Q.1 What is meant by mechanical energy ? State its two forms. State the law of conservation of energy. Give an example in which we observe a continuous change of one form of energy into another and vice-versa.
(b) Calculate the amount of work required to stop a car of 1000 kg moving with a speed of 72 km h”1. [SAII-2013]
Answer.
(a) It is the sum of KE and PE of an object. It states that energy can neither be created nor be destroyed. We observe a continuous change in energy in a simple pendulum and its : explanation. At the mean position, the energy is wholly kinetic while at the extreme position it is wholly potential. As the pendulum oscillates its energy continuously changes between kinetic and potential.

Q.2 (a)Derive an expression for kinetic energy of a body having mass m and moving with a velocity v.
(b)When velocity of a body is increased 5 times, what is the change in its kinetic energy ?
(c)Two masses m and 2m are dropped from heights h and 2h. On reaching the ground, which will have greater kinetic energy and why ? [SAII-2013]
Answer.
(a)For derivation see above questions.
(b)Kinetic energy is given by the expression
KE = 1/2 mv², therefore, if velocity is made 5 times KE will increase by 25 times.
(c)More the potential energy more will be the kinetic energy of the body when it falls. Hence, the body with mass 2m will have greater kinetic energy as it has more potential energy.

Question.3
(a)State the law of conservation of energy.
(b)What is the work done to increase the velocity of a car from 36 km h-1 to 72 km h~1 if the mass of the car is 1500 kg ? Does the work done by the force have a negative or a positive magnitude ?
(c)Where does an oscillating pendulum have maximum PE and KE ? [SAII-2013]
Answer.

Q.4 (a) Define power. Give its SI unit.
(b) Taking the example of a simple pendulum, explain the variations in the forms of energy and the; inter-conversions involved. [SA II-2011]
Answer.
(a) Power is defined as the rate of doing work. Its SI unit is watt.
(b) For a simple pendulum, the inter-conversion of energy is as shown in the table below :

Q.5 (a)What is the commercial unit of energy ? Give its relationship with SI unit of energy.
(b)An electric bulb of 60 W is lighted for 10 hours everyday. How many units of electrical energy is consumed by this bulb in one day ? [SAII-2013]
Answer.

Question.6 Define : (a) power (b) work done (c) kinetic energy. Give SI unit of each. [SAII-2014]
Answer.
(a) The rate of doing work is called power. Its SI unit is watt.
(b) Work is the product of force and displacement. Its SI unit is joule.
(c) It is the energy possessed by a body by virtue of its motion. Its SI unit is joule.

Question.7 The kinetic energy of an object of mass ‘m’ moving with a velocity of 5 ms-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?
Solution.

Question.8 Certain force acting on a 20 kg mass changes its velocity from 5 ms^-1 to 2 ms^-1. Calculate the work done by the force. [SAII-2013]
Solution.

Question.9 A body of mass 4 kg initially at rest is subjected to a force of I6 N. What is the kinetic energy acquired by the body at the end of 10 s? [SAII-2010]
Solution.

Question 10.A mass of 10 kg is dropped from a height of 50 cm. Find its : (i) Kinetic energy (ii) Velocity just as it reaches the ground. Does the velocity depend upon the mass of the particle? Explain. [Take g = 10 ms^-2] [SAll-2012]
Solution.

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