Question 1

Q1: Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'. - Suma is swimming...

Accepted Answer

According to the scientific definition of work, work is done when a force acts on an object and the object is displaced. Therefore: - Suma is swimming: Work is done because she is applying a force to move through the water, resulting in displacement. - A donkey is carrying a load on its back: Work i...

Question 2

Q10: An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kineti...

Accepted Answer

Given: Mass, $m = 40 	ext{ kg}$ Height, $h = 5 	ext{ m}$ Acceleration due to gravity, $g = 9.8 	ext{ m/s}^2$ (approx. $10 	ext{ m/s}^2$) To Find: Potential energy at height $h$, $E_p$ Kinetic energy at half-way down, $E_k$ Formula: Potential energy, $E_p = mgh$ Kinetic energy, $E_k = \frac{1}{2}...

Question 3

Q11: What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

Accepted Answer

The work done by the force of gravity on a satellite moving around the Earth is zero. This is because the gravitational force is always directed towards the center of the Earth, while the displacement of the satellite is tangential to its orbit. Since the force and displacement are perpendicular to...

Question 4

Q12: Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.

Accepted Answer

Yes, there can be displacement of an object in the absence of any net force acting on it, according to Newton's First Law of Motion (the law of inertia). If an object is already in motion with a constant velocity, it will continue to move with that velocity in the same direction unless acted upon by...

Question 5

Q13: A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.

Accepted Answer

According to the scientific definition of work, the person has not done any work on the bundle of hay. Although the person is exerting a force to hold the bundle, there is no displacement of the bundle. Therefore, the work done is zero. However, the person gets tired because their muscles are contra...

Question 6

Q14: An electric heater is rated 1500 W . How much energy does it use in 10 hours?

Accepted Answer

Given: Power, $P = 1500 	ext{ W} = 1.5 	ext{ kW}$ Time, $t = 10 	ext{ hours}$ To Find: Energy used, $E$ Formula: $$P = \frac{E}{t}$$  $$E = P 	imes t$$ Calculation: $$E = 1.5 	ext{ kW} 	imes 10 	ext{ hours} = 15 	ext{ kWh}$$ To convert to Joules: $$E = 15 	ext{ kWh} 	imes 3.6 	imes 10^6 \...

Question 7

Q15: Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to osci...

Accepted Answer

When a pendulum bob is drawn to one side and released, it oscillates due to the interconversion of potential and kinetic energy. At the highest point of its swing, the bob has maximum potential energy and zero kinetic energy. As it swings down, its potential energy is converted into kinetic energy,...

Question 8

Q16: An object of mass, $m$ is moving with a constant velocity, $v$. How much work should be done on the object in order to bring the object to rest?

Accepted Answer

The work that needs to be done on the object to bring it to rest is equal to the object's initial kinetic energy. The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy. Initially, the kinetic energy of the object is $\frac{1}{2}mv^2$. When the ob...

Question 9

Q17: Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of $60 \mathrm{~km} / \mathrm{h}$?

Accepted Answer

Given: Mass of car, $m = 1500 	ext{ kg}$ Velocity of car, $v = 60 	ext{ km/h} = 60 	imes \frac{1000}{3600} 	ext{ m/s} = \frac{50}{3} 	ext{ m/s}$ To Find: Work required to stop the car, $W$ Formula: Work done = Change in Kinetic Energy $$W = \frac{1}{2}mv^2 - \frac{1}{2}mu^2$$ Since the car is s...

Question 10

Q18: In each of the following a force, $F$ is acting on an object of mass, $m$. The direction of displacement is from west to east shown by the longer arro...

Accepted Answer

Let us analyze each case: - Case (i): The force is in the same direction as the displacement (west to east). Therefore, the work done is positive. - Case (ii): The force is perpendicular to the displacement. Therefore, the work done is zero. - Case (iii): The force is in the opposite direction to th...

Question 11

Q19: Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Accepted Answer

Yes, I agree with Soni. Acceleration is zero when the net force acting on the object is zero. This can happen even when several forces are acting on the object, provided that these forces are balanced. For example, if two equal and opposite forces act on an object, the net force is zero, and the acc...

Question 12

Q2: An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of...

Accepted Answer

The work done by the force of gravity on the object is zero. Work done by gravity depends on the vertical displacement. Since the initial and final points are on the same horizontal line, the vertical displacement is zero. Therefore, work done, $W = F 	imes s = mg 	imes 0 = 0$.

Question 13

Q20: Find the energy in joules consumed in 10 hours by four devices of power 500 W each.

Accepted Answer

Given: Power of each device, $P = 500 	ext{ W}$ Number of devices, $n = 4$ Total power, $P_{total} = 4 	imes 500 	ext{ W} = 2000 	ext{ W} = 2 	ext{ kW}$ Time, $t = 10 	ext{ hours}$ To Find: Energy consumed, $E$ Formula: $$P = \frac{E}{t}$$  $$E = P 	imes t$$ Calculation: $$E = 2 	ext{ kW} 	...

Question 14

Q21: A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy?

Accepted Answer

When a freely falling object reaches the ground and stops, its kinetic energy is converted into other forms of energy. The kinetic energy is primarily converted into: 1.  Sound energy: A sound is produced upon impact. 2.  Heat energy: The impact generates heat in the object and the ground. 3.  Defor...

Question 15

Q3: A battery lights a bulb. Describe the energy changes involved in the process.

Accepted Answer

The energy changes involved when a battery lights a bulb are as follows: 1.  Chemical energy in the battery is converted into electrical energy. 2.  Electrical energy flows through the circuit to the bulb. 3.  In the bulb, electrical energy is converted into light energy and heat energy. The light e...

Question 16

Q4: Certain force acting on a 20 kg mass changes its velocity from $5 \mathrm{~m} \mathrm{~s}^{-1}$ to $2 \mathrm{~m} \mathrm{~s}^{-1}$. Calculate the wor...

Accepted Answer

Given: Mass, $m = 20 	ext{ kg}$ Initial velocity, $u = 5 	ext{ m/s}$ Final velocity, $v = 2 	ext{ m/s}$ To Find: Work done, $W$ Formula: Work done = Change in Kinetic Energy $$W = \frac{1}{2}mv^2 - \frac{1}{2}mu^2$$ Calculation: $$W = \frac{1}{2} 	imes 20 	imes (2^2) - \frac{1}{2} 	imes 20 	i...

Question 17

Q5: A mass of 10 kg is at a point A on a table. It is moved to a point B . If the line joining A and B is horizontal, what is the work done on the object...

Accepted Answer

The work done on the object by the gravitational force is zero. Gravitational force acts vertically downwards, and the displacement of the object is horizontal. Since the force and displacement are perpendicular to each other, the work done is zero. Alternatively, since the change in height is zero,...

Question 18

Q6: The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Accepted Answer

No, this does not violate the law of conservation of energy. As the object falls, its potential energy decreases, but its kinetic energy increases by an equal amount. The total energy (potential energy + kinetic energy) remains constant throughout the fall. The potential energy is being converted in...

Question 19

Q7: What are the various energy transformations that occur when you are riding a bicycle?

Accepted Answer

When riding a bicycle, the following energy transformations occur: 1.  Muscular energy (chemical energy) from the rider is converted into kinetic energy to pedal the bicycle. 2.  The kinetic energy of the pedals is transferred to the wheels, causing them to rotate. 3.  Some energy is lost as heat du...

Question 20

Q8: Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Accepted Answer

No, transfer of energy to the rock does not take place because the rock does not move. Although you are exerting a force, there is no displacement. The energy you spend is converted into other forms of energy within your body, such as heat (due to muscle exertion) and is also used for internal body...

NCERT Solutions