Class 9 Science - Chapter Work and Energy NCERT Solutions | What is the work done by the force of gr

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 kinetic energy when it is half-way down.

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 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?

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

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.

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?

Which of the following is true for displacement?

(a) It cannot be zero.
(b) Its magnitude is greater than the distance travelled by the object.

The volume of 50 g of a substance is 20 cm³. If the density of water is 1 g cm^-3, will the substance float or sink?

What does a neuron look like?

What can you say about the motion of an object if its speed-time graph is a straight line parallel to the time axis?

How are the criteria for deciding divisions in plants different from the criteria for deciding the subgroups among animals?

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

Which one of the following is a correct electronic configuration of sodium?
(a) 2,8        (b) 8,2,1        (c) 2,1,8        (d) 2,8,1.

A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s^-1.

Give two practical applications of reflection of sound waves.

Why is the atmosphere essential for life?