KINESIOLOGY

M Farrukh Shahzad

BSPT, PPDPT

Faculty of Rehabilitation Sciences

Lesson 5

Inertia

Friction

Lever

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

Example

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

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

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

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

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

Friction

Friction plays an important role in safety

measures

Non slip floors in gymnasiums

Slopes, stairs, non slip foot wear, walking aids

etc

Levers

Lever

Lever types

Applications

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

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.)

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.

Example

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

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

2nd order lever

In this class the weight is

between the fulcrum and the

effort.

In this effort arm always exceed

the weight arm.

3rd order lever

The effort is between the fulcrum and the

weight.

So the weight arm therefore always exceed

than the effort arm.

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)

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.

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.

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

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.

Examples