Tag: the law of conservation of energy

Questions Related to the law of conservation of energy

State whether the given statement is True or False :

The potential energy of a free falling object decreases progressively.

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

  1. True

  2. False

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Correct Option: A
Explanation:

Because when body falls from a height potential energy decreases and kinetic energy progressively increases. During this total mechanical energy remains conserved.

A freely falling object eventually stops on reaching ground. What happens to its potential and kinetic energy while falling?

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

  1. Potential energy decreases and Kinetic energy increases

  2. Potential energy increases and Kinetic energy increases

  3. Potential energy increases and Kinetic energy decreases

  4. None

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Correct Option: A
Explanation:

Potential energy decreases and Kinetic energy increases when the object is falling but the total mechanical energy remains constant at every instant.

State whether the given statement is True or False :

Acceleration in an object could be zero even when several forces are acting on it.

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

  1. True

  2. False

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Correct Option: A
Explanation:

It can be zero when all forces cancel out each other i.e. net force acting on object is zero.

When a pendulum oscillates, it comes to rest after sometime because :

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

  1. energy lost by pendulum to overcome friction is gained by pendulum

  2. energy lost by pendulum to overcome its speed is gained by surrounding

  3. energy lost by pendulum to overcome friction is gained by surrounding

  4. energy lost by pendulum to overcome friction is gained by surrounding and pendulum

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Correct Option: C
Explanation:

When a pendulum oscillates, it comes to rest after sometime because energy lost by pendulum to overcome friction is gained by surrounding. Hence total energy of pendulum and surrounding system remains conserved.

Find energy in kWh consumed in 10 hours by four devices of power 500W each?

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

  1. 10 units

  2. 20 units

  3. 30 units

  4. 40 units

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Correct Option: B
Explanation:

$P=500W=0.50kW$

$t=10h$

$W=Power $\times$Time$

$=0.50 $\times$10 = 5kWh$

For 4 devices$ = 4 \times 5 = 20$ units

Does the Law of Conservation of energy apply to machines?

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

  1. No, as machines require a constant input of energy in order to work

  2. Yes, the energy required to maintain motion is balanced by the energy lost to friction

  3. No, as machines can create energy from motion

  4. Yes, but only to perpetual motion machines

  5. No, as the law would apply to only perpetual motion machines

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Correct Option: A
Explanation:

Machines require input work to be done on them, which increases the potential, or kinetic energy, or both of the system.

However this work is done on the machines by external systems that utilize their energies to convert them to work. 
So even though Law of Conservation of Energy holds for the entire system as a whole, it does not stand for the machines alone.

Based on the Law of Energy Conservation, which of the following would best maintain a machine's ability to do work without input of additional energy?

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

  1. Decrease the amount of contact between component parts

  2. Increase contact between component parts

  3. use solar energy as the source

  4. use chemical energy as the source

  5. increase the number of parts in the machine

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Correct Option: A
Explanation:

If we use solar energy or chemical energy it would mean input of additional energy , which is restricted in this case.

To increase the ability to do work of machine , we have to avoid the wastage of energy in the form of  heat produced , this heat is produced due to friction between component parts of machine .Friction between parts can be reduced by decreasing the amount of contact between parts .By doing that saved energy (which is not converted into heat) will be converted into work and this would best maintain the machine's ability.

A tennis ball dropped from a height of 2 m rebounds only 1.5 m after hitting the ground. What fraction of energy is lost in the impact?

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

  1. $\dfrac { 1 }{ 4 } $

  2. $\dfrac { 1 }{ 8 } $

  3. $\dfrac { 1 }{ 2 } $

  4. $\dfrac { 1 }{ 16 } $

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Correct Option: A
Explanation:


We know that the potential energy is dependent on the height. Let us consider the tennis ball dropped from height $H$ has P.E of $mgH$.

Then $mgh$ will be the potential energy while the ball rebounds.

Here,$H = 2m$ and $h = 1.5m$

Then, loss of energy  $ = \dfrac{mg(H-h)}{mgH}$

$=\dfrac { H-h }{ H } =\dfrac { 2-1.5 }{ 2 } =\dfrac { 1 }{ 4 } $

Therefore, fraction of energy lost in the impact $ = \dfrac { 1 }{ 4 } $

According to the law of conservation of energy, total energy 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. Constant

  4. None of these

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Correct Option: C

Before a rubber ball bounces off from the floor, the ball is in contact with the floor for a fraction of second. Which of the following statements are correct?

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

  1. Conservation of energy is not valid during this period

  2. Conservation of energy is valid during this period

  3. As ball is compressed, kinetic energy is converted to compressed potential energy

  4. None of these

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Correct Option: B
Explanation:

The law of conservation of energy is valid at any instant & in all circumstances.