MCQ Questions for Class 9 Physics Work And Energy Quiz 9

Two spring of spring constant k and 3k are stretched separately by same force. The ratio of the potential energy stored in them respectively will be

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3 : 1
0%
9 : 1
0%
1 : 3
0%
1 : 9

Which of the following is an example of zero work even when the body suffers a displacement from its initial position?

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A coolie carrying a load on his head and moving forward on a horizontal road
0%
A fielder catching a ball
0%
A child pushes a wall
0%
A child pulling a toy car

Explanation

When a coolie carries a load on his head, he applies a force upwards against the weight of the load, i.e., against the gravitational force. And the displacement is forward horizontally. So, the angle between the force and the displacement is

${90}^{\circ}$ and the work done is zero.
To catch a ball, work done is negative as the ball moves in the opposite direction of force before coming to rest.
A child pushing a wall is an example of zero work as the wall suffers zero displacements.
A child pulling a toy car is an example of positive work.

A crane pulls up a car of mass

$500kg$ to a vertical height of

$4m$ . So, work done by the crane is:

0%
$19.6J$
0%
$19.6kJ$
0%
$19600kJ$
0%
$4900J$

Explanation

To raise the car, the crane has to do work against the force of gravity.
Therefore, the force required to lift the car,

$F=mg=500\times{9.8}N=4900N$
Displacement,

$S=$ vertical height raised

$=4m$ .

$\therefore$ Work done,

$W=F.S=4900\times{4}J=19600J=19.6kJ$

Watt hour represents:

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Electric energy
0%
Current
0%
Voltage
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Power

Explanation

The watt-hour (Wh) is a unit of electric energy equivalent to one watt (1 W) of power expended for one hour (1 h) of time. Watt-hour is commonly used in electrical applications.

$1\ Wh = 1\times 60\times 60 = 3600\ J$

Unit used in selling electrical energy to consumer is:

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$Volt-Ampere$
0%
$Kilowatt-hour$
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$Volt/second$
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None of these

Explanation

The commercial unit of electrical energy is

$Kilowatt-hour\ (kWh)$ .

$1\ kWh$ is equal to

$1\ unit$ of energy.

A lamp rated

$20W$ and an electric iron rated

$50W$ are used for

$2$ hour everyday. Calculate the total energy consumed in

$20$ days.

0%
$14kwh$
0%
$2.8kwh$
0%
$40kwh$
0%
None of these

Explanation

Energy consumed by lamp in 2 hour

$E_l = 0.02\times 2 = 0.04$ kWh per day

Energy consumed by iron in 2 hour

$E_i = 0.05\times 2 = 0.1$ kWh per day

$\therefore$ Total energy consumed by both appliance in

$20$ days,

$E_T = (E_l+E_i)\times 20 = (0.04+0.1)\times 20 = 2.8$ kWh

Number of KWh in 1 Joule.

0%
$\displaystyle 3.6\times { 10 }^{ 6 }\ KWh$
0%
$\displaystyle 2.77\times { 10 }^{ -7 }\ KWh$
0%
$\displaystyle 600\ KWh$
0%
$\displaystyle 1.6\times { 10 }^{ -19 }\ KWh$

Explanation

We know that

$KWh$ is kilo watt-hour-

In SI unit-

$K = 10^3$

$hour = 3600\ s$

$1\ kWh = 3.6\times 10^6\ J$

$\therefore$

$1$

$J = \dfrac{1}{3.6\times 10^6} = 2.77\times 10^{-7}$

$kWh$

Which of these units of energy is the largest?

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$1\ Calorie$
0%
$1\ Joule$
0%
$1\ Erg$
0%
$1\ Kilowatt-hour$

Explanation

Energy is the ability to do work.

$1\ {calorie} = 4.186\ J$

$1\ erg= 1\times 10^{-7}\ J$

$1\ kWh=3.6\times 10^{6}$

$J$

$1\ kilowatt-hour$ is the largest unit of energy.

Calculate the power of a source if it produces energy of

$250\ J$ in

$10\ s$ .

0%
$2500\ W$
0%
$2.5\ W$
0%
$25\ W$
0%
Zero

Explanation

Given,

Energy,

$E = 250\ J$

Time,

$t = 10$ s

We know,

Power of the source,

$P = \dfrac{E}{t}$

$P = \dfrac{250}{10}$

$P = 25\ W$

Convert 1 kWh to SI unit of energy.

0%
$3.6\times 10^6J$
0%
$3.6\times 10^8J$
0%
$1.8\times 10^6J$
0%
$1.8\times 10^8J$

Explanation

We know that,

$1\ watt = 1\ joule/sec$ and

$1\ kW=1000\ watt$

Therefore,

$1\ kWh$

$= 1000\ watt\times 1\ hour = 1000\ joule/sec \times 3600\ sec$

$= 3.6\times 10^6$

$J$

Energy consumed in our house has a unit of

0%
watts
0%
horse power
0%
kilowatt-hours
0%
none

In which form, the energy is stored in a fuel?

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Potential energy
0%
Thermal energy
0%
Kinetic energy
0%
Chemical energy

Which one of the following energies cannot be possessed by a body at rest?

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Potential energy
0%
Kinetic energy
0%
Thermal energy
0%
Magnetic energy

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

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Potential energy decreases and Kinetic energy increases
0%
Potential energy increases and Kinetic energy increases
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Potential energy increases and Kinetic energy decreases
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None

What is practical unit of power?

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Watt
0%
Kilo watt
0%
Horse power
0%
None of these

Explanation

Practical unit of power is horse power

$\left(hp\right)$ .

$1\ hp = 746 W$ , where

$W$ is Watt (the SI unit of power).

A certain household has consumed

$250$ units of energy during a month. How much energy is this in joules?

0%
$9 \times 10^{10}\ J$
0%
$9 \times 10^{8}\ J$
0%
$9 \times 10^{9}\ J$
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$9 \times 10^{-8}\ J$

Explanation

Hint: Conversion of KWh to joule.

Step 1: Commercial unit of energy is KWh.

$1\; KWh = 10^3\; W = 10^3\; JS^{-1}$

$1\;hr = 60\times60 = 3600\;sec$

Therefore,

$1\;KWh = 1\; KW \times1\;hr$

$=10^3 \times 3600$

$=3.6 \times 10^6 \;J$

Step 2: Calculation according to question

Energy consumption

$=250\;KWh$

$= 250\times3.6\times10^6\;J$

$=900\times10^6\;J$

$=9\times10^8\;J$ or

$900\;MJ$

Hence, Option (B) is correct.

Find energy in kWh consumed in 10 hours by four devices of power

$500\ W$ each?

0%
$10\ kWh$
0%
$20\ kWh$
0%
$30\ kWh$
0%
$40\ kWh$

Explanation

Hint:

Power is the ratio of the work and the time. Energy consumed in four devices is four times the energy consumed in one device.

Step 1: Given data

Power consumed by each device, $P=500\ W=0.50\ kW$

Time, $t=10\ h$

Expression for the energy consumed is written as,

$W=P\times t$

Step 2: Calculations,

Energy consumed by each device, $W=0.50 \times 10 = 5kWh$

Energy consumed by four devices, $= 4 \times 5 = 20\ kWh$

Smallest commercial unit of energy is

0%
Kilowatt hour
0%
Watt second
0%
Watt hour
0%
Watt minutes

Explanation

Smallest commercial unit of energy is called Watt hour

$(Wh)$ . One watt hour is the energy consumed when

$1 \ watt$ of power is used for

$1 \ hour.$

Convert

$28.8 kJ$ into kilo watt hour.

0%
$8 \times {10}^{-3} kwh$
0%
$8 kwh$
0%
$8 \times {10}^{3} kwh$
0%
None of these

Explanation

We know that

$1 kWh=3.6 \times 10^6$

$Joules$

$E=28.8 \times 10^3$

$Joules$

$E = \dfrac{28.8 \times {10}^{3}}{3.6 \times {10}^{6}}$

$= 8 \times {10}^{-3} kwh$

An object is dropped from height

$h=2R$ on the surface of earth. Find the speed with which it will collide with ground, by neglecting effect of air. [

$R$ is radius of earth. Take mass of earth as

$M$ ]

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$2 \sqrt {\dfrac {GM} {3R}}$
0%
$\sqrt {\dfrac {2GM} {R}}$
0%
$\sqrt {\dfrac {GM} {R}}$
0%
$\sqrt {\dfrac {GM} {2}}$

Calculate the monthly bill if a heater of 100 watt is used at the rate of rs. 1 per unit for 1 hour daily.

0%
Rs. 200
0%
Rs. 6000
0%
Rs. 100
0%
Rs. 3

Explanation

Given :

$P=0.1$ kW

$t = 1$ hour per day

Energy consumed per day

$E = Pt = 0.1\times 1 = 0.1$ kWh per day

Total energy consumed per month

$E_T = 30\times 0.1 = 3$ kWh per month

Cost of electricity

$=Re$

$1$ per unit i.e per kWh

$\therefore$ Monthly bill

$= Re.$

$1\times 3 =$ Rs.

$3$ per month

Commercial unit of energy is .......................

0%
Joule
0%
Volt
0%
Kilowatt hour
0%
Watt second

Explanation

Commercial unit of electrical energy is kilowatt hour

$\left(kWh\right)$

$1 kwh = 3.6 \times {10}^{6} J$

What is law of conservation of energy?

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Energy can neither be created nor destroyed. It can only be transformed from one to another.
0%
Energy can only be created nor destroyed. It can only be transformed from one to another.
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Energy can neither be created but can be destroyed. It can only be transformed from one to another.
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None

An object of mass

$40\ kg$ is raised to a height of

$5\ m$ above ground. If the object is allowed to fall down find its kinetic energy when it is half-way down.

0%
1960 J
0%
980 J
0%
990 J
0%
1000 J

Explanation

Gravitational Potential energy, $W = mgh$

$h=$ vertical displacement $=5\ m$

$m =$ Mass of object $= 40\ Kg$

$g =$ Acceleration due to gravity $= 9.8\ ms^{-2}$

$Kinetic$ $Energy$ = $Loss$ $in$ $PE$ = $40 \times 5 \times 9.8= 1960\ J$

For half way down $=\dfrac{1960}{2}= 980\ J$

At this point i.e. half way object has an equal amount of potential and kinetic energy.

A weight-lifter lifts a weight of

$2000N$ straight up to a height of

$2.25\ m$ in

$65$ seconds. The weight-lifter kept holding the weight at the same height for the next

$4$ seconds before dropping it to the floor. Find out how much work did the weight lifter do while holding the weights overhead for

$4$ seconds ?

0%
$1300\ Joules$
0%
$3077\ Joules$
0%
$4500\ Joules$
0%
$8000\ Joules$
0%
No work was done

Explanation

We know that work done is given by ,

$W=F \times s$ where

$F$ is force

$s$ is displacement in direction of force

while the lifter is holding the weights overhead for

$4 \ s$ , there is no displacement , so work done

$W=F\times 0=0$

All types of energy that exist in the nature can be categorized as either kinetic or potential. Choose correct statement regarding the validity of above sentence.

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This statement is true for thermal and and mechanical, but not elastic
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This statement is true for all different types of energy
0%
This statement is false, except in the case of mechanical energy
0%
This statement is never true

A car standing at the top of a hill would be an example of what type of energy?

0%
Kinetic
0%
Topical
0%
Potential
0%
Beneficial
0%
Residual

Explanation

The car standing at the top of hill has attained a height above the ground level. Thus it has attained a gravitational potential energy given by

$mgh$

where

$m$ is the mass of the car,

$g$ is the gravitational constant,

$h$ is the height above ground that it has attained.

This energy can convert into the kinetic energy of the car if it starts coming down the hill, that is lose its potential energy.

Find out work done by a person in pushing a box with a force of

$10\ N$ over a distance of

$4.0\ m$ in the direction of the force?

0%
$0.4\ J$
0%
$4.0\ J$
0%
$40\ J$
0%
$400\ J$
0%
$4000\ J$

Explanation

$W = F \times s= 10 \times 4=40\ J$

$120J$ of work is done in

$2$ minutes by a water pump, the power of the pump is:

0%
$14.4\ KW$
0%
$240\ W$
0%
$60\ W$
0%
$1\ W$

Explanation

Given: Work done,

$W=120\ J$

Time,

$t=2\ min$

Power,

$P=\dfrac{W}{t}=\dfrac{120}{2 \times 60} = 1W$

A father holds his child on his shoulders during a parade. The father does no work during the parade because:

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No force acts on the child
0%
The momentum of the child is constant
0%
The potential energy of the child is varying
0%
The child's kinetic energy is constant
0%
The child's distance from the ground remains the same

Explanation

We know , work is given by ,

$W=fd \ cos\theta$

In the given example, father exerts force on child in upward direction and his displacement is in forward direction , therefore angle between force and displacement is

$90^{o}$ due to this work done is zero. Also there is no upward or downward displacement of child is taking place as work done is zero. Hence child's distance from the ground remains the same.

CBSE Questions for Class 9 Physics Work And Energy Quiz 9 - MCQExams.com

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Practice Class 9 Physics Quiz Questions and Answers

CBSE Questions for Class 9 Physics Work And Energy Quiz 9 - MCQExams.com

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Practice Class 9 Physics Quiz Questions and Answers

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