WORK-POWER-ENERGY-FITTER ONLINE NOTE

WORK-POWER-ENERGY

1. WORK

i)- Work is said to be done by a force

ii)- When its point of application moves in its own direction and is measured by the product of the force and the displacement of the force in its own direction.

iii)- The work is measured by the product of the force applied and the distance through which the body is displaced.

iv)- Applied force ‘F’ moves a body through a distance’s.

Work done = Force Applied x Distance Moved.

2. UNITS OF WORK

i)- Units of work are the combined units of force and displacement. 

ii)- Force may be expressed in absolute as well as practical units.

iii)- Absolute units of work are  joule .

iv)- The S.I. unit of work is 1 joule which is the work done by a force of moving the body through a distance of 1 metre. 

v)- Therefore joule = 1 N x 1 metre = 1 N.m( 1 joule)

1)- 1 dyne x 1 cm = 1 erg

2)- 1 newton x 1 metre= 1 joule

3)- 1 foot x 1 Pound = 1 foot Pound

4)- 1 gram x 1 cm = = 1 gram cm

5)- 1 kg x 1 metre = 1 kg metre

6)- 1 foot x 1 pound = 1 foot pound

7)- 1 kg metre = 9.8 joules

8)- 1 joule = 10' ergs

Example 1. 

A force of 250 newtons acted upon a body and the body has been moved through a distance of 15 metres. Calculate the work done ?

Solution:- 

Force applied= 250 newtons

Distance moved = 15 metres

Work done = Force applied x Distance moved

=250 x 15 =3750 joules Ans.

Example 2. 

A load of 15.5 kg is lifted through a height of 4.4 metres,Find out work done in (i) kg metre, (ii) joules.

Solution:-

Work done = Force x Distance moved

= 15.5 x 4.4

= 68.2 kg metres Ans.

But 1 kg metre = 9.8 joules( NOTE IT)

68.2 kg metres = 68.2 x 9.8

= 668.36 joules Ans.

Example 3. 

A body of mass 100 kg rests on a horizontal plane, the value of coefficient of friction between the body and plane being 0.025. Find the work done in moving the body through a distance of 10 metres along the plane ?

Solution:-

Force applied = Coefficient of friction x Normmal reaction

= 0.025 x 100 = 2.5 kg

Distance moved = 16 metres

Work done= Force applied x Distance moved

= 2.5 x 16 = 40 kg metres Ans.

Example 4. 

If a pump can raise 100 litres of water through a height of 200 metres in one minute, how much work it can do in one hour?

Solution: 

Weight of water lifted = 100 kg (1 litre water weight = 1 kg)

Height upto which water lifted = 200 metres

Work done in one minute = 100 kg x 200 metres = 20000 kg metres

Work done in one hour (i.e., 60 minutes)

= 20000, x 60 = 1200000 kg metres

Work done in one hour = 1200000 kg metres Ans.

2. POWER

i)- Power of a machine is the rate of doing work.

ii)- It is the work done in unit time.

Power = Work done/Time in seconds(FORMULA)

UNITS OF POWER

i)- The absolute unit of power is watt. 

ii)- One watt is that power which is required to perform 1 joule of work in 1 second.

iii)- 1 watt = 1 joule/second = 107 ergs/second

iv)- 1 kilowatt = 1000 watts.

v)- The practical unit of power is Horse Power (H.P.). 

vi)- One horse power (HP) is the amoun of work, a standard horse can do in one second. 

vii)- This value in metric system is 75 kg metres per second and in British system is 550

Foot Pounds/second.

viii)- 1 H.P. (Metric) = 75 kg metres per second or 735.5 watts(REMEMBER IT)

ix)- 1 H.P. (British) = 550 foot pounds/second or 746 watts(REMEMBER IT)

HORSE POWER OF ENGINES

The following terms are generally used in case of engines

Indicated Horse Power (I.H.P)

The actual power generated in the engine cylinder is called as the Indicated Horse Power (L.H.P.).

Brake Horse Power (B.H.P.)

i)- The Brake horse power is the power available to do useful work.

ii)-  B.H.P is always less than I.H.P. due to losses to overcome frictional resistance.

Mechanical Efficiency:-

i)- The ratio of Brake Horse Power to Indicated Horse Power is generally termed as mechanical efficiency. 

ii)- Mechanical efficiency is generally expressed in percentage.

iii)- Mechanical efficiency  = B.H.P/I.H.P x 100

iv)- Work done by a force = Magnitude of the force x distance moved by the body 

v)- Power = Total work done / total time taken

3. ENERGY

i)- Energy of a body is its capacity for doing work.

ii)- A system can have this energy in various forms, such as electrical, mechanical, heat chemical, atomic energy etc. 

iii)- Energy of one form can be transformed to other form, 

iv)- It cannot be created or destroyed. 

v)- If one form of energy disappears, it reappears in another from. This principle is known as law of conservation of energy.

USES OF ENERGY

i)- Energy of any form may be transferred to mechanical energy to do a mechanical work.

 ii)- For example, in heat engines, heat energy is converted into mechanical energy in moving the pistons and mechanical work is done in driving vehicles, pumps etc. 

iii)- In such a way, electrical energy supplied to a lathe is transformed to mechanical energy in rotating the spindle and mechanical work is done in turning a job.

POTENTIAL ENERGY AND KINETIC ENERGY

Mechanical energy may further be classified into potential energy and kinetic energy.

Potential energy is the energy, a body possesses because its position.

Potential energy = m.g.h. joules or m.h. kg metres (REMEMBER IT)

m = Mass of body in kilograms.

g = Acceleration due to gravity = 9.8 metres/second.

h = Height in metres.

Kinetic energy is the energy a body possesses because of its motion.

Kinetic energy = 1/2 mv2 joules or mv2/2g kg meter

m = Mass of body in kilogram

v = Velocity of the body in metres/second

g = Acceleration due to gravity = 9.8 metres/second2

EXAMPLES FOR POTENTIAL AND KINETIC ENERGIES

Examples for potential energy

(i) A body placed at a height possesses potential energy because it can do work when let fall.

(ii) Water in an overhead work.

(iii) Compressed gas in an overhead tank.

(iv) Wound up spring of watch etc.

Examples for kinetic energy:

(i) A moving train possesses kinetic energy because it can do work in pushing or pulling a body due to its motion.

(ii) Flowing water.

(ii) Blowing wind. 

(iv) Rushing steam.

(v) Rotating wheels etc.

TRANSMISSION OF POWER BY A BELT-PULLEY DRIVE

i)- When the applied force causes or tends to cause the rotation of a body about center, the action is called "Twisting Moment" or "Turning Moment or simply torque. 

ii)- Torque is the product of the applied force and the perpendicular distance between the line of action of the force and the moment center.

iii)- Torque= Applied force x Perpendicular distance

H.P transmitted = 2πNT/4500

H.P transmitted is also given by = (T1- T2)in kg x velocity in meters per minut/4500

T1-T2 = Net tension in the belt in the kilograms

N = Rotational speed in RPM

T = Torque in kg metres

1 kg 9.8 Newtons.