# Work and Energy - Class 9 - Science

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## Back Questions - Work and Energy | NCERT | Science | Class 9

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 in a pond.

A donkey is carrying a load on its back.

A wind-mill is lifting water from a well.

A green plant is carrying out photosynthesis.

An engine is pulling a train.

Food grains are getting dried in the sun.

A sailboat is moving due to wind energy.

- **Suma is swimming in a pond.** While swimming, Suma applies force against the water, and there is a displacement in the direction of this force, meaning **work is done** by Suma on the water.

- **A donkey is carrying a load on its back.** If the donkey is just standing still or moving horizontally without ascending or descending a slope, no work is done against gravity (which would be the primary force considered if the load were being lifted). However, any horizontal movement doesn't count towards work against gravity. In this simplified context, **no work is done** so long as there is no vertical displacement of the load.

- **A wind-mill is lifting water from a well.** The windmill applies a force to lift water, causing a vertical displacement of water in the direction of the force. Therefore, **work is done** on the water by the windmill.

- **A green plant is carrying out photosynthesis.** Photosynthesis involves the conversion of light energy into chemical energy within the plant. Since this process doesn't involve applying a force to move an object in the direction of that force, **no work is done** in the mechanical sense.

- **An engine is pulling a train.** The engine exerts a force on the train which moves the train in the direction of this force, indicating that **work is done** by the engine on the train.

- **Food grains are getting dried in the sun.** Drying involves the removal of water due to evaporation, a process driven by heat from the sun, but no physical displacement of the grains by the applied force of the sun's rays in a specific direction. So, **no work is done** on the grains in the context of the physical concept of work.

- **A sailboat is moving due to wind energy.** The wind exerts a force on the sail which moves the boat in the direction of the force. Here, **work is done** on the sailboat by the wind.

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 the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

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Sign up nowA battery lights a bulb. Describe the energy changes involved in the process.

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Sign up nowCertain force acting on a $20 \mathrm{~kg}$ mass changes its velocity from $5 \mathrm{~m} \mathrm{~s}^{-1}$ to $2 \mathrm{~m} \mathrm{~s}^{-1}$. Calculate the work done by the force.

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Sign up nowA mass of $10 \mathrm{~kg}$ is at a point $\mathrm{A}$ 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 by the gravitational force? Explain your answer.

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

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

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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?

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A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

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An object of mass $40 \mathrm{~kg}$ is raised to a height of $5 \mathrm{~m}$ above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

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

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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.

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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.

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An electric heater is rated $1500 \mathrm{~W}$. How much energy does it use in 10 hours?

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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 oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?

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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? 100 words

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

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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?

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Find the energy in joules consumed in 10 hours by four devices of power $500 \mathrm{~W}$ each.

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

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## Extra Questions - Work and Energy | NCERT | Science | Class 9

An electric heater is rated at $2 \mathrm{~kW}$. Electrical energy costs Rs 4 per $\mathrm{kWh}$. What is the cost of using the heater for 3 hours?

(a) Rs 12

(b) Rs 24

(c) Rs 36

(d) Rs 48

To calculate the cost of using the electric heater, we first determine the **total energy consumption** over the period of use. The energy consumed by the electric heater is calculated using the formula:
$$
\text{Energy (kWh)} = \text{Power (kW)} \times \text{Time (hours)}
$$

For a heater rated at **2 kW**, used for **3 hours**:

$$ \text{Energy} = 2 , \text{kW} \times 3 , \text{hours} = 6 , \text{kWh} $$

Next, we calculate the cost of this energy consumption. Given that the cost of electrical energy is **Rs 4 per kWh**:

$$ \text{Cost} = 6 , \text{kWh} \times 4 , \text{Rs/kWh} = 24 , \text{Rs} $$

Thus, the cost of using the electric heater for 3 hours is **Rs 24**.

**Answer: (b) Rs 24**

A $40 \ \mathrm{kg}$ girl is swinging on a swing from rest. Then, the power delivered when moving with a velocity of $2 \ \mathrm{m/s}$ upwards in a direction making an angle $60^\circ$ with the vertical is

A) $248 \sqrt{3} \ \mathrm{W}$

B) $392 \sqrt{3} \ \mathrm{W}$

C) $448 \ \mathrm{W}$

D) $580 \ \mathrm{W}$

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The distance $x$ moved by a body of mass $0.5 \text{ kg}$ due to a force varies with time $t$ as $x = 3t^2 + 4t + 5$, where $x$ is expressed in meters and $t$ in seconds. What is the work done by the force in the first 2 seconds?

A) 25 J

B) 50 J

C) 75 J

D) 100 J

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Displacement of an object of mass $2 \mathrm{~kg}$ varies as $\mathrm{s} = 3 \mathrm{t}^{2} + 5$. Find work done by man from $t = 0$ to $t = 5 \mathrm{sec}$.

A) 100 $\mathrm{~J}$ B) 200 $\mathrm{~J}$ C) 500 $\mathrm{~J}$ D) 900 $\mathrm{~J}$

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Two blocks of different masses are hanging on two ends of a string passing over a frictionless pulley. The heavier block is twice that of the lighter one. The tension in the string is $60 \mathrm{~N}$. Find the decrease in potential energy during the first second after the system is released.

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What is the work to be done to increase the velocity of a car from $30 \mathrm{~km}$ per hour to $60 \mathrm{~km}$ per hour if the mass of the car is $1500 \mathrm{~kg}$?