Work
Work is said to be done when the point of application of force is displaced.
Work done depends on two factors.
- Force applied
- Distance travelled by the body in the direction of force.
The work done (W) by the force is measured by the product of magnitude of force (F) and the displacement (S) from of the point of application, in the direction of force.
Some other important units of work are erg, eV, MeV, kWh.
The relation among the units is given by
Power
The work done in unit time is called power
If the force "F" acts on the body which has velocity "V" at that instant, then the instantaneous power due to "F" is given by
Energy
Energy is the capacity to do work.
The unit of energy is same as that of "work"
Energy can be further classified into various well defined forms such as
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In many process that occur in nature, energy may be transformed from one form to other.
Mass can also be transformed into energy and vice versa
Mass can also be transformed into energy and vice versa
Kinematic energy :
Kinetic energy (K.E.) is that capacity of a body to do work by virtue of its motion.
If a body of mass "m" is moving with a velocity "v", then the energy required to bring it to rest is its kinetic energy.
since all values in the product will be positive, K.E. is always positive.
It does not depends on the direction of motion.
Kinetic energy depends on the frame of reference.
It does not depends on the direction of motion.
Kinetic energy depends on the frame of reference.
Potential energy :
The energy possessed by a body (or) system by virtue of its position (or) configuration is known as the "potential energy".
Example : A block attached to a compressed or elongated spring possesses potential energy.
Example : A block attached to a compressed or elongated spring possesses potential energy.
Note :
- Potential energy is defined for a conservative force only. For non-conservative force it has no meaning.
- Potential energy is independent of frame of reference.
Three types of potentials are introduced here. They are :
- Gravitational potential energy
- Elastic potential energy
- Electrical potential energy
Gravitational potential energy :
Gravitational potential energy is associated with the state of separation between objects which attract one another via - the gravitational force
The gravitational potential energy of two particles separated by distance "r" is given by
If the force between two objects is attractive, then PE is negative. If the forces between two objects is repulsive then PE is positive
If a body of mass "m" is raised to a height "h" from the surface of the earth, the change in PE of the system = mgh
[ when h <<< R (radius of the earth) ]
If a body of mass "m" is raised to a height "h" from the surface of the earth, the change in PE of the system = mgh
[ when h <<< R (radius of the earth) ]
Elastic potential energy :
Electric potential energy :
The electric potential energy of two point charges separated by distance "r" in vacuum is given by
If the force of interaction between two charges is attractive, then PE is negative.
Electrical potential energy depends upon medium between charges.
Electrical potential energy depends upon medium between charges.
Mechanical Energy :
The mechanical energy of a system is the sum of the potential energy and the kinetic energy.
A body can have mechanical energy without having either kinetic energy or potential energy
Heat energy (Thermal energy)
Heat is the energy that is transformed between a system and its environment because of a temperature difference that exists between them.
Heat is an internal energy that consists of the kinetic and potential energies associated with the random motion of the atoms, molecules and other microscopic bodies within the object.
Heat is an internal energy that consists of the kinetic and potential energies associated with the random motion of the atoms, molecules and other microscopic bodies within the object.
Heat absorbed or liberated :
sound :
Sound is a form of energy, that is produced by a body when it is in the state of vibration. It propagates in the form of longitudinal waves through elastic media through elastic media and causes sensation of hearing.
Light :
Light is a form of energy, which causes sensation of vision. Light travels from one place to another place in the form of electromagnetic waves. Electromagnetic wave can transport energy and deliver it to a body on which it falls. The amount of energy per unit time is transported by electromagnetic wave in a medium.
Velocity of light is different in different media and is maximum in vacuum.
Mass Energy :
Mass is also a form of energy.
Example : When atom bomb explodes, a heavier atom splits into lighter atoms. In this process, some of the mass disappears. This mass reappears in the form of heat., light and sound energies.