M Farrukh Shahzad
Faculty of Rehabilitation Sciences
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
Man standing in a train, holding the handle for
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
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
The force which opposes motion when one
surface slides upon another
Can be sufficient to prevent movement
Rough substances such as rubber when they
are in contact with one another i.e tyres
moving on a road
Frictional resistance obtained during
movement is said to be less compared to
Limiting friction is the friction obtained just as
sliding is about to set in
Friction plays an important role in safety
Non slip floors in gymnasiums
Slopes, stairs, non slip foot wear, walking aids
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
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
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.
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
In this effort arm always exceed
the weight arm.
19. 3rd order lever
The effort is between the fulcrum and the
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
22. Length of arm exceed weight
Less effort would be required to raise the
Advantage would be gained by the use of the
levers and this is known as mechanical
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
Mechanical advantage is the ratio of the
weight to the effort
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.