Class 9 Science - Chapter Work and Energy NCERT Solutions | Illustrate the law of conservation of en
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?
The law of conservation of energy states that energy can neither be created nor be destroyed. It can change from one form to another. Consider the case of an oscillating pendulum.
When a pendulum moves from its mean position P to either of its extreme positions A or B, it rises through a height h above the mean level P. At this point, the kinetic energy of the bob changes completely into potential energy. The kinetic energy becomes zero, and the bob possesses only potential energy. As it moves towards point P, its potential energy decreases progressively. Accordingly, the kinetic energy increases. As the bob reaches point P, its potential energy becomes zero and the bob possesses only kinetic energy. This process is repeated as long as the pendulum oscillates.
The bob does not oscillate forever. It comes to rest because air resistance resists its motion. The pendulum loses its kinetic energy to overcome this friction and stops after some time.
The law of conservation of energy is not violated because the energy lost by the pendulum to overcome friction is gained by its surroundings. Hence, the total energy of the pendulum and the surrounding system remain conserved.
More Questions From Class 9 Science - Chapter Work and Energy
- Q:-
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.
- Q:-
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?
- Q:-
A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy?
- Q:-
Define 1 J of work.
- Q:-
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.
- Q:-
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?
- Q:-
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
- Q:-
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?
- Q:-
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.
- Q:-
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Popular Questions of Class 9 Science
- Q:-
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?
- Q:-
State the universal law of gravitation.
- Q:-
Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume.
- Q:-
What is meant by a pure substance?
- Q:-
How does the sound produced by a vibrating object in a medium reach your ear?
- Q:-
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
- Q:-
What are canal rays?
- Q:-
State any two conditions essential for good health.
- Q:-
How is our atmosphere different from the atmospheres on Venus and Mars?
- Q:-
Who discovered cells, and how?
Recently Viewed Questions of Class 9 Science
- Q:-
Which of the following materials fall in the category of a “pure substance”?
(a) Ice
(b) Milk
(c) Iron
(d) Hydrochloric acid
(e) Calcium oxide
(f) Mercury
(g) Brick
(h) Wood
(i) Air. - Q:-
State which of the following situations are possible and give an example for each of these:
(a) an object with a constant acceleration but with zero velocity
(b) an object moving in a certain direction with an acceleration in the perpendicular direction. - Q:-
(a) Tabulate the differences in the characterisitcs of states of matter.
(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density. - Q:-
A motorboat starting from rest on a lake accelerates in a straight line at a constant rate of 3.0 m s-2 for 8.0 s. How far does the boat travel during this time?
- Q:-
Name the regions in which parenchyma tissue is present.
- Q:-
A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
- Q:-
Convert the following temperatures to the Celsius scale.
(a) 300 K (b) 573 K.
- Q:-
What are macro-nutrients and why are they called macro-nutrients?
- Q:-
What do you mean by acceleration due to gravity?
- Q:-
What are the desirable characters of bee varieties suitable for honey production?
- All Chapters Of Class 9 Science
- All Subjects Of Class 9