WORK, ENERGY AND POWER
Work can be defined as the product of force and distance moved in the direction of the force. Work is a scalar quantity. The unit of work is joules. Note that 1 Newton-metre is equivalent to 1joule (1Nm = 1J).
Workdone = force * distance
When the body is on an inclined plane like the diagram below
If the force is acting on the body at a particular angle,Then, Workdone = fdcosᶱ
Where ᶱ is the angle of inclination.
Energy can be defined as the ability to do work. A body without substantial amount of energy will not be able to do work.
- Mechanical Energy
- Sound Energy
- Heat Energy
- Light Energy
- Chemical Energy
- Electrical Energy
- Nuclear Energy
This is the sum of kinetic and potential energy.
i.e Mechanical Energy = K.E + P.E
Let's us take them one after the other and see their meaning and how they can be calculated.
This is the energy possessed by a body by virtue of its motion.i.e a moving body possesses kinetic energy. It can be calculated by using this formula
Where ‘m’ is the mass in kg and ‘v’ is the velocity in m/s
P.E = mgh
Where ‘m’ is the mass in kg,’g’ is the acceleration due to gravity and ‘h’ is the height in metre.
This is produced when an object vibrates. The sound vibrations cause waves of pressure that travel through a medium such as air,water, wood or metal. Sound is produced by a vibrating body. It is an example of mechanical wave.
Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases. Heat energy is the energy that is transferred from a hot body to a cold body as a result of temperature difference. Transfer of heat between two bodies can be carried out using a device called calorimeter.
Light energy is a form of electromagnetic radiation. Light is the only form of energy visible to the eyes.
This is the energy that is stored in the bonds of chemical compounds. Chemical energy is released during a chemical reaction. It is also the energy release or absorbed during the formation of chemical compounds.
Electrical energy is the energy caused by the movement of electric charges called electrons. The faster the charges move, the more electrical energy they carry.
This is the energy released during nuclear fission or fusion. Nuclear fission is the breaking down of heavy nucleus to form two lighter nuclei of approximate equal masses with the release of energy and radiations while Nuclear fusion is the combination of two lighter nuclei to form heavy nucleus. The energy released is called Nuclear Energy.
From the diagram above,it can be seen that at the top, the potential energy possessed by the body is maximum. Few seconds before the body hits the ground the kinetic energy possessed by the body is maximum. At a certain height, the potential energy is equal to kinetic energy. i.e P.E = K.E
Devices that uses the principle of conservation of energy are:
- Solar cell: this convert solar energy to electrical energy.
- Electric cell: this convert chemical energy to electrical energy.
- Electric cooker: this convert electrical energy to heat energy.
- Electric iron: this convert electrical energy to heat energy.
- Mouthpiece: this convert sound energy to electrical energy.
- Dynamo : this converts mechanical energy to electrical energy.
- Electric motor: this converts electrical energy to mechanical energy.
- Microphone: this converts sound energy to electrical energy.
- Electric bulb: this converts electrical energy to light and heat energy.
- Loud speaker: this converts electrical energy to sound energy.
Power is the rate at which work is done. The unit of power is watts. Power can be calculated using the following formulae.
- If the cage containing a truck of coal weighing 70kg is raised to height of 50m in 1min, what is the total power expended? (g=10m/s²)
- If a pump is capable of lifting 200kg of water through a vertical height of 20m in 10min, the power of the pump is
- In the figure below, the workdone by the force of 20N inclined at an angle of 60° to the object dragged horizontally to a distance of 4m is
Solution
- A body of mass 6kg is acted on by a constant force of 12N for 2sec. The kinetic energy gained by the body at the end of the time is?
- Renewable Energy Resources: These are resources that can be replaced after they are used. Examples are solar, hydro, wind etc
- Non-Renewable Energy Resources:These are resources that cannot be replaced after they are used. Examples are Nuclear, petroleum, coal etc
- A man of mass 50kg ascends a flight of stairs 5m high in 5secs. If acceleration due to gravity is 10m/s². The power expended?
- A girl whose mass is 20kg climbs up 25steps each of height 15cm in 10secs. Calculate the power expended (g= 10m/s²)
- A boy drags a bag of rice along a smooth horizontal floor with a force of 2N applied at an angle of 60° to the floor. The workdone after a distance of 3m is
- If a pump is capable of lifting 5000kg of water through a vertical height of 60m in 15min, the power of the pump is
- An object is moving with a velocity of 5m/s. At what height must a similar body be situated to have a potential energy equal in value with the kinetic energy of the moving body?(g=10m/s²)
- A force of 6N acts horizontally on a stationary mass of 2kg for 4s. the kinetic energy gained by the mass is ?