CBSE-IX-Science
15: 2-Work and Energy
- Qstn #15Illustrate 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?Ans : The bob eventually comes to rest due to the frictional force offered by the air and the rigid support holding the thread.
It is not a violation of the law of conservation of energy since mechanical energy can get converted into another form of energy which cannot be utilised for useful work. This loss of energy is called dissipation of energy.
- Qstn #16An 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?Ans : In order to bring the object to rest the work done must be equal to the kinetic energy of the moving object
- Qstn #17Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?Ans : Initial velocity of the car (u) = 60 km/h
Final velocity (v) = 0 (object has to be stopped)
Initial kinetic energy
= 208333.30 J
Final kinetic energy
= 0
Therefore, work done = change in kinetic energy = 208333.30 ― 0 = 208333.30 J
- Qstn #18In 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.
Ans : (i) Since in this diagram displacement is perpendicular to the direction of force, so work done is zero.
(ii) Since in this diagram displacement is in the direction of force, so work done is positive.
(iii) Since in this diagram displacement is in the opposite direction of the force applied hence work done is negative.
- Qstn #19Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?Ans : Yes we agree with her statement. Because when many balanced forces act on the object its displacement becomes zero.
- Qstn #20Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.Ans : Since Energy = power x time
= 20000 Wh = 20 kWh
- Qstn #21A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?Ans : A free falling object eventually stops on reaching the ground since on striking the ground its kinetic energy is transmitted to the ground.
NCERT Solutions Class 9 Science