Engineering Mechanics

Chapter 30 : Work, Power and Energy „„„„„ 601

* First of all, the term horse power was introduced by James Watt, during his experiments on his engine.

He chose a normal horse and found that it could do a work of 33 000 ft-lb (converted to 4 500 kg-m in

metric units) in one minute. He thus, adopted this measure for comparing the performance of his

engines.

We know that distance travelled in 5 minutes

s = 75 × 5 = 375 m

and work done by the horse, W = Force × Distance = 300 × 375 = 112 500 N-m

= 112.5 kN-m = 112.5 kJ Ans.
Example 30·2. A spring is stretched by 50 mm by the application of a force. Find the work
done, if the force required to stretch 1 mm of the spring is 10 N.

Solution. Given : Spring stretched by the application of force (s) = 50 mm ; Stretching of
spring = 1 mm and force = 10 N.

We know that force required to stretch the spring by 50 mm

= 10 × 50 = 500 N

∴

500

Average force 250 N

2

==

and work done = Average force × Distance = 250 × 50 = 12 500 N-mm

= 12.5 N-m = 12.5 J Ans.

30.4. POWER

The power may be defined as the rate of doing work. It is thus the measure of performance
of engines. e.g. an engine doing a certain amount of work, in one second, will be twice as powerful
as an engine doing the same amount of work in two seconds.

30.5. UNITS OF POWER

In S.I. units, the unit of *power is watt (briefly written as W) which is equal to 1 N-m/s or
1 J/s. Generally, a bigger unit of power (kW) is used, which is equal to 10^3 W. Sometimes, a still
bigger unit of power (MW) is also used, which is equal to 10^6 W.

30.6. TYPES OF ENGINE POWERS

In the case of engines, the following two terms are commonly used for power.

1. Indicated power. 2. Brake power.

30.7. INDICATED POWER

The actual power generated in the engine cylinder is called indicated power (briefly written as
I.P.). Sometimes, the indicated power is also defined as the power, which is fed into the engine
in the form of steam or calorific value of the fuel.

30.8. BRAKE POWER

It has been observed that the entire power, generated by the engine cylinder, is not available
for useful work. This happens because a part of it is always utilized in overcoming internal friction
of the moving parts of the engine. The net output of the engine (i.e. I.P. – Losses) is called brake
power (briefly written as B.P).