effort of magnitude equal to that of weight
will be required to lift it.
 No advantage is gained but the machine is
useful for measuring weights e.g common
balance.

22. Length of arm exceed weight
 Less effort would be required to raise the
weight.
 Advantage would be gained by the use of the
levers and this is known as mechanical
advantage.

23. Mechanical advantage
 Mechanical advantage gained by first order
lever when fulcrum is nearer to weight.
 In all levers of 2nd order.
 Never gained any mechanical advantage in 3rd
order levers.
 Mechanical advantage is the ratio of the
weight to the effort
M.A= W/E

24. Mechanical disadvantage
 Where the weight arm exceeds the effort arm
the mechanical disadvantage occurs.
 This happens in 1st order levers when the
fulcrum is near the effort.
 In all 3rd order levers.
 Mechanical disadvantage means that we have
to put more force than the actual weight to
accomplish our task.

25. Examples