Class 9 Science Work & Energy (Questions from board papers)

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Science

Chapter-11

Work and Energy

(Questions from Board Papers)

Question 1:
Identify and state the type of transformation of energy in the following cases:
(a) When coal is burnt.
(b) In a thermal power plant [CBSE 2014]

Answer 1:
(a) Chemical energy stored in coal is converted to heat energy.
(b) Chemical energy of fuel is first converted into heat energy than to kinetic energy and finally to electrical energy.

Question 2:
(a) What is meant by potential energy of a body?

(b) A body of mass m is raised to a vertical height h through two different paths A and B. What will be the potential energy of the body in the two cases? Give reason for your answer. [CBSE 2014]

Answer 2:
(a) Potential energy is the energy possessed by a body due to its position and shape.
(b) The potential energy in both the cases is equal.
P.E. = m.g.h
It is independent of the path followed for attaining the height.

Question 3:
Calculate the total energy consumed in the month of November in a household in which four devices of power 500 W each are used daily for 10 h. [CBSE 2013]

Answer 3:
Given,
Power, P = 500 W;
Time, t = (4 × 10 × 30) h = 1200 h
Total energy consumed in the month of November
= P × t
= 500 × 1200
= 600000 Wh = 600 kWh [ ∵ 1 kWh =1000W]
= 600 unit. [ ∵ 1 kWh =1 unit ]

Question 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? [CBSE 2012]

Answer 4:
Work done = Force × displacement
= 140 × 15
= 2100 J = 2.1 kJ

Question 5:
A body of mass 2 kg is thrown up with a speed of 25 m/s. Find its maximum potential energy. [CBSE 2012]

Answer 5:
Maximum kinetic energy of the body = Maximum potential energy of the body.
Given,
Mass of the body, m = 2 kg
Speed, v = 25 m/s
Therefore, K.E. =1/2 𝑚𝑣2
=1/2 × 2 × 25 × 25
= 625 J.

Question 6:
Define work. How is work done measured?

Answer 6:
Work is said to be done whenever a force acts on a body causing its motion in the direction of the force.

Work done = Force × Displacement

For example, when a force F acts on a body it moves through a distance ‘s’ in the direction of the force.

Therefore, the work done, W = F × s

Question 7:
Is there any work done on a body in uniform circular motion? Justify your answer.

[CBSE 2012]

Answer 7:
If a body moves in uniform circular motion, the centripetal force ‘F’ acts along the radius towards the Centre and the displacement ‘s’ acts along the tangent to the circle. Thus, the angle between ‘F’ and‘s’ is 90°.

Therefore,

W = 0 [ ∵ Force is perpendicular to the displacement ]
Since W= 0, no work is done by the body in uniform circular motion.

Question 8:
Given below is a few situations, study them and state in which of the given cases work is said to be done. Give reason for your answer. [CBSE 2012]

(a) A person pushing hard a huge rock but the rock does not move.
(b) A bullock pulling a cart up to 1 km on road.
(c) A girl pulling a trolley for about 2 m distance.
(d) A person standing with a heavy bag on his head.

Answer 8:
(a) Work done = 0,
Reason- As the rock has not moved so displacement is zero.

(b) Work done = Positive
Reason – As the motion is in the direction of force applied.

(d) Work done = 0
Reason – As there is no movement in the position of the bag, therefore displacement is zero.

Question 9:
What is power? What is its SI unit? [CBSE 2010]

Answer 9:
The rate at which work is done is called power.

Power = work done / time. Its SI unit is Watt.

Question 10:
Name two devices which convert electrical energy into sound. [CBSE 2012]

Answer 10:
Loudspeaker and microphone.

Question 11:
Identify the kind of energy possessed by a running athlete. [CBSE 2011]

Answer 11: Kinetic energy.

Question 12:
The potential energy of a free falling object decreases progressively. Does this violate the law of conservation of energy? Why? [CBSE 2011]

Answer 12:
Decrease in P.E. = Increase in K.E. So there is no loss in the energy by the object. There is only the transformation of the energy.

Question 13:
When a ball is thrown vertically upwards, its velocity goes on decreasing. What happens to its potential energy as its velocity becomes zero? [CBSE 2011]

Answer 13:
According to the law of conservation, energy can neither be created nor be destroyed but can transform from one form to another.

Therefore, K.E. Lost by the ball = P.E. gained by the ball

So, as velocity of ball becomes zero then K.E. would be zero and the P.E.
would be maximum.

Question 14:
If the heart works 60 joules in one minute, what is its power? [CBSE 2011]

Answer 14:

Power = Work done / Time = 60 J/ 60 s = 1 W [ 1 Watt, W = J/s ]

Question 15:
A coolie is walking on a railway platform with a load of 30 kg on his head. How much work is done by coolie? [CBSE 2011]

Answer 15:
Coolie is carrying a load on his head so force exerted by it under gravity and displacement of the load makes 90 degree angle.

W = 0 [ ∵ Force is perpendicular to the displacement ]

Therefore, Work done by the coolie is zero.

Question 16:
A horse of mass 210 kg and a dog of mass 25 kg are running at the same speed. Which of the two possesses more kinetic energy? How? [CBSE 2011]

Answer 16:
Kinetic energy, K.E. = 1/2 𝑚𝑣2
If the velocity is constant, Kinetic energy is directly proportional to mass. So as mass increases, kinetic energy also increases. Here, mass of horse is greater than that of dog, so kinetic energy of the horse is greater than dog when speed is constant.

Question 17:
What will cause greater change in kinetic energy of a body? Changing its mass or changing its velocity. [CBSE 2010]

Answer 8:

Therefore, the change in velocity of a body will cause greater change in kinetic energy.

Question 18:
When a force retards the motion of a body, what is the nature of work done by the force? State reason. List two examples of such a situation. [CBSE 2014, 2013]

Answer 18:
The work done by the force is negative because this force acts opposite tothe direction of movement of the body, thereby hindering the motion.

Example:
1. Work done by the force of friction acting opposite to the direction of football.
2. Work done in Tug of war by losing team.

Question 19:
A lift carries a maximum weight of 2400 N to a height of 10 m with constant speed of 2 m/s. Find out the power and work done. [CBSE 2013]

Answer 19:
Given,
Force, F= 2400N
Height, h = 10 m
Speed, v = 2 m/s
Therefore, Work done = F × s = 2400 × 10 = 24000 J
And, Power = F × v = 2400 × 2 = 4800 W

Question 20:
What is meant by work done by a force? When do we call work done to be positive or negative? In a game of ‘tug of war’ state the type of work done by the winning and the losing teams. [CBSE 2013]

Answer 20:
When force applied on a body causes the body to move then the work is said to be done by the force.

Positive work done: when the body is displaced in the same direction that of the force then work done is positive. That is the angle between the force applied and displacement is 0.

Negative work done: when the body is displaced in the direction opposite to that of the force then work done is negative. Here the angle between the force applied and displacement is 180.

In the game of ‘tug of war’ work done by the winning team is positive and the work done by the losing team is negative.

Question 21:
Four persons jointly lift a 250 kg box to a height of 1 m and hold it.
(a) Calculate the work done by the persons in lifting the box.
(b) How much work is done for just holding the box?
(c) Why do they get tired while holding it? (g = 10 m/s^2). [CBSE 2012]

Answer 21:
(a) Given, Force, F = 250 × 10 = 2500 N
Displacement, s = 1 m
Work done = F × s = 2500 × 1 = 2500 J

(b) As there is no displacement in just holding the box, so no work is done i.e. Work done is zero.

(c) To hold the box men are applying force against the gravity, which involves their muscles to work, hence they get tired holding it.

Question 22:
(a) Name the type of energy possessed by a freely falling body of mass ‘m’ at the highest point ‘h’.
(b) Define the energy.
(c) A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy? [CBSE 2012]

Answer 22:
(a) Potential energy, P.E. is the energy possessed by a body of mass, m
after reaching a height, h. it is given by, P.E. = mgh
(b) The capacity to do work is known as Energy.
(c) K.E.of the object is converted into heat and sound energy.

Question 23:
Two masses m and 2 m are dropped from heights h and 2h. On reaching the ground, which body will have a greater kinetic energy and why? [CBSE 2012]

Answer 23:
K.E. of first mass, m = P.E. lost by mass m = mgh
K.E. of second mass, 2 m = P.E. lost by mass 2 m
= 2 m × g × 2h = 4 mgh
Therefore, mass 2 m will have 4 times greater kinetic energy on reaching
the ground.

Question 24:
Water is falling on the blades of a turbine at the rate of 8 × 102 kg per minute, height of fall is 50 m. Calculate the power given to turbine. (g = 10 m/s^2). [CBSE 2012]

Answer 7:
Given,
Height of fall, h = 50 m
Acceleration due to gravity, g = 10 m/s2
Mass of water, m = 8 × 102 kg
Time taken, t = 1 min = 60 s

Therefore, the power given to turbine = 6.67 × 10^3 W. [ using, Power = mgh/t ]