## A freely falling object eventually stops on reaching the ground.

[question] Question. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy ? [/question] [solution] Solution: The kinetic energy on reaching the ground changes into heat energy, sound energy, etc. and therefore gets dissipated to the ground and air [solution]...

## Find the energy in kWh

[question] Question. Find the energy in kWh consumed in 10 hours by four devices of power 500 W each. [/question] [solution] Solution: Total power of 4 devices = 4 × 500 W $=2000 \mathrm{~W}=\frac{2000 \mathrm{~W}}{1000}=2 \mathrm{~kW}$ Time $=10 \mathrm{~h}$ Energy consumed = Power × time = 2 kW × 10 h = 20 kWh [/solution]...

## Soni says that the acceleration in an object could be zero

[question] Question. 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 ? [/question] [solution] Solution: Yes, we do agree. When a number of forces act on a body, such that they constitute balanced forces, then net force acting on the body is zero. In such a situation no acceleration occurs in the object. [/solution]...

## A battery lights a bulb.

[question] Question. A battery lights a bulb. Describe the energy changes involved in the process. [question] [solution] Solution: Within the electric cell of the battery, the chemical energy changes into electric energy. The electric energy on flowing through the filament of the bulb, first changes into heat energy and then into the light energy. [/solution]...

## An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground.

[question] Question. 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? [/question] [solution] Solution: The object returns to its level of projection, therefore displacement is in horizontal direction, while force of gravity acts in vertically downward direction. Thus, force and displacement a...

## Look at the activities listed below.

[question] Question. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'. (a) Suma is swimming in a pond. (b) A donkey is carrying a load on its back. (c) A wind-mill is lifting water from a well. (d) A green plant is carrying out photosynthesis. (e) An engine is pulling a train. (f) Food grains are getting dried in the sun. (g) A sailboat is moving due to wind energy. [/question] [solution] Solution: (a) Work is done ...

## Define average power.

[question] Question. Define average power [/question] [solution] Solution: The ratio of the total work done or total energy consumed to the total time taken is called average power. $P_{\text {average }}=\frac{\text { total work done }}{\text { total time taken }}$ The concept of average power is useful when the power of a device varies with time. [/solution]...

## A lamp consumes 1000 J of electrical energy in 10 s.

[question] Question. A lamp consumes 1000 J of electrical energy in 10 s. What is its power ? [/question] [solution] Solution: Power $=\frac{\text { Energy }}{\text { Time }}=\frac{1000}{10}=100 \mathbf{W}$ [/solution]...

## Define 1 watt of power.

[question] Question. Define 1 watt of power. [/question] [solution] Solution: When a work of 1 joule is done in 1 s, the power is said to be one watt. [/solution]...

## What is power ?

[question] Question. What is power ? [/question] [solution] Solution: Power is defined as the rate of doing work. Power $=\frac{\text { work done }}{\text { time taken }}$ [/solution]...