Work Power And Energy(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

WORK POWER AND ENERGY MCQs

Work And Energy, Sources Of Energy, Electric Potential Energy Potential And Dipoles

Total Questions : 109 | Page 11 of 11 pages

Question 101. A ball, when kept on a spring attains a certain height after it is compressed and released. For which of the following height attained by the ball, maximum of his energy is required to compress the spring?

Discuss Question

Answer: Option D. -> 9 m
:
D
Maximum energy depends on the work done in compressing the spring which is directly proportional to the height attained by the ball after the spring is released. The gravitational potential energy is expressed as "m x g x h” and it would be the maximum for the height of

9 m

. So, he has to domaximum work to compress the spring when the ball rises to a height of

9 m

Question 102. A box of mass 2 kg is lifted diagonally from point A to point B as shown in the given figure. Given that the acceleration due to gravity is 10

m s^{- 2}

, what is the gravitational potential energy gained by the box?

A Box Of Mass 2 Kg Is Lifted Diagonally From Point A To Poin...

20 J
60 J
80 J
100 J

Discuss Question

Answer: Option B. -> 60 J
:
B
Given, mass of the box,

m = 2 k g

, vertical height,

h = 3 m

and acceleration due to gravity,

g = 10 m s^{- 2}

.
We know that, gravitational potential energy =

m g h = 2 \times 10 \times 3 = 60 J o u l e

Question 103. A man lifts a luggage of

10 k g

from the ground and puts it on his head

1.7 m

above the ground. Calculate work done by him on the luggage. Take acceleration due to gravity

= 10 m s^{- 2}

150J
170J
180J
200J

Discuss Question

Answer: Option B. -> 170J
:
B
Given:
Mass,

m = 10 k g

,
Acceleration due to gravity,

g = 10 m s^{- 2}

,
Displacement,

s = 1.7 m

.
Force

F

is calculated as

F = m \times g = 10 \times 10 = 100 N

Work done by the man onluggage

= F o r c e \times d i s p l a c e m e n t

W = 100 \times 1.7 = 170 J

Hence, the required work done is

170 J

.
It can be noted that the work is done by the man on the luggage because he lifts it against the gravitational force of the earth.

Question 104. A child is moving on a skateboard at the speed of 2

m s^{- 1}

. After a force acts on him, his speed becomes 3

m s^{- 1}

. Find the work done by the external force on the child, given the mass of the child is 20 kg.

100 J
50 J
20 J
320 J

Discuss Question

Answer: Option B. -> 50 J
:
B
Let initial velocity of the child be

v_{1}

and final velocity be

v_{2}

Given,

v_{1} = 2 m s^{- 1}

v_{2} = 3 m s^{- 1}

and massof the child

= 20 k g

.
Since the child is in motion, he will have a kinetic energy of

\frac{m v^{2}}{2}

, which will be increased after an application of force by an external agent on him.
Now, Applying work energy theorem ,
Work done by the external force on the child

= \frac{m v_{2}^{2}}{2} - \frac{m v_{1}^{2}}{2}

= \frac{20 \times (9 - 4)}{2}

= 10 \times 5 = 50 J

Question 105. When a lift is going up, the potential energy of the person with respect to earth inside a lift remains the same.

True
False
50 W
70 W

Discuss Question

Answer: Option B. -> False
:
B
Potential energy depends on the position of an object with respect to a reference point. In this case, the earth is the reference point. A man on a lift going up, gains height with respect to the earth.
Thus, the potential energy of the person inside the lift increases.

Question 106. Moving train has complete mechanical energy in the form of potential energy.

True
False
Just after touching the ground floor
Just before touching the ground floor

Discuss Question

Answer: Option B. -> False
:
B
Moving train has velocity and will have only kinetic energy. A moving train will not gain any height and hence has no potential energy. Therefore, the given statement is wrong.

Question 107. An object is falling from a terrace of height

20 m

. At what point does it have maximum kinetic energy?

20m height
10m height
Just after touching the ground floor
Just before touching the ground floor

Discuss Question

Answer: Option D. -> Just before touching the ground floor
:
D
When the object isat a height of

20 m

above the ground, it has only potential energy.
Since the velocity of the object increases when it comes down, its kinetic energy will increase. When theobject is just about to touchthe ground, its potential energy will be zeroand kinetic energy will be maximum.

Question 108. Hulk tried to move the wall of a fortress, but unfortunately he was unable to do it. But he applied a very large amount of force to do so, hence work done is positive.

True
False
8.3 m
7.3 m

Discuss Question

Answer: Option B. -> False
:
B
Work done by a force is defined as the product of force and displacement of the body along the direction of force. Since Hulk was unable to move the wall, the displacement is zero and irrespective of the large amount of force he applied, work done will be zero.

Question 109. An electric bulb consumes 7.2 kJ of electrical energy in 2 minutes. What is the power of the electric bulb?