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 2 of 11 pages

Question 11. A moving body of mass m and velocity 3 km/h collides with a body at rest of mass 2m and sticks to it. Now the combined mass starts to move. What will be the combined velocity

3 km/h
2 km/h
1 km/h
4 km/h

Discuss Question

Answer: Option C. -> 1 km/h
:
C

A Moving Body Of Mass M And Velocity 3 Km/h Collides With A ...

Initial momentum =

m \times 3 + 2 m \times 0 = 3 m

Final momentum =

3 m \times V

By the law of conservation of momentum

3 m = 3 m \times V \Rightarrow V = 1 k m / h

Question 12. The velocity - time graph of a particle moving in a straight line is shown in figure. The mass of the particle is 2kg.Work done by all the forces acting on the particle in time interval between t = 0 to t = 10 s is

300 J
−300 J
400 J
−400 J

Discuss Question

Answer: Option A. -> 300 J
:
A
From work - energy theorem,
W =

△ K E = K_{f} - K_{i} = \frac{1}{2} m (v_{f}^{2} - v_{i}^{2})

\frac{1}{2} \times 2 [(- 20)^{2} - (10)^{2}] = 300 J

Question 13. When two bodies collide elastically, then

Kinetic energy of the system alone is conserved
Only momentum is conserved
Both energy and momentum are conserved
Neither energy nor momentum is conserved

Discuss Question

Answer: Option C. -> Both energy and momentum are conserved
:
C
By definition of perfectly elastic collision, both Momentum and Kinetic energy of the system are conserved.

Question 14. A particle of mass m moves with a variable velocity v, which changes with distance covered x along a straight line as

v = k \sqrt{x}

,where k is a positive constant . The work done by all the forces acting on the particle, during the first t second is

mk4t2
mk4t24
mk4t28
mk4t216

Discuss Question

Answer: Option C. -> mk4t28
:
C
GIven

v = k \sqrt{x}

\frac{d x}{d t} = k \sqrt{x}

x^{- \frac{1}{2}} d x = k d t

Integrating both sides, we get

\frac{x^{\frac{1}{2}}}{\frac{1}{2}} = k t + C

; Assuming x(0) = 0
Therefore, C = 0

2 \sqrt{x} = k t \Rightarrow x = \frac{k^{2} t^{2}}{4}

v = \frac{k^{2} t}{2}

Therefore, work done,

△ W

= Increase in KE
=

\frac{1}{2} m v^{2} - \frac{1}{2} m (0)^{2} = \frac{1}{2} m {[\frac{k^{2} t}{2}]}^{2} = \frac{m k^{4} t^{2}}{8}

Question 15. A lorry and a car moving with the same K.E. are brought to rest by applying the same retarding force, then

Lorry will come to rest in a shorter distance
Car will come to rest in a shorter distance
Both come to rest in a same distance
None of the above

Discuss Question

Answer: Option C. -> Both come to rest in a same distance
:
C
Stopping distance =

\frac{k i n e t i c e n e r g y}{r e t a r d i n g f o r c e} \Rightarrow s = \frac{1}{2} \frac{m u^{2}}{F}

If lorry and car both possess same kinetic energy and retarding force is also equal then both come to rest in the same distance.

Question 16. A moving railway compartment has a spring of constant k fixed to its front wall. A boy stretches this spring by distance x and in the mean time the compartment moves by a distance s. The work done by boy w.r.t. earth is

12kx2
12(kx)(s+x)
12kxs
12kx(s+x+s)

Discuss Question

Answer: Option A. -> 12kx2
:
A
Total work done is equal to the energy stored in the spring.

Question 17. What is the primary source of wind energy?

Moon
Sun
Hot magma
Gravitation

Discuss Question

Answer: Option B. -> Sun
:
B
Solar energy is the primary source of energy for almost all the energy conversion.
Wind is caused by differences in the atmospheric pressure which is causeddue to differential heating by sun light.When a difference in atmospheric pressure exists, air moves from the higher to the lower pressure area.

Question 18. If a turbine is used in a fast flowing stream of river without any water storage facility. What energy conversion would take place for power generation?

Potential energy to electrical energy
Heat energy to electrical energy
Wind energy to electrical energy
Kinetic energy to electrical energy

Discuss Question

Answer: Option D. -> Kinetic energy to electrical energy
:
D
As there is no storage facility, there is no potential energy. Thefast flowingstreamofthe riverhaskinetic energy.The turbine is used to convert the kinetic energy of the water directlyinto electrical energy.

Question 19. How high temperature is achieved in solar cooker by the use of mirrors?

Mirrors reflect sunlight into the cooker.
Mirrors absorb more sunlight falling on the cooker.
Mirrors diffuse light in all direction.
Mirrors refract sunlight on the cooker.

Discuss Question

Answer: Option A. -> Mirrors reflect sunlight into the cooker.
:
A
Solar cookers achieve higher temperature by using mirrors to focus the rays of the sun.
Concave mirrors are the most preferred ones as they can converge sunlight over a small area.

Question 20. Which of the following is not combustible?

Oxygen
Hydrogen
Butane
Methane

Discuss Question

Answer: Option A. -> Oxygen
:
A
Combustible materialisthesubstance that can be burned to provide heat or power. Oxygen cannot be burned to produce heat.In fact, other substances areburned inthe presenceof oxygen to produce heat energy. Hydrogen, butane, and methane are fuels used to produceheat.