force , work and energy

7/30/2019 Force , Work and Energy

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Force,Work

and Energy


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Force and Work The force acting on a body is directly proportional to the

product of its mass and acceleration.

The SI unit of force is newton ( N )

Consider ,a force F acts on a body ,moves the bodythrough a distance S , then

Thus ,the work done is equal to the product of force and

distance moved by body in the direction of force. If the direction of displacement S is inclined to F at an

angle of ,then , W = F cos x S

The SI unit of work is Joule.

F = ma

work done (W) = F x S


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Power and Energy

The rate of doing work is called power.

power = work done (w) / time

The unit is watt (W) . 1 W = 1 joule /sec.

Energy is the ability to do work.

It is measured by the amount of work that can be performed.

Unit is joule.

There are many forms of energy : mechanical energy , Heat

energy ,light energy ,electrical energy , chemical and atomic

energy.


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2 forms of mechanical energy

1. Potential energy : The energy the body possess by virtue of its

position or state of strain.

e.x. Stretched rubber ,compressed air, water stored up in reservoir

2. Kinetic energy : The energy possessed by the body by virtue of

its motion.

e.x. Working of saw mill ,moving bullet

# Law of conservation of energy :

Energy can neither be created nor be destroyed ,but energy

can be transformed from one form to another form.

e.x. Cycle ,fan, microphone ,when we ignite crackers


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Forces of the Body

The term force is defined as a push or a pull.

Dynamics is a study of forces acting on a moving body.

Statics is the study of forces on bodies on rest.

Both dynamics & statics together are called mechanics.

In statics the force is a graphical representation , which has

magnitude ,direction and point of application.

Moment of a force : It is defined as the turning effect of a

force on a body and is equal to the product of force and

the perpendicular distance between the axis of rotation &

the direction of force.


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Parallel forces :

If 2 equal forces are acting in the same direction ,they

are called like parallel forces ;and If in opposite direction

are called unlike parallel forces.

A resultant is a single force acting on a body that

effectively replace all other forces acting on the body.

Forces on a Rigid body : A rigid body is one which does

not undergo any change in shape and volumem,when

external forces are acting on it.

Couple : 2 equal unlike parallel forces acting on a rigid

body constitute a couple . It tends to rotate the body


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The moment of the couple is equal to the product of one of

the force and the perpendicular distance between thedirection of the 2 forces.

The moment of the couple is called torque.

e.x turning a water tap ,opening the cap of bottel.

Friction : When a body slides over the surface of another

body ,there is an opposing force acting opposite to the

direction of motion. This opposing force is friction.

Benefits : breaks in automobiles ,while walking.

Friction provides lots of wastage. e.x wear & tear of machines

, produces heat , reduces efficiency of machines.


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Simple Machines A simple machine is a device in which a force is applied at 1

point in one direction is made available at some other point

in some other direction.

E.x are : levers , pulley ,screw , gears , wheal and axle , and

the inclined plane.

The force applied on simple machine is called effort /power.

The force overcome by machine is called load / weight.

Principle of simple machine : when effort is applied on a

machine , the point of application moves through a certain

distance . Work is done by machine .ie input . The load

moved by machine through a certain distance. This work

done by machine is output.


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Mechanical advantage (M.A) :

M.A = Load / effort

Velocity ratio ( V.R) :

V.R = distance moved by effort

distance moved by load

Efficiency () :

- The efficiency of simple machine is define as the ratio of the

work done on the load to the work done by the effort.

= M.A x 100 %

V.R


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Traction : It is a clinical practice in which parts of a human

body is applied with a mechanical pull ,in order to extend or

hold it in a desired position for a long time.

It works on principals of Newtons third law of motion.

Usually traction is applied on the head or body or limbs in two

directions.

Various types pulleys are employed in application of traction.

The fraction is applied in the direction in which the weight is

applied.

To counter traction ,the bed is elevated.


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Body Mechanics

Bones of the body acts as levers for movement.

The fulcrum is situated at the joint where movement occurs.

The load is the weight of the part to be moved ,acting at its

center of gravity and the effort is provided by the muscles

producing the movements ,acting at their point of insertion.

Examples are :

1. first order lever : skull balance at atlanto-occipital joint

2. Second order lever : Action of brachioradialis as flexor of

elbow

3. Third order : flexion of hip joint :


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Applications of forces in nursing

Forces acting on human body :

- Inside forces : from the action of muscles

- Outside force : gravity , friction , forces from medical applications

Friction : In human ,bony surfaces are covered with smooth

hyaline cartilage and lubricated with synovial fluid.

Inclined plane : the principal of inclined plane may be applied in

the use of flat board for exercising muscles.

Angel of pull of a muscle

Pulley systems : are used for raising patients trunk in sling

suspension.

force , work and energy

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