Mechanical Engineering Principles

WORK, ENERGY AND POWER 157

=5820 kN mm=5820 N m

=5820 J

Hence the work done as the tool moves through
100 mm is5.82 kJ

Now try the following exercise

Exercise 68 Further problems on work

1. Determine the work done when a force of
   50 N pushes an object 1.5 km in the same
   direction as the force. [75 kJ]


2. Calculate the work done when a mass
   of weight 200 N is lifted vertically by a
   crane to a height of 100 m. [20 kJ]


3. A motor supplies a constant force of
   2 kN to move a load 10 m. The force is
   then changed to a constant 1.5 kN and
   the load is moved a further 20 m. Draw
   the force/distance graph for the complete
   operation, and, from the graph, determine
   the total work done by the motor.
   [50 kJ]


4. A spring, initially relaxed, is extended
   80 mm. Draw a work diagram and hence
   determine the work done if the spring
   requires a force of 0.5 N/mm of stretch.
   [1.6 J]


5. A spring requires a force of 50 N to cause
   an extension of 100 mm.
   Determine the work done in extending
   the spring (a) from 0 to 100 mm, and
   (b) from 40 mm to 100 mm.
   [
   (a) 2.5 J (b) 2.1 J

]

6. The resistance to a cutting tool varies
   during the cutting stroke of 800 mm
   as follows: (i) the resistance increases
   uniformly from an initial 5000 N to
   10000 N as the tool moves 500 mm,
   and (ii) the resistance falls uniformly
   from 10000 N to 6000 N as the tool
   moves 300 mm.

Draw the work diagram and calculate the

work done in one cutting stroke.

[6.15 kJ]

14.2 Energy

Energy is the capacity, or ability, to do work. The

unit of energy is the joule, the same as for work.

Energy is expended when work is done. There are

several forms of energy and these include:

(i) Mechanical energy

(ii) Heat or thermal energy

(iii) Electrical energy

(iv) Chemical energy

(v) Nuclear energy

(vi) Light energy

(vii) Sound energy

Energy may be converted from one form to another.

The principle of conservation of energystates that

the total amount of energy remains the same in

such conversions, i.e. energy cannot be created or

destroyed.

Some examples of energy conversions include:

(i) Mechanical energy is converted to electrical

energy by a generator

(ii) Electrical energy is converted to mechanical

energy by a motor

(iii) Heat energy is converted to mechanical

energy by a steam engine

(iv) Mechanical energy is converted to heat

energy by friction

(v) Heat energy is converted to electrical energy

by a solar cell

(vi) Electrical energy is converted to heat energy

by an electric fire

(vii) Heat energy is converted to chemical energy

by living plants

(viii) Chemical energy is converted to heat energy

by burning fuels

(ix) Heat energy is converted to electrical energy

by a thermocouple

(x) Chemical energy is converted to electrical

energy by batteries

(xi) Electrical energy is converted to light energy

by a light bulb