Class 9 Science Chapter 10 – Work and Energy Intext Solutions |

Find here the NCERT Class 9 Science Chapter 10 – Work and Energy Intext Solutions with detailed and simple explanations. All intext questions and answers are prepared according to the latest NCERT textbook. These Class 9 Science solutions help students understand key concepts like work, energy, power, and their relationship. Useful for quick revision, homework, and exam preparation.
Download the Chapter 4 NCERT Solutions PDF and study offline anytime.

Class 9 Science Ch 9 :- Work and Energy Intext Solution :- PDF

-----------------------------------------------------------------------------------------------------------------------------------------------------------

 

--------------------------------------------------------------------------------------------------------------------------------------------------------------

WORK AND ENERGY 

INTEXT QUESTION ANSWER

Ques 1:- A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case ?

Ans :- When a force F acts on an object to displace it through a distance in its direction, then the work done (W) on the body by the force is given by :

Work done = Force x Displacement

Given:

Force exerted (F) = 7 N,    

Displacement (S) = 8 m

Therefore, work done, W = F x S

W = 7 N x 8 m    === > W = 56 Nm

Work done = 56 J

Thus the work done by the force is 56 J. 

Page NO. :- 116

Ques 1:- When do we say that work is done?

Ans :- Work is said to be done when force is applied on an object and the

object moves through a distance in the direction of application of force.

--- It is equal to the product of force and displacement.

Work done = Force x displacement

W = F x S

Ques 2 :- Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Ans :- When a force F displaces a body through a distance (S) in the direction

of the applied force, then the work done (W) on the body is given by the expression:

==> Work done = Force × Displacement

==> W = F × S

The SI unit of work is Joule(J)

Ques 3 :- Define 1 J of work.

Ans :- 1 J is the amount of work done by a force of 1 N on an object that displaces it through a distance of 1 m in the direction of the applied force.

1 Joule = 1 Newton (N) x 1 metre (m)

Ques 4 :- A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Ans :- Given Applied force (F) = 140 N , Displacement (d) = 15 m

Work done by the bullocks is :

W= F × d

W = 140 N x 15m = 2100 Nm

W = 2100 J (1 J = 1 Nm)

Therefore, 2100 J of work is done in ploughing the length of the field.

Page No. :- 119

Ques 1 :-  What is the kinetic energy of an object?

Ans :- The energy possessed by a body by virtue of its motion is called kinetic energy.

--- All moving objects possess kinetic energy and this energy can be transformed into other forms to perform tasks.

The formula for kinetic energy is:

E_k = ½ mv²

Where: -

E_k = kinetic energy, m = mass of the object, v = velocity of the object

Example: - (i) A speeding bullet has kinetic energy.

(ii) wind has kinetic energy. It is due to this energy that the wind can turn the blades of a windmill.

(iii) Flowing rivers water has kinetic energy.

Ques 2 :- Write an expression for the kinetic energy of an object.

Ans :- If a body mass (m) is moving with a velocity (v) then its kinetic energy 

(E_k ) is given by the expression.

E_k = ½ mv²

Ques 3 :- The kinetic energy of an object of mass (m) moving with a velocity of 5 m/s is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Ans :- Given Kinetic Energy of the object = 25 J

Velocity of the object (v) = 5 m/s

K.E. = ½ mv² ==> 25 = ½ x m x (5)²

==>25 x 2  = 25 x m

==> m = 50/25    

==> m = 2 kg

Mass of the object = 2 kg

(i) Now when velocity is doubled

V₁ = v x 2 = 5 x 2 = 10 m/s

==> m = 2 kg

\ K.E. = ½ (mv₁)²

K.E. = ½ x 2 x (10)²

==> K.E. = ½ x 2 x 10 x 10  

==> K.E. = 10 x 10

\ K.E. = 100 J

(ii) When velocity is increased three times then

v₂ = v x 3 = 5 x 3 = 15 m/s  

==> m = 2 kg  m = 2 kg

K.E. = ½ m (mv2)²

K.E. = (½) x 2 x  (15)²

==> K.E. = ½ x 2 x 15 x 15 

==> K.E. = 15 x 15

K.E. = 225 J

SI unit of Kinetic energy is Joule (J).

Page No. :- 123

 

Ques 1 :- What is power?

Ans :- Power is the rate of doing work or the rate of transfer of energy. If w is the amount of work in time t , then power is given by:

SI unit of Power is Watt (W).

--- The power may vary with time

Ques 2:- Define 1 watt of power.

Ans :- Power is 1 watt when a work of 1 Joule is done in 1 second.

OR

Power is 1 watt when the rate of consumption of energy is 1 joule per second.

Ques 3 :- A lamp consumes 1000 J of electrical energy in 10 s. What is its power ?

Ans :- Electrical Energy = work done = 1000 J  , Time = 10 s 

==> Power (P) = 100 W

Hence the power of the lamp is 100 W.

Ques 4:- Define average power.

Ans :- Average power is defined as the ratio of total work done by the body

to the total time taken by the body.