Case study ( 4 Marks )

Question 14 Marks

A more powerful vehiclewould complete a journey in a shorter timethan a less powerful one. We talk of the powerof machines like motorbikes and motorcars.The speed with which these vehicles changeenergy or do work is a basis for theirclassification. Power measures the speed ofwork done, that is, how fast or slow work isdone. Power is defined as the rate of doingwork or the rate of transfer of energy. If anagent does a work $W$ in time $t$, then power isgiven by$P$= work/time
$P= W/T$. The unit of power is watt.
(i) The rate of doing work is defined as
$(a)$ Energy
$(b)$ Force
$(c)$ Power
$(d)$ None of these
(ii) Total energy consumed divided by total time taken is called as
$(a)$ Average power
$(b)$ Instantaneous power
$(c)$ Both $a$ and $b$
$(d)$ None of these
(iii) Let A and B having same weight start climbing the rope and reach height of 10m. Let A takes 10sec while B takes 12sec then work done
$(a)$ By both will be same
$(b)$ By $A$ is more than work done by $B$
$(c)$ By $B$ is more than work done by $A$
$(d)$ None of these
(iv) Define 1 Watt of power
(v) An electric bulb of 20W is used for 5h per day. Calculate the ‘units’ of energy consumed in one day by the bulb.

Answer

$(i) c$
$(ii) a$
$(iii) a$
$(iv)$ A power is said to be $1$ watt when $1$ joule of work is done within $1$ second of time.
$(v)$ Power of electric bulb $= 20\ W$
$= 0.02\ kW.$
Time used, $t = 5\ h$
Energy = power $\times $ time taken
$= 0.02kW \times 5 h$
$= 0.10 kW\ h$
$= 0.10$‘units’.
The energy consumed by the bulb $0.10$ units

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Question 24 Marks

The form of energy can be changed from one form to another. What happens to the totalenergy of a system during or after the process?Whenever energy gets transformed, the totalenergy remains unchanged. This is the law ofconservation of energy. According to this law, energy can only be converted from one form to another it can neither be created nor destroyed. The total energy before and after the transformation remains the same.The lawof conservation of energy is valid inall situations and for all kinds of transformations. Thus during motion the sum of the potential energy and kinetic energy of the object would be the same at all points. That is, potential energy + kinetic energy = constant.Andcalled as mechanical energy.
(i) Which of the energy conversion occur in electric iron?
$(a)$ Electric energy converted into heat energy
$(b)$ Electric energy converted into light energy
$(c)$ Heat energy converted into electrical energy>
$(d)$ None of these
(ii) When ball drops from height which of the energy conversion takes place
$(a)$ Gravitational potential energy converted into kinetic energy
$(b)$ Kinetic energy converted into Gravitational potential energy
$(c)$ Heat energy converted into electrical energy
$(d)$ None of these
(iii) When ball is thrown vertically upward which of the following quantity remains constant?
$(a)$ Energy
$(b)$ Force
$(c)$ Potential energy
$(d)$ None of these
(iv) State law of conservation of energy.
(v) In hydroelectric power plant which energy conversion happens?

Answer

$(i) a$
$(ii) a$
$(iii) a$
$(iv)$ This is the law ofconservation of energy. According to this law,energy can only be converted from one form to another it can neither be created nor destroyed. The total energy before and after the transformation remains the same.
$(v)$ In hydroelectric power plant potential energy of water reservoir is converted into electric energy.

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Question 34 Marks

Lift an object through a certain height. The object can now do work. It begins to fall when released. This implies that it has acquired some energy. If raised to a greater height it can do more work and hence possesses more energy. From where did it get the energy? In the above situations, the energy gets stored due to the work done on the object. The energy transferred to an object is storedas potential energy if it is not used to cause a change in the velocity or speed of the object.An object increases its energy when raisedthrough a height. This is because work isdone on it against gravity while it is being raised. The energy present in such an objectis the gravitational potential energy.The gravitational potential energy of anobject at a point above the ground is definedas the work done in raising it from the ground by height hto that point against gravity.Let the work done on the object against gravity beW. That is,
work done, $W$ = force $\times $ displacement
$= mg \times h$
Therefore potential energy $(PE)= mg*h.$
(i) Energy possessed by body due to its position is called
$(a)$ Potential energy
$(b)$ Kinetic energy
$(c)$ Nuclear energy
$(d)$ None of these
(ii) SI unit of potential energy is
$(a)$ Joule$(J)$
$(b)$ Newton meter$(N-m)$
$(c)$ both $a$ and$ b$
$(d)$ None of these
(iii)You do work while winding the key of a toy car. The energy transferred to the spring inside is stored as
$(a)$ Potential energy
$(b)$ Kinetic energy
$(c)$ Nuclear energy
$(d)$ None of these
(iv)Find the energy possessed by an object of mass 5kg when it is at a height of 10 m above the ground. Given, $g = 9.8 m/s^2.$
(v)Find the work done by Gravity on an object of mass 5 kg which moves from height 10m to ground when it is released from height of 10 m. Given, $g = 9.8 m/s^2.$

Answer

$(i)$ $a$
$(ii)$ $c$
$(iii)$ $a$
$(iv)$ we have potential energy as
$= mg \times h$
$=5 \times 9.8 \times 10$
$=490 J$
$(v)$ work done, $W$ = force \times displacement
$= mg \times h$
$= 5 \times 9.8 \times 10$
$=490 J$

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Question 44 Marks

A moving object can do work. An object moving faster can do more work than an identical object moving relatively slow. A moving bullet, blowing wind, a rotating wheel, a speeding stone can do work. How does a bullet pierce the target? How does the wind move the blades of a windmill? Objects in motion possess energy. We call this energy kinetic energy.Thus, the kinetic energy possessed by an object of mass, m and moving with a uniform velocity, $v$ is
$\mathrm{KE}=1 / 2 \times \mathrm{mv}^2$
The energy possessed by an object is thus measured in terms of its capacity of doing work. The unit of energy is, therefore, the same as that of work, that is, joule $(J)$.
(i) Energy possessed by body which is in motion is called
$(a)$ Potential energy
$(b)$ Kinetic energy
$(c)$ Nuclear energy
$(d)$ None of these
(ii) Which of the following has same unit?
$(a)$ Potential energy and Force
$(b)$ Kinetic energy and work
$(c)$ Both $a$ and $b$
$(d)$ None of these
(iii) Kinetic energy depends
$(a)$ Inversely on velocity of body
$(b)$ Directly on square of velocity of body
$(c)$ Directly on velocity of body
$(d)$ None of these
(iv) Define kinetic energy of body. Give its SI unit
(v) Is kinetic energy scalar or vector? Justify your answer

Answer

$(i) b$
$(ii) b$
$(iii) b$
$(iv)$ Energy possessed by object due to its motion is called as kinetic energy. Its $SI$ unit is $N$-m or Joule$(J)$.
$(v)$ kinetic energy is scalar quantity as it is a work done and work done is scalar quantity hence kinetic energy is also scalar quantity and doesn’t have any direction.

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Question 54 Marks

Work done by force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force. Work has only magnitude and no direction. Work done is negative when the force acts opposite to the direction of displacement. Work done is positive when the force is in the direction of displacement.The unit of work is newton-metre ($N$ m)or joule $(J)$.
(i) Work done is
$(a)$ Scalar quantity
$(b)$ Vector quantity
$(c)$ Tensor quantity
$(d)$ None of these
(ii) When force acts against the direction of displacement then work done will be
$(a)$ positive
$(b)$ negative
$(c)$ both a and b can possible
$(d)$ None of these
(iii) SI unit of work is
$(a)$ Joule$(J)$
$(b)$ Newton meter$(N-m)$
$(c)$ both $a$ and $b$
$(d)$ None of these
(iv)You are lifting stone from floor. Work is done by theforce exerted by you on the stone. Theobject moves upwards. The force youexerted is in the direction ofdisplacement. However, there is theforce of gravity acting on the object. Which one of these forces is doingpositive work?
Which one is doing negative work?
(v) Define 1J of work.

Answer

$(i) a$
$(ii) b$
$(iii) c$
$(iv)$ Here work done by you is positive work as work is being done in the direction of displacement unlike in case of gravitational force which acts in downward direction against the direction of displacement which is in upward direction.
$(v)$ When $1$ Newton of force acts on body and body displaces from its position by $1$ meter then the work done is said to be $1$ joule $(J)$.

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Science Case study ( 4 Marks )

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