UNIT 6:

WORK AND ENERGY...

Q. Define work. What is its SI unit?

Ans. Work is done when force is acting on a body displaces it in the direction of force.

o   Its SI unit is Joule ( J ).

Q. When does a force do work? Explain.

Ans. A work is said to be done when a force acting on a body displaces in its direction.

Q. Why do we need energy?

Ans. We need energy to do work. Without energy no work can be done. So no progress can be made without energy.

Q. Define energy. Give two types of mechanical energy.

Ans. The ability of a body to do work is called energy.

There are two types of mechanical energy.

o   Kinetic energy

o   Potential energy

Q. Define kinetic energy and derive its relation.

Ans. The energy possessed by a body due to its motion is called kinetic energy.

Derivation:

            Consider a body of mass “m” moving with initial velocity “”, after covering a distance “s” stops due to some force of friction and its final velocity becomes equal to zero “”. The body possesses kinetic energy and is capable of doing work against force of friction.

Therefore,

Kinetic energy of the body = work done by it due to its motion

As       

Since motion of the object is opposed, hence “a” is negative.

Using 3^rd equation of motion,

From equation 1 and 2,

Q. Define potential energy and derive its relation.

Ans. The energy possessed by a body due to its position is called potential energy.

Derivation:

            Let a body of mass “m” be raised up through height “h” from the ground. The body will acquire potential energy equal to the work done in lifting it to the height “h”.

Potential energy of the body = work done in lifting the body to the height “h”

P.E. = F x h

Her F = w,

P.E. = w x h

And w = mg. so,

P.E. = mgh

Q. Why fossils fuels are called non-renewable form of energy?

Ans. Fossils fuels took millions of years for their generation and are used once to get energy and are consumed completely. They cannot be generated artificially in short time. That is why they are called non-renewable form of energy.

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Q. Which form of energy is most preferred and why?

Ans. Solar energy is more preferable because it can directly be obtained from sun and do not have any kind of pollution.

Q. How is energy converted from one form to another? Explain.

Ans. Energy converts from one form to another when something or an action happens. For instance when climbing stairs, chemical energy in food is changed into kinetic energy by muscles, and into potential energy as the body is raised against gravity.

Q. Name the five devices that convert electrical energy into mechanical energy.

Ans.   

i. Electric motor                              ii. Electric fan

            iii. Elevator                                       iv. Electric vibrator

            v. Grinder

Q. Name a device that converts mechanical energy into electrical energy.

Ans. A.C generator converts mechanical energy into electrical energy.

Q. What is meant by the efficiency of a system?

Ans. Efficiency of the system is the ratio of required form of energy obtained from a system as output to the total energy given to it as input.

Q. How can you find the efficiency of a system?

Ans. Efficiency of the system can be found by the following formula,

Q. What is meant by the term power?

Ans. Power is defined as “rate of doing work”.

Q. Define watt.

Ans. It is the SI unit of power and defined as “the power of the body is one watt if it does work at the rate of 1 Joule per second ()”.