Tag: work and energy

Quizzes (20) Questions (90)

Questions Related to work and energy

Multiple choice physics work and power commercial unit of energy power 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 :

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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

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

1 kWh is equal to

  1. $3.6 \times 10^6 MJ$
  2. $3.6 \times 10^5 MJ$
  3. $3.6 \times 10^2 MJ$
  4. $3.6 MJ$
Reveal answer Fill a bubble to check yourself
D Correct answer
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)
Multiple choice physics work and energy commercial unit of energy power work and power

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

  1. time in seconds

  2. time in minutes

  3. time in hours

  4. time in days
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

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


Answer-(C)

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

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?

  1. $ 0.11$
  2. $ 2.70$
  3. $ 64.80$
  4. $ 108.00$
Reveal answer Fill a bubble to check yourself
A Correct answer
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 =  $0.54 \times 0.2 = 0.11$
Multiple choice physics work and power commercial unit of energy power work and energy

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:

  1. $t^{1/2}$
  2. $t^{-1/2}$
  3. $t/\sqrt{m}$
  4. $t^{2}P _{0}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Power =F.V


Power delivered as at 

Cont. magnitude $P _0$

$P _0=F.V$

$P _0=ma\times V$

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

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

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

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

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

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

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

$1$ kWh$=$ ______________J.

  1. $3.6\times 10^6$
  2. $36\times 10^6$
  3. $3.6\times 10^7$
  4. $3.6\times 10^5$
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

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

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

  1. 8 dyne

  2. 4 dyne

  3. 16 dyne

  4. 32 dyne
Reveal answer Fill a bubble to check yourself
D Correct answer
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.



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

Name the quantity which is measured in $Wh$.

  1. Force

  2. Power

  3. Energy

  4. All the above
Reveal answer Fill a bubble to check yourself
C Correct answer
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 $Wh$.