Tag: work and energy

Questions Related to work and energy

A body projected vertically from the earth reaches a height equal to earth's radius before returning to the earth. The power exerted by the gravitational force is greatest :

physics work and power commercial unit of energy power work and energy

  1. at the highest position of the body

  2. at the instant just before the body hits the earth

  3. it remains constant throughout

  4. at the instant just after the body is projected

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Correct Option: B
Explanation:

$Power,$ $P$ $=$$\overrightarrow{F}$.$\overrightarrow{v}$ $=$ $Fv$ $cos$$\theta$
$Just$ $before$ $hitting$ $the$ $earth$ $θ$ $=$ $0°.$ $Hence,$ $the$ $power$ $exerted$ $by$ $the$ $gravitational$ $force$ $is$ $greatest$ $at$ $the$ $instant$ $just$ $before$ $the$ $body$ $hits$ $the$ $earth.$

1 kWh is equal to

physics work and energy commercial unit of energy power work and power

  1. $3.6 \times 10^6 MJ$

  2. $3.6 \times 10^5 MJ$

  3. $3.6 \times 10^2 MJ$

  4. $3.6 MJ$

Show answer
Correct Option: D
Explanation:
1 kilowatt hour is the energy produced by 1 kilowatt  power source in 1 hour.

$1kWh=1kW\times 1hour=1000\times 3600 W.s$

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

$\implies 1kWh=3.6MJ$

Answer-(D)

Number of kilowatt-hours =$\dfrac { volt\times ampere\times time }{ 1000 } $. Then:

physics work and energy commercial unit of energy power work and power

  1. time in seconds

  2. time in minutes

  3. time in hours

  4. time in days

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Correct Option: C
Explanation:

Kilowatt-hours is the power generated in one hour=$\dfrac{volt\times current\times time( hour)}{1000}$


Answer-(C)

If 1 unit of electricity cost $0.20$, how much does it cost to switch on a heater marked $120 V$, $3 A$ for $90$ min?

physics work and energy commercial unit of energy power work and power

  1. $ 0.11$

  2. $ 2.70$

  3. $ 64.80$

  4. $ 108.00$

Show answer
Correct Option: A
Explanation:
Voltage across the heater $V = .12$ kilo-volts 
Current flowing through the heater $I = 3 A$
Thus power of the heater $P = VI$
$\therefore$ $P = 0.12 \times 3 = 0.36 $ $kW$
Time of operation $t = 90$ min $ = 1.5 $ hr
Thus energy consumed $E = Pt$
$\implies$ $E = 0.36 \times 1.5 = 0.54$ $kWhr$
Cost to switch on heater =  $0.54 \times 0.2 = 0.11$

A car of mass m starts from rest and accelerates so that the instantaneous power delivered to the car has a constant magnitude $P _{0}$. The instantaneous velocity of this car is proportional to:

physics work and power commercial unit of energy power work and energy

  1. $t^{1/2}$

  2. $t^{-1/2}$

  3. $t/\sqrt{m}$

  4. $t^{2}P _{0}$

Show answer
Correct Option: A
Explanation:

Power =F.V


Power delivered as at 

Cont. magnitude $P _0$

$P _0=F.V$

$P _0=ma\times V$

$\dfrac{P _0}{m}=\dfrac{dv}{dt}\times V$

$\displaystyle \left(\dfrac{P _0}{m}\right)\int^t _0 dt=\int^v _0 vdv$

$\dfrac{P _0t}{m}=\dfrac{V^2}{2}$

$V^2=\left(\dfrac{2P _0}{m}\right)t$

$V=\sqrt{\dfrac{2P _0}{m}}\times t^{\dfrac{1}{2}}$

$\boxed{V\alpha\,t^{\dfrac{1}{2}}}$

$1$ kWh$=$ ______________J.

physics work and energy commercial unit of energy power work and power

  1. $3.6\times 10^6$

  2. $36\times 10^6$

  3. $3.6\times 10^7$

  4. $3.6\times 10^5$

Show answer
Correct Option: A
Explanation:

$1$ kilowatt-hour(kWh) is a unit of energy. Normally, we want energy to be expressed in joules(J) and time in seconds(s).
Energy(kWh)$=$Power(kW)$\times$(h)$=1000$W$\times 3600$s$=1000$J/s$\times 3600$s$=3600000$J$=3.6\times 10^6$J.

The force acting on a 4gm mass in the energy region ${ U=8x }^{ 2 }$ at x= -2 cm is :

physics energy and its forms idea of energy introduction to work and energy work and energy

  1. 8 dyne

  2. 4 dyne

  3. 16 dyne

  4. 32 dyne

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Correct Option: D
Explanation:

Given,

$m=4gm$
$U=8x^2$

The force acting on mass $m$ in the potential energy $U$  at $x=-2cm$ will be

$F=-\dfrac{dU}{dx}$
$F=-\dfrac{d(8x^2)}{dx}$
$F=-16x$
$|F| _{x=-2cm}=-16\times(-2)=32dyne$
The correct option is D.



Which of the following pairs represent units of the same physical quantity?

physics energy and its forms idea of energy introduction to work and energy work and energy

  1. kelvin and joule

  2. kelvin and calorie

  3. newton and calorie

  4. joule and calorie

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Correct Option: D
Explanation:

joule and calorie are both units of energy .
$1\  joule = 0.24 \ calories$

What is the equivalent of $1$ joule in calorie?

physics energy and its forms idea of energy introduction to work and energy work and energy

  1. $0.24$

  2. $2.4$

  3. $24$

  4. $240$

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Correct Option: A
Explanation:
$1 joule = 1/4.18 \ cal= 0.24 \ calorie$

Name the quantity which is measured in $Wh$.

physics energy and its forms idea of energy introduction to work and energy work and energy

  1. Force

  2. Power

  3. Energy

  4. All the above

Show answer
Correct Option: C
Explanation:

We know that $ power = work/ time$.

Power has unit $W$ and time can have either seconds or minutes or hours.Hence work can have either $W\ min$ or $Wh$ or $Ws$. We in general consider work is nothing but energy. Hence energy can have unit $Wh$.