#### Energy

21. Potential energy and kinetic energy are types of

(a) Electrical energy      (b) Magnetic energy                  (c) Thermal energy       (d) Mechanical energy

Note: Mechanical energy is one which involves motion, and hence change of position. As such, K.E and P.E are two forms of mechanical energy.

22. Energy cannot be

(a) Converted or transferred                                           (b) destroyed or transferred                                (c) created or transferred                                              (d) created or destroyed

23. Solar energy is converted by plants in process of photosynthesis into

(a) Chemical Potential energy                                      (b) Elastic Potential energy

(b) Gravitational Potential energy                                  (d) Kinetic energy

Hint: Chemical potential energy is the energy stored in the chemical bonds of a substance. Apart from photosynthesis, food we eat, gasoline and dynamite contain such energies . This energy is released and used by humans and plants.

24. Food we eat contains

(a) Elastic Potential energy                                            (b) Gravitational Potential energy

(c) Kinetic energy                                                         (d) Chemical Potential energy

25. Wind and waves are used by turbines to convert

(a) Mechanical energy into Electrical energy                  (b) Kinetic energy into Electrical energy

(c) Potential energy into Electrical energy                      (d) Potential energy into Kinetic energy

Explanation: Both wind and water waves have kinetic energy due to their motion. Turbines are a mechanism in which this kinetic energy is converted into useful electrical energy.

26. As pendulum bob swings, some of its energy is converted into

(a) Sound energy          (b) Thermal energy      (c) Light energy                        (d) Kinetic energy

Explanation: When a pendulum bob is in motion a continuous interchange of K.E and P.E takes place. However, at the same time the bob is also dissipating some energy due to the presence of surrounding air which obstructs the motion of the pendulum. This energy is converted as increasing the temperature of the surrounding. Consequently, the amplitude of the pendulum is continuously decreasing until it comes to a stop after some time.

27. Pendulum bob cannot attain its initial height because

(a) It continues to lose energy in thermal form             (b) it continues to lose energy in sound form

(c) It continues to lose energy in light form                   (d) it continues to lose energy in potential form

Explanation: As above

28. When a pendulum bob is displaced to one side, it gains

(a) Chemical Potential energy                                       (b) Elastic Potential energy

(c) Gravitational Potential energy                                (d) Kinetic energy

Explanation: When a pendulum bob is displaced from its mean position, its vertical height from the earth also increases. It is because the pendulum is moving along a circular arc around the point of suspension. It can’t move in a straight line. With increased height from the ground, its potential energy also increases. (Similarly, we note there are many kinds of potential energy).

29. A pendulum bob moves at a speed of 2 m s-1 and mass of 0.5 kg, its K.E would be

(a) 1 J              (b) 0.25 J                      (c) 0.5 J                        (d) 2 J

Solution: Given that                 velocity= 2 m/s             mass = 0.5 kg

Apply the formula, K.E = ½ mv2, we have K.E= ½ * 0.5 * 2 * 2 = ½ * 2 = 1 J

30.   The type of energy possessed by a simple pendulum, when it is at the mean position is

(a) Kinetic energy                    (b) Potential energy      (c) P.E +K.E     (d) Sound energy

Explanation: When a pendulum is vibrating about its mean position, its energy is continuously interchanged between K.E and P.E. At the extreme points, when it is at maximum distance from the mean position all its energy is P.E. At the mean position all its energy is K.E.

31.   Which form of energy does the flowing water possess?

(a) Electric energy         (b) Potential energy      (c) Gravitational energy (d) Kinetic energy

Explanation: Flowing water has its energy due to its motion.

32. If the speed of an object is doubled its kinetic energy is

(a) Doubled                  (b) 4 times increased    (c) Halved                     (d) Tripled

Solution

Let the speed of an object is ‘v’. Its K.E1 = ½ mv2. When its speed is doubled, then speed is 2v. Its K.E2 = ½ m(2v)2 = ½ . 4v2 = 4(1/2 mv2) = 4(K.E1)

33. A bullet fired from a gun can pierce a target due to its

(a) Mechanical energy   (b) Heat energy             (c) Kinetic energy        (d) acceleration

Explanation: Energy of a body by virtue of its motion is called kinetic energy. The moving object possessing this type of energy is capable of doing work. Bullet fired from a gun also posses K.E and is capable of doing work. When it pierces into a body, its K.E is converted to doing work against the body by piercing into it.

34. A car stopped screeching to avoid crash with a van, changes involved in the process is

(a) K.E is converted into sound energy              (b) K.E is converted into sound and thermal energy

(c) P.E is converted into sound, heat and K.E    (d) K.E and P.E is converted into thermal and sound

Explanation: The car posses K.E while moving. When it stopped, the K.E it possessed is converted to heat of rubbing the tires with the road and sound of screech.

35. A car of mass 1.5 tones travels at a constant speed of 40 km ⁄ h, calculate its kinetic energy in kJ.

(a) 9.259 KJ                  (b) 92.59 KJ                 (c) 925.9 KJ                  (d) None of a, b, c

Solve yourself.

Hint:

1 ton = 1000 kg, therefore, mass of the car = 1.5 * 1000 = 1500 kg

Velocity = v = 40 km/h = 40 * 1000 m /3600 s = 400 m/36 s = 11.111 m/s

And 1 Kj = 1000 j

36.   A 1 kg mass has a kinetic energy of 1 joule when its speed is

(a) 0.45 m/s                  (b) 1 m/s                                   (c) 1.4 m/s                    (d) 4.4 m/s

Solution:          Mass = 1 kg                  K.E = 1 J          v = ?

K.E = ½ m v2      OR       1 = ½ * 1 * v2   OR 2 = v2   OR

37. If a bicycle of mass 15 kg moves at a speed of 50 m s-1, K.E of bicycle is

(a) 18750 J                   (b) 5625 J                     (c) 37500 J                    (d) J

Solution

K.E = ½ m v2 = ½ * 15 * 50 * 50 = 18750 J

38. A shopping cart moves at a speed of 3 m s-1 with a mass of 2 kg, K.E of shopping cart is

(a) 6 J                           (b) 9 J                          (c) 18 J                         (d) 12 J

Solve yourself.

39. When spring or rubber band is released it converts potential energy into

(a) Mechanical energy   (b) Electrical energy      (c) Thermal energy       (d) Kinetic energy

Explanation: When a spring or rubber is stretched, work is done on them. This work stores as elastic P.E in the spring or rubber. When it is released, the elastic P.E is converted to K.E so that the spring or rubber restores its original position once again.

40. If a body has kinetic energy, then it must have

(a) Speed                      (b) impulse                   (c) acceleration             (d) force

Explanation: K.E is the energy possessed by a body by the virtue of its motion. Hence a body having speed is moving and possessing K.E.

41. Change in Gravitational P.E can be written as

(a) mgh                        (b) mh                          (c) mg                          (d) gh

Did you know? When we raise a body in the gravitational field, what we do is in fact is changing the P.E possessed by the body.

42. When a body is raised to a height 2h in the gravitational field, the change in P.E is

(a) ½ (mgh)                 (b) mgh                                    (c) 2mgh                      (d) (mg)/(2h)

Solution: Since P.E. = mgh. Put h = 2h, we have gravitational potential energy= 2mgh

43. A compressed spring possesses

(a) Kinetic energy         (b) Elastic Potential energy      (c) Gravitational P.E     (d) Sound energy

Explanation: When a spring is compressed, work is done upon it. The work stores in the spring as elastic potential energy and when the spring is released, the elastic energy stored in the spring is converted to work.

44. As object gains speed, it’s G.P.E (Gravitational Potential Energy)

(a) increases      (b) remains constant                 (c) decreases                 (d) varies depending on altitude

Explanation: When a body is in motion and there is a change in the altitude, its distance from the center of earth is varying. Since the gravitational potential energy of the body depends upon its distance from the center of earth, therefore, there is a change in the P.E of the body. If the body moves on a leveled smooth surface, there is no change in P.E, however.

45:   An object of mass 1 kg has potential energy of 1 joule relative to the ground when it is at a height of

(a) 0.102 m                   (b) 1 m                         (c) 9.8 m                                  (d) 32 m

Explanation: P.E = mgh   OR h = P.E/mg (Solve yourself)

46. An object is lifted 5 m above ground level, mass of object is 20 kg and gravitational pull is 10 ms-2, P.E of the object is

(a) 40 J                         (b) 1000 J                     (c) 2.5 J                        (d) 0.4 J

Solution:          mass = 20 kg                height = 5 m     g = 10 ms-2

P.E = mgh = 20 * 10 * 5 = 1000 J

47. Mass of a car is 500 kg, which is lifted up by a crane at a height of 200 m; gravity is 10 ms-2. P.E of car will be

(a) 1000000 J               (b) 10000 J                   (c) 100 J                       (d) 25 J

(Solve yourself.)

48:   A man of mass 50 kg jumps to a height of 1 m. His potential energy at the highest point is (g = 10 m/s2)

(a) 1.50 J                      (b) 60 J                         (c) 500 J                                   (d) 600 J

(Solve yourself.)