Back to: PHYSICS SS1
Welcome to class!
In today’s class, we will be talking about work done by the force field. Enjoy the class!
Work Done by Force Field
The work done by the force of gravity is the same whether a man comes down from a certain height (h) using a staircase or along the slope or he comes down from the same height using a lift or elevator. When one is lifting an object he exerts a force equal to the weight of the object in the vertical direction. Hence work is done. When you climb a stair, you exert a force equal to your weight to lift yourself upwards.
Hence, work done in lifting objects given by:
Workdone = Force (weight of the object) x height moved
Workdone (WD) = Force x distance
Worddone (WD) = mg x h
Worddone (WD) = mgh
Worked example
(1) A man of mass 70 kg climbs a ladder to the top of a building 40 m high. If g = 10m/s2, calculate the work done by the man in lifting himself to the height.
Solution
Force exerted = weight of the man = mgh
Distance = height climbed = 40 m
Work done = F x h
Word done (WD) = 70 x 10 x 40
Word done (WD) = 28,000J
(2) Calculate the work done in lifting a 50 kg bag of cement from the ground to the top of building 25 m high. (Take g = 10m/s2)
Solution
Force = weight = mg = 50 x 10 = 500N
Distance = height = 25 m
Work done = F x h
Word done (WD) = 500 x 25
Word done (WD) = 25,000J
Types of energy
Energy exists in different forms. These are:
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Mechanical energy
Mechanical energy is of two types: Potential and Kinetic.
Potential energy (P.E):
It is the energy a body possesses because of its position or condition. For instance, a body at a height above the ground has potential energy by virtue of its position. Also, a stretched catapult possesses potential energy by virtue of its condition.
Potential energy = mass x acceleration due to gravity x height
P.E = mgh
Kinetic energy (K.E):
This is the energy of a moving body.
K.E = ½ mv2
Where m = mass of the body
v = velocity of the body
Worked examples
(1) Calculate the Potential energy of a 5kg stone at a height of 10m. (Take g = 10m/s2)
Solution
P.E = mgh
Where m = 5kg, h = 10m and g = 10m/s2
P.E = 5 x 10 x 10
P.E = 500J
(2) What is the kinetic of a 2kg javelin moving with a velocity of 20m/s?
Solution
K.E = ½ mv2
Where m = 2kg, v = 20m/s and g = 10m/s2
K.E = ½ x 2 x (20)2
K.E = ½ x 2 x 400
K.E = 400J
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Chemical energy
Chemical energy is stored in food substances. We can also get chemical energy from fuel such as coal, petroleum products, etc. the chemical energy we obtain from the food eaten is utilized to do work when we move about and undertake various physical or mental activities. Also when fuel is burst, energy stored in it or released for various uses e.g. to drive a car.
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Heat energy
Heat energy is common in nature. We derive heat from the Sun and we use it for drying and for other purposes.
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Electrical energy
This is derived from electric charges whether in motion (current electricity) or at rest (Static electricity). Electrical energy is the most useful form of energy in modern technology. Most appliances such as television sets, a radio set, refrigerators, computers etc can only function when there is electricity.
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Sound energy
This is a form of energy, which stimulates the sensation of hearing. The vibration of the medium through which it is passing produces it.
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Light energy
Light energy enables us to see. We can obtain light by burning material, or by heating metal to a very high temperature. Light energy is also obtained from the Sun.
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Nuclear energy
This is the energy obtained from an atom either by breaking it up or by fusing its constituent particles together. This is called a nuclear reaction. The nuclear reaction includes radioactive decay, nuclear fission and nuclear fusion.
Energy conversion
Energy conversion also termed as the energy transformation, is the process of changing one form of energy into another. Energy conversion occurs everywhere and every minute of the day. There are numerous forms of energy like thermal energy, electrical energy, nuclear energy, electromagnetic energy, mechanical energy, chemical energy, sound energy etc. On the other hand, the term Energy Transformation is used when energy changes forms from one form to another. Whether the energy is transferred or transformed, the total amount of energy doesn’t change and this is known as the Law of Conservation of Energy.
The law of energy conversion
Thermodynamics is the study of how energy gets converted from one form to another. Details on how the laws of thermodynamics apply to the conversion of energy are given below.
The first law of thermodynamics states that
Energy can neither be created nor destroyed; it can only be transformed from one form to another.
This is also known as the law of conservation of energy or the law of energy conversion. There are various types and forms of energy. Some examples of everyday energy conversions are provided below.
| Scenario | Energy conversions involved |
| Rubbing both hands together for warmth | Kinetic Energy to Thermal Energy |
| A falling object speeding up | Gravitational Potential Energy to Kinetic Energy |
| Using battery-powered torchlight | In the battery: Chemical to Electrical Energy In the bulb: Electrical to Radiant Energy |
| In Geothermal Power Plant | Heat Energy to Electrical Energy |
| In Thermocouple | Heat Energy to Electrical Energy |
| In Hydroelectric Dams | Gravitational potential energy to Electric Energy |
| In Electric Generator | Kinetic energy / Mechanical work to Electric Energy |
| In Windmills | Wind Energy to Mechanical Energy or Electric Energy |
| In OTEC | Heat Energy to Electric Energy or Mechanical Energy |
| Using Microphone | Sound Energy to Electric Energy |
| Photosynthesis in Plants | Solar Energy to Chemical Energy |
| In Piezoelectrics | Strain to Electric Energy |
| In Electric lamp | Electric Energy to Heat and Light Energy |
| Burning of wood | Chemical energy to Heat and Light Energy |
| In Fuel cells | Chemical Energy to Electric Energy |
| In steam engine | The heat energy to Mechanical Energy |
| In Electric heater | Electric Energy to Heat |
In our next class, we will be talking about Viscosity. We hope you enjoyed the class.
Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.
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