Tag: work, energy and machines

Questions Related to work, energy and machines

According to law of conservation of energy the total energy of the universe is:

physics types of energy law of conservation of energy the law of conservation of energy work, energy and machines

  1. Increasing

  2. Decreasing

  3. Remains constant

  4. Above law does not say anything about the universe

Show answer
Correct Option: C
Explanation:

Law of conservation of energy states that Energy can neither be created nor destroyed. Hence according to the law of conservation of energy the total energy remains constant. This law is applied everywhere in the universe.

According to the law of conservation of energy.

physics types of energy law of conservation of energy the law of conservation of energy work, energy and machines

  1. energy exists in only one form

  2. energy can be created but no destroyed, and it can be transformed from one form to another

  3. energy exists in many forms but it cannot be transformed

  4. energy can neither be produced nor be destroyed, it can only be transformed from one form to another form

Show answer
Correct Option: D
Explanation:

Law of conservation of energy.

The temperature of water at the bottom of a large waterfall is higher than that of the water at the top, because

physics types of energy law of conservation of energy the law of conservation of energy work, energy and machines

  1. the falling water absorbs heat from the sun.

  2. the KE of the falling water is converted into heat.

  3. the water at the bottom has greater PE.

  4. rocks on the bed of the river give out heat.

Show answer
Correct Option: B
Explanation:

As the water falls, the Kinetic Energy of the water gets converted to heat energy increasing its temperature at the bottom than at the top.

Law of conservation of energy states that :

physics types of energy law of conservation of energy the law of conservation of energy work, energy and machines

  1. energy cannot be destroyed but can be created and transformed from one form to another

  2. enerry exists in only one form

  3. energy exists in many forms but it cannot be transformed

  4. energy can neither be created nor be destroyed but can be transformed from one form to another

Show answer
Correct Option: D
Explanation:

The law of conservation of energy states that energy can neither be created nor be destroyed but it can be converted from one form to another i.e total energy in the universe will never change.

Eg:- Potential energy of a gas or oil is changed into energy to heat a building.

A thorium nucleus emits an alpha particle. Which of the following fundamental physics principles can be used to explain why the direction of the daughter nucleus recoil must be in the opposite direction of the alpha emission?
I. Newtons third law
II. Conversation of momentum
III. Conversation of energy

physics types of energy law of conservation of energy the law of conservation of energy work, energy and machines

  1. Only II

  2. Only III

  3. I and II only

  4. II and III only

Show answer
Correct Option: C
Explanation:

By Newton's third law of motion, every action has equal and opposite reaction. Hence, the direction of the daughter nucleus recoil must be in the opposite direction to the direction of the alpha emission.


By law of conservation of momentum, the net momentum of the system should remain constant as no external force is acting on the system. Hence, momentum of the daughter nucleus must be opposite to the direction of the alpha particle so that the net momentum of the system remains zero.

By law of conservation of energy, the total energy of an isolated system remains zero. It cannot be used to conclude the direction of motion of the particles.

A ball is dropped from a height of $10\ m$. If the energy of the ball reduces by $40\%$ after striking the ground, how much high can the ball bounce back? $(g = 10 m s^{-2})$ :

physics types of energy law of conservation of energy the law of conservation of energy work, energy and machines

  1. $6\ m$

  2. $4\ m$

  3. $10\ m$

  4. None of the above

Show answer
Correct Option: A
Explanation:

$M g h = M \times 10 \times 10 = 100 M\  J.$
Energy is reduced by $40\%$ then the remaining energy is $60M\ J$.
Then, $60M=M\times 10\times {h}'$

or, ${h}' = 6\ m$

A stone of mass $m$ kg is whirled in a vertical circle of radius $20$ cm. The difference in the kinetic energies at the lowest and the topmost position is:

physics types of energy law of conservation of energy the law of conservation of energy work, energy and machines

  1. $4 \ mg\  joule$

  2. $0.4\ mg\ joule$

  3. $40 \ mg \ joule$

  4. None of these

Show answer
Correct Option: B
Explanation:

According to the law of conservation of energy 
Total energy of the system (KE $+$ PE) will be equal at both  , topmost and bottom most point.
Therefore, $\bigtriangleup PE =\bigtriangleup KE$
 $KE _{b}-KE _{t} = PE _{t} - PE _{b}$
$PE _{t} - PE _{b} = m\times g\times (2\times r)$
$\bigtriangleup KE = 0.4 \ mg$ $joule$

When a men walks on a horizontal surface  then work done by

physics work, energy and machines concept of energy energy for everything forms of energy

  1. friciton is zero

  2. contact force is non-zero

  3. gravity is non-zero

  4. None of these

Show answer
Correct Option: B
Explanation:

When the man moves on a horizontal surface, then work done by gravity is zero since gravitational force acts perpendicular to the displacement. 


And the normal force will be also perpendicular to displacement so work done by It also is zero. - - - - - - - - 1

But frictional force is acting opposite to displacement so work done by frictional force is negative. - - - - - - - - - - - 2

Using statement (1) and (2) we can say that work done by contact force is non - zero