Class 9 Science - Chapter Work and Energy NCERT Solutions | An object of mass 40 kg is raised to a h

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?

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?

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

Define 1 J of work.

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 arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.

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?

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

The kinetic energy of an object of mass, m moving with a velocity of 5 m s^-1 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?

Certain force acting on a 20 kg mass changes its velocity from 5 m s^-1 to 2 m s^-1. Calculate the work done by the force.

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

Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupees coin and a one-rupee coin?

State the universal law of gravitation.

Which of the following are matter?

Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume.

What is meant by a pure substance?

How does the sound produced by a vibrating object in a medium reach your ear?

In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium observations are in agreement with the law of conservation of mass.

sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

What are canal rays?

State any two conditions essential for good health.

How is our atmosphere different from the atmospheres on Venus and Mars?

Who discovered cells, and how?

How many atoms are present in a

(i) H₂S molecule and
(ii) PO₄^3- ion?

What is the primary characteristic on which the first division of organisms is made?

The speed-time graph for a car is shown is Fig. 8.12.

          Fig. 8.12

(a) Find how far does the car travel in the first 4 seconds. Shade the area on the graph that represents the distance travelled by the car during the period.
(b) Which part of the graph represents uniform motion of the car?

Abdul, while driving to school, computes the average speed for his trip to be 20 km h^-1. On his return trip along the same route, there is less traffic and the average speed is 40 km h^-1. What is the average speed for Abdul’s trip?

Suggest a method to liquefy atmospheric gases.

What is the nature of the distance-time graphs for uniform and non-uniform motion of an object?

What is pasturage and how is it related to honey production?

Why should preventive measures and biological control methods be preferred for protecting crops?

Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.

A trolley, while going down an inclined plane, has an acceleration of 2 cm s^-2. What will be its velocity 3 s after the start?