[1 Mark Question Answer]

Question 11 Mark

A lead pellet of mass 4 g leaves an air gun with a velocity of $50 ms^{-1}$. What is the magnitude of potential energy stored by the spring of air gun?

Answer

Given mass $(m)=4 g=\frac{4}{1000} kg$; Velocity $(V)=50 ms^{-1}$
$\therefore$ Kinetic energy of pellet $=\frac{1}{2} m V^2=\frac{1}{2} \times \frac{4}{1000} \times 50 \times 50=5 J$,
Now, potential energy = kinetic energy = $5 ~ J$.

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Question 21 Mark

The heart of running athlete is beating 100 times a minute. If the work done by the heart per beat is 1.2 J , calculate the power of the heart.

Answer

Number of beats made by heart in 1 minute (60s) $=100$
$\therefore$ Number of beats made by heart in 1 second $=\frac{100}{60}$
So, Power of heart $=$ number of beats per sec $\times$ work done per beat $=\frac{100}{60} \times 1.2=2 W$.

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Question 31 Mark

For how long must an electric motor pump of $2 HP$ operate, so as to pump $5 m^3$ of water from a depth of $15 m ? [$Take $g =10 N / kg , 1 m^3$ of water $=10^3 kg, 1 HP =750 W$ ]

Answer

Mass of water $=5 \times 10^3 kg$
$\text { Work }=m g h=5 \times 10^3 \ kg \times 10 N / \ kg \times 15 m=75 \times 10^4 J$
$\text { Time }=\frac{\text { Work }}{\text { Power }}=\frac{75 \times 10^4 J}{2 HP}=\frac{75 \times 10^4 J}{2 \times 750 W}=0.5 \times 10^3 s=500 s .$

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Question 41 Mark

What force must be applied to a body through a distance of $10 m$ , such that it does a work of $4000 J$ ? If the mass of body is $20 \ kg$ , what is the acceleration of the body?

Answer

$S=10 m, W=4000 J, m=20 \ kg$
$\text { Force }=\frac{W}{S}=\frac{4000 J}{10 m}=400 N$
$\text { Acceleration }=\frac{\text { Force }}{\text { mass }}=\frac{400 N}{20 \ kg}=20 ms^{-2}$

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Question 51 Mark

A boy of mass 55 kg runs up a flight of 40 stairs, each measuring 0.15 m in 15 s calculate: [Take $g =10 ms^{-2}$ ]
(a) work done by the boy
(b) power developed by the boy

Answer

(a) Work $= F \times S =\left(55 kg \times 10 ms^{-2}\right) \times(40 \times 0.15) m = 3 3 0 0 J$.
(b) Power $=\frac{ W }{t}=\frac{3300}{15 s}=220 W$.

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Question 61 Mark

(a) Define electron volt and express it in joule.
(b) Give an example when electric energy changes into magnetic energy.

Answer

(a) 1 electron volt $( eV )$ is defined as the amount of work done when an electron moves through an electric field with a potential difference of 1 volt.
$1 eV=\text { charge on one electron } \times 1 V=1.6 \times 10^{-19} C \times 1 V=1.6 \times 10^{-19} J$
(b) Electromagnet

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Question 71 Mark

Define kilowatt hour and convert it into joule.

Answer

Kilowatt hour (kWh) : When an electric power of one kilowatt flows through a conductor for one hour, then the electrical energy which flows through the conductor is one kilowatt hour.
$1 kWh=1 kW \times 1 \text { hour }=1000 \frac{J}{s} \times 3600 s=3,600,000 J=3.6 \times 10^6 J= 3 . 6 ~ M J$

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Question 81 Mark

Give example of :
(a) When heat energy changes into kinetic energy.
(b) When sound energy changes into electric energy.

Answer

(a) In heat engines, the heat energy changes to kinetic energy.
(b) Microphone

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Question 91 Mark

(a) State the absolute unit of power in SI system.
(b) What is horse power? What is its magnitude in SI unit?

Answer

(a) Unit of power in SI system is watt.
(b) Horse power is a bigger unit of power. $1 HP =746 W$ (in SI unit).

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Question 101 Mark