Class 9th
Physics
Chapter 06
Work & Energy
Student Learning Objectives (SLO)
-
Energy| Forms of energy | Work-energy thoerem | Law of conservation of energy:
The energy of body is defined as ” The ability of body to do work is known as energy. ”
Every matter in the universe needs energy for doing work. Energy is either in the form of kinetic or potential energy.
- Unit of energy:
The unit of energy is same as that a work. i.e Joule which is equal to N.m.
- Forms of energy:
In this universe energy is found in different forms.
- Chemical Energy:
The energy which is found in the bonds of atoms when the chemically react with each other is
Chemical Energy. This energy can be in different forms.i.e energy found in carbohydrates in food and energy in gasoline is chemical energy.
- Radiant Energy:
Radiant energy is the energy which travels in the form of electromagnetic waves. Radiant (light
energy ) is necessary for the survival of life on earth. Solar cells convert Radiant energy directly into electrical energy.
- Electrical energy:
When a charge is moving it contains electrical energy. When an electron moves from
negatively to positively charged objects it produces electricity.
- Heat energy:
It is that form of which travels from hot objects to cold objects due to difference of
temperature. e.g fire burning in the room transfer he to hold room and keeps it warm.
- Sound energy:
When the molecules of an object vibrate back and force it produces energy in the form of
wave which is known as sound wave. This energy is known as sound energy. It causes ear drum of humans to vibrate.
- Nucleur energy:
Energy which is obtained from nucleus of an atom is known as nuclear energy. Process of
fission and fusion produces nuclear energy.
Work-energy thoerem:
Work energy theorem shows equivalence between work and energy. This theorem state
that ” Work and energy are same physical entities and can be converted into one another. When we do work on an object and it moves through a displacement S this work is converted into Kinetic energy of body and if we lift the object to a height this work is stored in the form of potential energy in the object.
work done = change in energy
Law of conservation of energy:
This law states that , ” energy cannot be created and cannot be destroyed in any
physical or chemical process. It is always converted from one form to another form but the total amount of energy remains constant. ” e.g when an object is dropped from a certain height it’s potential energy is converted into kinetic energy . As soon as it approaches the earth it’s potential energy converts into Kinetic energy but Total amount of energy remains constant.
- Applications of Law of conservation of energy:
There are following applications of law of conservation of energy.
- A diver on a spring board:
The diver has stored chemical potential energy in his/her muscles. He uses this energy
to bend the diving board. When diving board is bent , elastic potential energy is stored in the board , which is then converted into kinetic energy as upward push.
- Generation of electricity:
Water is moved to a certain height, it gains gravitational potential energy. When water
falls from height this Gravitational potential energy is converted to Kinetic energy of turbine. Turbine is connected to generator which converts this kinetic energy into electrical energy.
- Pole Vault:
When a pole vaulter initially runs , he gains Kinetic energy. Vaulter the plants the pole
and this Kinetic energy of Vaulter is converted to elastic potential energy of Pole. Pole vaulter is then lifted by the elastic potential energy of Pole getting gravitational potential energy. His gravitational potential energy is converted to Kinetic energy of athlete and then this energy is dissipated into deforming that cushion where he lands.
SLO Question:
What will be energy conversions when an object is thrown vertically upward and then comes back to hit the ground?
Answer :
When an object is thrown vertically upward it has kinetic energy. As it’s speed is slowing down so as it gains height it’s kinetic energy decreases and Potential Energy increases . At the highest points the body stops for a moment. It’s Velocity at that point becomes zero so Kinetic energy will be zero . At that point all the Kinetic energy which body possessed is converted to Potential Energy.
After that body starts moving downward, height decreases so Potential Energy decreases and Kinetic energy increases as speed of body increases .When body strikes the earth surface it has maximum kinetic energy and zero potential energy . When it strikes the surface of Earth then all the energy it possesses is dissipated to different forms such as sound energy, heat energy and other forms.
