Physics Chapter 6|Work Energy 9th Class||tutoronlines

Work and Energy

Table of contents:

Number Name of topics
1 Work
2 Energy
3 Kinetic energy
4 Potential energy
5 Forms of energy
6 Interconversion of energy
7 Major source of energy
8 Efficiency
9 Power

Chapter short Questions

Q.1 The rope is parallel to the road?

Ans. Work is given by W=FScos

If the rope is parallel to the road. Than

So W=100×10×cos0

W=100× 10×1



Q.2 The rope is making angel of 30° with road?

Ans. As we know that W= FScos




Exercise short Questions

Q.1 Define work . What is its SI unit?

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

Unit of work:

In system international. Its unit is joule(J)

Q.2 When does a force do work? Explain.

Ans. When force act on the body and body cover some distance in the direction of force then we say work is done. And this work can be calculated by the formula.


Q.3 Why do we need energy?

Ans. We need energy to do different types of work in our daily life. When we say that body has energy, we mean that it has the ability to do work.


Energy is required to move

Energy is required to stop the moving objects.

Q.4 Define energy. Give two types of mechanical energy?

Ans. A body possesses energy if it is capable to do work.


Ability of a body to do work is known as energy.

Type of mechanical energy.

Mechanical energy possessed by a body is of two types.

Kinetic energy

Potential energy

Q.5 Define K.E and derive its relation?
Ans. Kinetic energy:

Kinetic energy is define as the energy possessed by a body due to its motion. An object that has motion, whether it may be vertical or horizontal motion, has kinetic energy. Kinetic energy depends on mass(m) of the object and speed (v) of the object.

The equation used to represent kinetic energy is:


Derivation of kinetic energy :

The formula of Work is,


where F is the force and s is the displacement in the direction of the force.

We know that,



W=m.a.s ——-(1)

From the equation of motion, we have,


As the body was initially at rest so u = 0


1/2v²=as ——- (2)

Putting the value of ‘as’ from eqn.(2) into eqn.(1) we get,


As kinetic energy is equal to work.



Q.6 Define potential energy and drive its relation?

Ans. The derivation of potential energy is provided here. Potential energy is defined as the energy that is held by an object because of its position with respect to other objects. The SI unit of potential energy is joule whose symbol is J. Potential energy term was introduced by Scottish engineer and physicist William Rankine in the 19th century.

The gravitational potential energy of an object, elastic potential energy of an extended spring, and electric potential energy of an electric charge are the most common type of potential energies observed.


Potential energy is the completed work to take a body to a certain height. For a body with mass m, h is the distance to which it is raised and g is the gravitational force acting on the body, then work done W is given as:

W = force * displacement

W = mg * h

∴ W = mgh

As the work done is equal to mgh which is gained by the object, potential energy P.Eis given as:

P.E= mgh

Therefore, the above is the derivation of potential energy.

Q.7 Why fossils fuels are called non-renewable from energy?

Ans. The fossils fuels took million of year for their formation. Thus, we call it non renewable resources.

Q.8 Which form of energy is most preferred and why?

Ans. Solar energy is the most preferred because it is the ultimate source of energy for life and sun rays do not pollute the environment. It is huge source of energy and if we find a suitable method to use a fraction of the solar energy reaching the Earth, then it would be enough to fulfil our energy requirements.

Q.9 How is energy converted from one form to another?

Ans. Note; If this question asked as a short Questions then write only one Example….As follows:

Energy cannot be destroyed however it can be converted into some other forms. For example, when we rub our hands together quickly, We will feel them warm. In this action, we use our muscular energy in rubbing hands as a result production of heat occur. In the process of rubbing hands, mechanical energy converts into heat energy.

Note: if ask as a long question then: Explain.

See 5 logo question

Q.10 Name the five devices that convert electrical energy into mechanical energy?

Ans. (i) Electric motor

(ii) Electric Fan

(iii) Drill machine

(iv) Grinder

( v) Sewing machine

(vi) Elevator

(vii) Juicer machine

Q.11 Name a device that converts mechanical energy into electrical energy?

Ans. Electric Generator is device which is used to convert the mechanic energy into electrical energy.

Q.12 What is efficiency of a system?

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

We can find efficiency by formula

% Efficiency= required form of output/total input energy×100

Q.13 How can you find the efficiency of a system?

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


Efficiency= Required form of output/ total input energy


% Efficiency= required form of output/ Total input energy×100

Q.14 What is power?

Ans. Rate of doing work with respect to time is called term power.



If we represented power by ‘P’, work by ‘W’ and time by ‘t’ then


Q.15 Define watt?

Ans. In system international, the unit of power is watt (W)

One watt:

A body has power of one watt if it does work at rate of 1 joule per second (1Js-1)

Bigger unit:




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