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CBSE Questions for Class 9 Physics Work And Energy Quiz 2 - MCQExams.com
CBSE
Class 9 Physics
Work And Energy
Quiz 2
The SI unit of energy is:
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newton
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pascal
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joule
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watt
Explanation
As energy is defined via work so,the SI unit for
energy is the same as the unit of work that is
j
oule (J)
1 kilowatt hour $$=$$ ___________ .
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$$10.6 \times 10^6$$Joule
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$$3.6 \times 10^6$$Joule
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$$30.6 \times 10^6$$Joule
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$$3.6 \times 10^5$$Joule
Explanation
$$1\ watt = 3600\ joule$$
$$1\ kW = 1000\ watt$$
$$1\ h=60\times 60 = 3600\ s$$
$$1\ kWh= 1\ kW\times 1\ hour=1000\ W\times 3600\ s = 1000 \times 3600 joule = 3.6 \times 10^{6}\ J$$
Find the energy in $$kWh$$ consumed in $$10$$ hours by four devices of power $$500\ W$$ each.
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$$30 \:kWh$$
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$$25 \:kWh$$
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$$20 \:kWh$$
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$$35 \:kWh$$
Explanation
Hint: Calculate the energy consumed by each device.
Correct answer is an option (C).
Step 1: Calculate energy consumed by each device
The power of each device is $$500W$$.
The time for which each device is used is $$10hr$$
Energy consumed by each device $$=500\times 10$$=
$$5000Wh$$
= $$5kWh$$
Step 2: Calculate energy consumed by four devices.
Energy consumed by one device $$=5kWh$$
Energy consumed by four devices $$=5\times 4$$
$$=20kWh$$
Hence, the energy consumed by four devices in $$kWh$$ is $$20kWh$$.
The correct answer is an option (C).
Work is said to be done when:
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(i) a force should act on an object, and (ii) the object must be displaced.
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(i) no force should act on an object, and (ii) the object must be displaced.
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(i) a force should act on an object, and (ii) the object must not be displaced.
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(i) no force should act on an object, and (ii) the object must not be displaced.
Explanation
Work is done whenever the given condition is satisfied:
(i) A force should act on an object
(ii) There is a displacement of the body caused by the applied force along the direction of the applied force.
The expression for work done, when the displacement is in the direction of the force is:
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$$ F \times S$$
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$$ 0$$
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$$- \dfrac{F}{ S}$$
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$$ \dfrac{F \times S}{2}$$
Explanation
In this case, the force and the displacement acts along the same direction. So, work done is positive. Therefore, the expression for work done is given as $$Work\ done=Force(F)\times Displacement(S)$$.
The CGS unit for energy is:
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dyne
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erg
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centimeter
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joule
Explanation
The CGS unit for energy is erg.
$$1\ erg = 10^{-7} J$$
The power output of a motor is $$5 \ kW.$$ What is the work done by the motor in $$5 \ s$$?
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2.5 k J
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25 kJ
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250 k J
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25000 k J
Explanation
Power $$=\dfrac{Work }{Time}$$
So Work $$={Power}\times Time$$
Given:
$$P=5 \ kW$$
$$t= 5 \ sec$$
$$W=5\times 5 =25 \ kJ$$
Hence, option B is correct.
The capacity of a body to do work is called ______ .
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Stamina
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Potential
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Velocity
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E
nergy
Explanation
Energy is the capacity of a physical system to perform work. Energy exists in several forms such as heat energy, kinetic or mechanical energy, light energy, potential energy, electrical energy, or other forms. According to the law of conservation of energy the total energy of a system remains constant, though energy may transform into another form.
The SI unit of energy is the joule (J) or newton-meter (N-m). Joule is also the SI unit of work.
The rate of doing work is called ........
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power
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energy
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acceleration
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velocity
Explanation
power is the rate of doing work. It is equivalent to an amount of energy consumed per unit time. In the SI system, the unit of power is the joule per second (J/s).
Energy can be defined as:
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The capacity to do work
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Work done in unit time
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A push or a pull
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Attraction between two oppositely charged particles
Explanation
Energy is the capacity of a system to perform work. The various forms of energy include potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.
Which among the following is the commercial unit of electrical energy?
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Volt
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Watt
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Ampere
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KWh
Explanation
Answer is D.
The kilowatt-hour is a unit of energy equal to 1,000 watt-hours, or 3.6 mega-joules. If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt-hours is the product of the power in kilowatts and the time in hours. The kilowatt-hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.
When heated, molecules absorb ____ and their capacity to do ____ increases.
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Work ,energy
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Energy ,work
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Heat, work
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Energy ,heat
Explanation
When heated, molecules absorb the $$energy$$, and their capacity to do $$work$$ increases.
When the time taken to complete a given amount of work increases, then :
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power generated increases
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power
generated
decreases
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energy increases
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energy decreases
Explanation
Power is the rate of energy generation(or absorption).
The relation between time and power is P = $$\dfrac{E}{t}$$.
So power and time are inversely proportional to each other. Hence when the time to complete a given amount of work increases then the power generated decreases.
Power is measured in :
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joule per second
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joule per minute
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joule per square second
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none of these
Explanation
As, Power$$=\dfrac{Energy(J)}{Time(S)}$$
So, Power is measured in joule per second. It is known as $$watt$$.
Power spent by a machine depends on:
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the amount of work done by the machine
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time taken by the machine to do the work
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both A and B
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neither of A and B
Explanation
$$Power= \dfrac{Work\quad done}{time \quad taken}$$
Power spent by a machine depends on two factors:
(i) the amount of work done by the machine and
(ii) the time taken by the machine to do the work.
If a machine performs a given amount of work at a faster rate, more power is spent by the machine.
If $$1J$$ of work is done in $$1s$$, the power spent is equal to:
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$$1\ Js$$
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$$1\ MW$$
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$$1\ W$$
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None of these
Explanation
If $$1\ J$$ of work is done in $$1\ s$$, the power spent is said to be $$1\ W$$.
$$Power = \dfrac{work}{time}$$
$$\Rightarrow 1\ W=\dfrac{1J}{1s}$$
$$\Rightarrow 1\ W=1{J}{s}^{-1}$$
Unit of power is
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Hp
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Kilowatt hour
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Watt
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None
Explanation
The power of the appliance is defined as the rate of consumption of energy by the appliance.
Power,
$$P = \dfrac{Energy}{time}$$
The unit of power is $$Js^{-1}$$ or watt $$(W)$$.
Work is expressed by the formula as
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Work $$=$$ Force (N) $$\times$$ Distance (m)
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Work $$=$$ Force (g) $$\times$$ Distance (m)
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Work $$=$$ Force (mg) $$\times$$ Distance (m)
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Work $$=$$ Force (kg) $$\times$$ Distance (cm)
Explanation
Unit of force is Newton (N).
For a heater rated at 4.4 KW; 220 V, calculate the energy consumed by the heater in 4 hours?
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12 KWh
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17.6 KWh
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0 KWh
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4.4 KWh
Explanation
Given : $$P = 4.4$$ KW, $$t = 4$$ h
$$\therefore$$ Energy consumed in 4 h
$$\Rightarrow E= P\times t =4.4\times 4 = 17.6$$ kWh
One megawatt is equivalent to
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$$\displaystyle 10^{2}W$$
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$$\displaystyle 10^{6}W$$
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$$\displaystyle 10^{4}W$$
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$$\displaystyle 10^{8}W$$
Explanation
Prefix Mega means $$ 10^6 $$.
So, 1 Megawatt = $$1 \times 10^6$$ W
1 Megawatt = a
1 Horsepower = b
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$$\displaystyle a={ 10 }^{ 3 }W,b=1.34W$$
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$$\displaystyle a={ 10 }^{ 6 }W,b=746W$$
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$$\displaystyle a={ 10 }^{ 6 }J,b=0W$$
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$$\displaystyle a={ 10W,b={ 10 }^{ 3 }W }$$
Explanation
Mathematically ,1 Megawatt $$ = 10^6$$ W
One horsepower is equal to 746 watt i.e $$1$$ hp $$ = 746$$ W
An agent does $$4$$ J of work in $$2$$ s. Find the power of the agent.
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$$1$$ W
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$$3$$ W
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$$2$$ W
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$$6$$ W
Explanation
Given: Work done $$W=4\ J$$, time $$t=2\ s$$
We know that $$Power=\dfrac{Work}{Time}=\dfrac{4}{2}=2\ W$$
Practical uit of power
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kw
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mw
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gw
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hp
Explanation
Practical unit of power is horsepower $$(hp)$$.
One horsepower is equal to 746 watts.
whereas Watt is the SI unit of power.
The practical unit for the measurement of electric power is:
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watt
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kilowatt
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kilowatt hour
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horse power
Explanation
SI unit of power is $$J/s \ (watt).$$
Watt can be represented as $$kilowatt \ (10^3$$watt).
Kilowatt hour
is unit of energy (or watt sec, Kilowatt can be changed to watt and hour to sec) so this is unit of energy.
Horsepower is similarly another unit of power. But it is not very convenient to use since we are using the SI units of energy (joules) and time (seconds) to calculate power. We will have to divide the result by $$746$$ to obtain power in Horsepower. $$(\because 1 \ HP=746 \ W)$$
So, the best possible answer is watt, i.e. option A.
Which of the following will lead to a change in kinetic energy of a body?
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change in its mass
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change in its velocity
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all of the above
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none of the above
Explanation
Kinetic energy of the body $$K = \dfrac{1}{2}mv^2$$
$$\implies$$ $$K \propto m$$ and $$K \propto v^2$$
Thus changing either the mass of the body or its velocity will lead to a change in kinetic energy of a body.
What is potential energy?
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Energy of an object due to its position or arrangement in a system
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Energy of an object due to its nature or arrangement in a system
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Energy of an object due to its shape or arrangement in a system
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None
Explanation
The potential energy is the stored energy of an object due to its position.
some examples of potential energies are gravitational potential energy, Electrostatic potential energy and elastic energy etc.
A body placed at ground will have less gravitational potential energy than a body placed at some height. therefore, potential energy changes by changing the position of object. Hence, Option A is correct.
Which of the following is NOT a correct unit for work?
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erg
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watt
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joule
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Nm
Explanation
Option A
erg- It is the unit of work in CGS system
Option B-
Watt- It is the unit of power which work per unit time
Option C-
joule- It is the unit of work in MKS system in terms of energy
Option D-
Nm- Newton meter is the unit of work in terms of force and displacement.
Unit of work is
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Joule
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Newton
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Tesla
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None of these
Explanation
Work is defined as the product of force applied and the displacement of the object multiplied by the $$cos$$ of angle between the force applied and the displacement.
Work done $$W = F s \cos\theta$$
SI unit of work is Joule $$(J)$$.
State whether the given statement is True or False :
The potential energy of a free falling object decreases progressively.
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True
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False
Explanation
Because when body falls from a height potential energy decreases and kinetic energy progressively increases. During this total mechanical energy remains conserved.
If Rahul has done the same amount work in less time compared to Rohan then:
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Rahul has more power
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Rohan has more power
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Both Rahul and Rohan have equal power
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Rahul has more energy than Rohan
Explanation
The relation between power P and work W is given by $$P=W/t$$,
where, $$t$$ be the time
For same amount of doing work, Power $$P\propto \dfrac 1t$$
$$\because$$ Rahul is taking less time in compare to Rohan so, Rahul has more power.
Option A
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