Work Energy

, & Power Essay, Research Paper

Work

Work is said to be done when the point of application of a force moves and is measured by the product of the force and the distance moved in the direction of the force.

Work = force x distance moved in direction of the force

The SI unit of work is called the joule (J) and is the work done when the point of application of a force of 1 Newton (N) moves through 1 meter (m) in the direction of the force. Larger units used are the kilojoules (kJ) and the mega joule (MJ).

1 kJ = 1,000 J (or 103 J)

1 MJ = 1,000,000 J (or 106 J)

It follows that an engine which exerts a force of 9000 N over a distance of 6 m will do 9000 x 6 = 54 000 J, or 54 kJ.

Energy

Anything which is able to do work, as defined above, is said to possess energy, and therefore

Energy is the capacity to perform work.

It may take many forms, such as nuclear, chemical, heat, mechanical or electrical energy. If we ignore the theoretical atomic physics, which never affects electrical-craft work, it is true to say that, whereas energy can be converted from one from one form to another, it can be neither created nor destroyed. For instance, coal, or oil, containing chemical energy, is burned in the boilers of a power station, and produces heat. This heat evaporates water to become steam under pressure, which is fed to a turbine where mechanical energy is produced heat. This turbine drives an alternator, which produces electrical energy. The unit of energy is the same as the work it is capable of performing, i.e. the meter Newton (joule) or in other words

Mechanical Energy

In mechanics, energy is divided into two kinds called potential and kinetic energy respectively (abbreviated p.e. and k.e.).

Kinetic energy is the energy, which a body has by reason of its motion.

Potential energy is the energy something has by reason of its position or state.

k.e. = ?mv2 and p.e. = Mgh

Where:

M = mass (in kilograms)

g = gravity (9.81)

h = height (in meters)

v = velocity (in m/s)

Obvious examples of kinetic energy are moving bullets or hammer heads. These are able to do work by overcoming forces when they strike something. A heavy flywheel stores energy in the form of rotational kinetic energy and so keeps an engine running smoothly in between the working strokes of its piston.

One of the commonest forms of potential energy is that possessed by a body when it is above the level of the earth’s surface. When something is lifted vertically, work is done against its weight and this work becomes stored up in the body as gravitational potential energy.

Another example is the elastic potential energy stored up in a wound clock-spring.

Electrical Energy

Essentially, an electric current is a movement or flow of minute particles called electrons. Each electron has the same charge of electricity and the movement of these particles leads to the development of kinetic of potential energy.

Chemical Energy

Energy that is stored in matter and released during a chemical reaction, such as the combustion of a fuel.

Thermal Energy

A form of energy directly associated with and proportional to the random movement of molecules of a substance. This movement may be variously generated by combustion, friction, chemical action, radiation, etc.

Transformation of energy from one kind to another

The law of conservation of energy which was mentioned earlier states that energy is neither created nor destroyed but only changes from one form to another. Work and energy are, both measured in the same units, namely, joules. The world we live in provides energy in many different forms, of which the most important has been, chemical energy. The utilization of the latent chemical energy in coal, oil and gas, released in the form of heat to drive stream turbines and internal-combustion engines, has been a major factor in the development of modern civilization.

Power and its units

Machines may be classified by the speed with which they do work; thus there are motorcar engines of small “power”, as they are rated, or large power.

Power is defined as the rate of doing work,

or average power = work done

Time taken

The SI units of power is called the watt (W) and is a rate of working of 1 joule per second

Thus, 1 W = 1 J/s

Larger units used are the kilowatt (kW) and the megawatt (MW)

1 kW = 1 000 W (or 103 W)

1 MW = 1 000 000 W (or 106 W)

WORKSHEET

1) Calculate the power of a pump which can lift 200 kg of water through a vertical height of 6 m in 10 s. (Assuming g = 9.81 m/s2)

2) A boy whose mass is 40 kg finds that he can run up a flight of 45 steps, each 16 cm high, in 5.2 s. (Assuming g = 9.81 m/s2)

3) A man whose mass is 75 kg walks up a flight of 12 steps each 20 cm high in 5 s. Find the power he develops in watts?

4) A vertical force of 58 720 N lifts a vehicle through a distance of 1.5 m on a hydraulic ramp of hoist. Find the power required to lift it in (a) ? min, (b) ? min, (c) 1 min.

5) A bundle of conduit has a mass of 200 kg. What is its weight? What work must be done in lifting the conduit from the floor on to a table rack 2 m high?

6) A force of 100 N will just move a van on a level road. What work will be expended in pushing the van 15 m?

7) An electric motor drives a pump which lifts 1000 L (liters) of water each minute to a tank 20 m above normal water level. What power must the motor provide if the pump is 50% efficient? 1L of water weighs 9.81 N.

8) A sling is marked as having a safe working load of 2000 kg. What weight will it support safetly?

9) A machine requires a force of 500 N to lift it. What is its mass?

10) The petrol engine of a builder’s hoist is to be replaced with an electric motor. What should be he rating of the motor if it must be capable of lifting 2400 N through 32 m in 24 seconds, the hoist gear being 80% efficient?

Note that efficiency is:

Efficiency = input x 100%

Output

Bibliography

Motor Vehicle Mechanics : Third Edition

By Technitians Examinations of the City and Guilds of LOndon Institute.