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Page No 212:

Question 1:

While performing an experiment on verifying the laws of reflection of sound, a student is to choose between
(i) a narrow or a wide tube, and
(ii) a strong or a faint source of the sound.
The observed experimental difference, between the values of angle of incidence and angle of reflection, is likely to be minimum when he chooses a :
(1) narrow tube and a faint source
(2) wide tube and a faint source
(3) narrow tube and a strong source 
(4) wide tube and a strong source

Answer:

The observed experimental difference, between the values of angle of incidence and angle of reflection, is likely to be minimum when he chooses a narrow tube and a strong source. Because narrow tube allows the selected sound waves to travel clearly so that more distinct sound can be heard. Also, a strong source is always preferred over a faint source because the sound should be clear and audible to verify the laws of reflection of sound.

Hence, the correct answer is option 3.

Page No 212:

Question 2:

For verifying the laws of reflection of sound, a student has to choose from :
(i) a black polished metal sheet or a white thermocol sheet.
(ii) a 0.5 m long tube of diameter 3 cm or a 1.5 m long tube of diameter 20 cm
He should prefer to choose the :
(1) metal sheet and the 0.5 m long tube
(2) metal sheet and the 1.5 m long tube
(3) thermocol sheet and the 0.5 m long tube
(4) thermocol sheet and the 1.5 m long tube

Answer:

Metal surfaces reflect the highest percentage of the incident sound waves, whereas the thermocol sheets absorb the sound waves. Also, a narrow tube allows the selected sound wave to travel more clearly. That's why to verify the laws of reflection, the student should choose a metal sheet with a 0.5 m long tube of 3 cm diameter.

Hence, the correct answer is option 1.

Page No 212:

Question 3:

A student sets up a slinky on a smooth tabletop in the manner shown here.




How can he produce transverse waves in the slinky by moving its free end Q?
(1) at an angle of 45º with the table top
(2) backward and forward along the length of the slinky
(3) up and down
(4) left and right

Answer:

In a transverse wave, the particles are displaced perpendicular to the direction the wave travels. The student can generate a transverse wave by moving the free end Q of the slinky, vertically up and down.

Hence, the correct answer is option 3.

Page No 212:

Question 4:

A student lists the following precautions for the experiment on determining the velocity of a pulse propagated through a stretched string :
(A) The string should not be stretched too tight.
(B) The counting of the pulse journeys must start from zero and not from one.
(C) The string should be stretched straight in contact with the table.
(D) The amplitude of the pulse should be kept appreciably high.
The incorrect entry, in this list of precautions, is the precaution listed as :
(1) A
(2) B
(3) C
(4) D

Answer:

The incorrect entry in the above list of precautions is C, because the table should not in contact with the string, as it would cause a disturbance in the pulse propagation. when a pulse travels on a string, particles of the string move perpendicularly up and down, so there should be enough space both side the rope to particle moves up and down.

Hence, the correct answer is option 3.

Page No 212:

Question 5:

For plotting temperature-time graph for a hot body, as it cools to room temperature, a student is to choose one each from each of the following pairs.
A : Calorimeter
(i) blackened from outside
(ii) polished from outside

B : Base for keeping the calorimeter
(i) insulated
(ii) metallic
In order to get the correct graph he should prefer to choose :
(1) A (i), B (ii)
(2) A (ii), B (ii)
(3) A (i) , B (i)
(4) A (ii), B (i)

Answer:

For plotting temperature-time graph for a hot body, the student needs to choose a calorimeter which is blackened from outside as it would absorb more heat and the temperature change could be observed more accurately, also the base for keeping the calorie meter should be insulated so that the temperature of the surface where it is put does not interfere with the experiment readings.

Hence, the correct answer is option 3.



Page No 213:

Question 6:

The magnitude of zero error of the spring balance and least count of the measuring cylinder, shown here, are, respectively :



(1) 2.5g and 0.1 mL
(2) 5.0g and 0.1 mL
(3) 2.5g and 0.2 mL
(4) 5.0g and 0.2 mL

Answer:

Total number of divisions on the scale of spring balance = 100

Least count of the spring balance =250100= 2.5 g

Zero mark lags behind by 2 divisions on the main scale, so the zero error = 2.5×2=5.0 g

Total number of divisions on the scale of measuring cylinder = 50

Least count =1050=0.2 mL

Hence, the correct answer is option 4. 

Page No 213:

Question 7:

A student takes some water in a beaker and heats it over a flame for determining its boiling point. He keeps on taking its temperature readings. He would observe that the temperature of water :
(1) keeps on increasing regularly
(2) keeps on increasing irregularly
(3) first increases slowly, then decreases rapidly and eventually becomes constant
(4) first increases gradually and then becomes constant.

Answer:

When the water is heated over the flame, it's temperature increases continuously, until it starts boiling, after reaching boiling point, the temperature of the water does not change, it becomes constant, as water starts vapourising. 

Hence, the correct answer is option 4.

Page No 213:

Question 8:

The zero-error in the spring balance shown and the correct weight of the solid, suspended from it, are equal, respectively, to :



(1) + 2g wt; 19g wt
(2) + 2g wt; 15g wt
(3) − 2g wt; 19g wt
(4) − 2g wt; 15g wt

Answer:

Total number of the divisions on the scale = 30

Least count=3030=1 g

As the zero mark of the scale is leading from its main position by two divisions, so the zero error of the scale = 2×1=2 g

Weight according to the reading = 17 g wt

Actual weight = Main reading - zero error = 17 g wt - 2 g wt = 15 g wt

Hence, the correct answer is option 2.

Page No 213:

Question 9:

Four students A, B, C, and D while performing an experiment on establishing the relation between the loss of weight of a small solid when fully immersed in tap water, and the weight of water displaced by it, used four different shapes of overflow cans containing water as shown.



The arrangement, that would give correct results, is that of student :
(1) (A)
(2) (B)
(3) (C)
(4) (D)

Answer:

In the given experiment, when the body is immersed in water, the water displaced should be moved out of the small made in the can, so its weight can be measured for experiment purposes. But in cans A, B, and D, the water level is not matching with the height of the hole. So when the body will be immersed in those cans it will not release the exact amount of water which the body displaced, So all the cans except C would give false results, but C would give correct results, as the water level is exactly matching with the height of the hole. 

Hence, the correct answer is option 3.



Page No 214:

Question 10:

Two students X and Y have to do their experiment on plotting the temperature-time graph for a hot body as it cools to room temperature. They do their experiments in the same lab, using completely identical apparatus, take equal amounts of tap water heated up to the same temperature, start their observations simultaneously and note the temperature values at identically spaced intervals of time. Their temperature-time graphs, plotted on a given graph paper, with the same give choices of scales along the axis, are however, as shown :



The following could be the reason for this difference :
(A) Use of an overhead fan by student X.
(B) Use of an overhead fan by student Y.
(C) Less frequent stirring of water by student X.
(D) Less frequent stirring of water by student Y.
The most likely reason is :
(1) A
(2) B
(3) C
(4) D

Answer:

As we can see from the graph made by student X, the body cools down fast in comparison with the graph made by student Y. The body cools down slowly in the case of student B. As both students performed the same experiment under the same laboratory, the only factor of faster cooling in the case of student X can be the use of an overhead fan. So due to the use of an overhead fan by student X, the temperature of the body falls faster, it cools down quicker.

Hence, the correct answer is option 1.

Page No 214:

Question 11:

In the set up shown the weight of the body was measured in air and in water. The reading of mark X in the spring balance would be :



(1) 36g wt
(2) 30g wt
(3) 24g wt
(4) 6g wt

Answer:

When a body is immersed in a liquid, it feels an upward force, which is known as the buoyant force. The measure of the buoyant force is equal to the weight of the liquid displaced by the body. As we can see from the given figure, the volume of the liquid displaced is 6 mL, so the weight of the water displaced it 6 g wt. (Density of the water = 1 g/mL)

When the body is immersed in water, the spring balance will show the net weight of the body.

Net weight of the body in liquid = Actual weight - Buoyant force = 30 g wt - 6 g wt. = 24 g wt.

Hence, the correct answer is option 3.

Page No 214:

Question 12:

A student uses a spring balance of least count 10g wt and range 500g wt. He records the weight of small iron cube in air, in tap water and in a concentrated solution of common salt in water. If his three readings taken in this order are W1, W2 and W3, he is likely to observe that :
(1) W1 > W2 > W3 
(2) W3 > W2  > W1
(3) W1 > W3 > W2
(4) WW> W3 

Answer:

When a body is immersed in liquid, it feels an upward force, which is known as buoyant force. The measure of the buoyant force is equal to the weight of the liquid displaced by the body.

When immersed in the liquid, the spring balance will show the net weight reading of the body. So due to buoyant force, the weight of the body in the liquid will be less than the weight of the body in the air. As the density of the salt water is more than the normal water, hence the weight of the displaced salt water will be more than the same volume of the normal tap water, hence the buoyant force will be more when the body will be immersed in the salt water.

The body will have the least weight when immersed in salt water, and most when it is in the air. So we can say, W1 > W2 >W3

Hence, the correct answer is option 1.

Page No 214:

Question 13:

While performing an experiment on verifying the laws of reflection of sound, the 'reflected sound' can be detected better by keeping one ear :
(1) near the end of the tube and keeping the other ear closed
(2) near the end of the tube and keeping the other ear open
(3) at about 5 cm from the end of the tube and keeping the other ear closed
(4) at about 5 cm from the end of the tube and keeping the other ear open

Answer:

The reflected sound can be detected better by keeping one ear near the end of the tube and the other ear closed. Because by doing so, other environmental sounds can be excluded and one can focus only on the sound coming from the tube.

Hence, the correct answer is option 1.

Page No 214:

Question 14:

In an experiment to determine the densities, four solids A, B, C and D are immersed in a liquid contained in a measuring cylinder, one by one. The volumes of water displaced by the solids A, B, C and D are 100 cm3, 100 cm3, 80 cmand 80 cm3, respectively. When weighed in air, the masses of solids A, B, C, and D were found to be 80g, 100g, 100g and 80g, respectively. The two solids having identical densities are :
(1) A and C
(2) B and D
(3) C and D
(4) A and D

Answer:

Weights of the liquid displaced by A, B, C, and D are 100 g, 100 g, 80 g, and 80 g respectively, as the density of the water is 1 g/cm3. Now we can see the solid B and D displaces the liquid equal to their own weights, that means buoyant force is equal to the weight of the body in case of solids b and D. By this we can conclude that the density of both B and D is equal to the density of water, which makes their density identical, that's why they are experiencing the same buoyant force equal to their own weight.

Hence, the correct answer is option 2. 



Page No 215:

Question 15:

A student is given an iron cube of side 1 cm, a measuring cylinder of range 100 mL and least count 1 mL, and a spring balance of range 100g wt and least count 1g wt. He can use these to measure :
(1) both the mass and the volume of the given iron cube
(2) neither the mass nor the volume of the given iron cube
(3) only the mass of the given iron cube but not its volume
(4) only the volume of the given iron cube but not its mass

Answer:

The student can measure the mass of the cube by using spring balance. Also, he can measure the volume of the cube by immersing the whole spring balance system with the attached cube, inside the measuring cylinder. The volume of the water displaced will be equal to the volume of the body. 

Hence, the correct answer is option 1.

Page No 215:

Question 16:

Four beakers are labelled as P, Q, R and S. A student puts salt solutions of different concentrations in the four beakers without noting which beaker contains solution of which concentration. A solid suspended from the hook of a spring balance gives a reading of 150g in air. When this solid, while still suspended from the hook of spring balance, is fully immersed in beakers P, Q, R, and S, the readings shown by the spring balance in the four beakers P, Q, R and S are 110g, 130g, 140g and 120g respectively. The most concentrated solution is contained in the beaker labelled as :
(1) P
(2) Q
(3) R
(4) S

Answer:

The buoyant force depends on the weight of the liquid displaced, as the density of the different salt solution is different, so the buoyant force acting on them will be different too. More the density of the liquid more will be the weight of the liquid displaced, hence more will be the buoyant force acting in an upward direction on the body. Therefore, the most concentrated solution will show the least reading on the spring balance when immersed in liquid.

Hence, the correct answer is option 1.

Page No 215:

Question 17:

While doing an experiment on plotting the temperature-time graph of hot water as it cools, we can get a good graph :
(1) only by noting the temperature of hot water every 30 seconds throughout
(2) only by noting the temperature of hot water every 1 minute throughout
(3) by noting the temperature of hot water every 1 minute to start with and every 2 minutes later on
(4) by noting the temperature of hot water every 2 minutes to start with and every 1 minutes later on

Answer:

When hot water cools down, Initially its temperature falls at a higher rate but later the rate of fall of temperature decreases. So, in starting the temperature will fall faster but after sometime, it will fall a little slower. Hence, to plot a temperature-time graph for cooling of hot water, we have to note the temperature of hot water every 1 minute to start with and every 2 minutes later on. 

Hence, the correct answer is option 4.

Page No 215:

Question 18:

A student heats some amount of tap water in a calorimeter to a temperature of nearly 50ºC above the room temperature. He then records the temperature of this water as it is cooling down at regular intervals of 2 minutes each. He tabulates his observations as follows :

Time (in minutes) → 0 2 4 6 8 10
Temperature (in ºC) → t1 t2 t3 t4 t5 t6
He is likely to observe that :
(1) (t− t2) > (t3  t4) > (t5  t6)
(2)
 (t− t2) = (t t4) = (t5  t6)
(3) 
(t− t2) < (t t4) < (t5  t6)
(4) 
(t− t2) < (t t4) > (t5  t6)

Answer:

When hot water cools down, initially its temperature falls at a higher rate but later the rate of fall of temperature decreases. So, for every two minutes, the temperature difference would be greater in starting and will decrease with time. Therefore, we can say that (t− t2) > (t t4) > (t5  t6). 

Hence, the correct answer is option 1.

Page No 215:

Question 19:

When an object is fully immersed in a liquid, the apparent loss in weight :
(1) is less than the weight of liquid displaced by it
(2) is more than the weight of liquid displaced by it
(3) is equal to the weight of liquid displaced by it
(4) does not depend on the density of the liquid displaced by it

Answer:

When a body is immersed in liquid, an upside force acts on it, which is known as the buoyant force. The measure of the buoyant force is equal to the weight of the liquid displaced by the body. As the apparent weight loss in the body happens due to the buoyant force acting in the opposite direction of its weight, so the apparent weight loss is equal to the weight of the liquid displaced by the body. 

Hence, the correct answer is option 3.

Page No 215:

Question 20:

Four metal balls A, B, C and D having radius of 2.5 cm each are made of copper, aluminium, gold and iron respectively. The densities of copper, aluminium, gold and iron are 8.9 g/cm3, 2.7 g/cm3, 19.3 g/cm3 and 7.8 g/cm3 respectively. When the balls A, B, C and D are tied to threads, suspended from the hook of a spring balance and immersed completely in strong salty water, one by one, the apparent loss in weight will be :
(1) maximum in ball B
(2) maximum in ball C
(3) minimum in ball B
(4) same in all the balls

Answer:

When a body is immersed in a liquid, the apparent loss in the weight of the body occurs due to the Buoyant force acting on it. The buoyant force acts in an upward direction and is equal to the weight of water displaced by the body. All the four balls are of the same size, so they will replace the same amount of water. That's why the apparent loss in weight will be the same for all the balls.

Hence, the correct answer is option 4.

Page No 215:

Question 21:

A glass ball hanging from the hook of a sensitive spring balance is completely submerged in highly salty water and tap water, one by one. If the readings of spring balance when the ball is in highly salty water and tap water are x and y respectively, then :
(1) x < y
(2) x > y
(3) x = y
(4) x = 2y

Answer:

When a body is immersed in a liquid it feels an upward force, which is known as buoyant force. The buoyant force depends upon the weight of the liquid displaced by the body. 

In the given case, the same glass ball is being submerged in both the liquids, so the volume of water displaced will be the same in both cases. But the density of the highly salty water is more than the normal tap water, so the weight of the liquid displaced in case of highly salty water will be more. As a result, the glass ball will feel a more upward force when immersed in salt water. So the spring balance reading will be less for salty water and more for tap water. i.e. x < y.

Hence, the correct answer is option 1.

Page No 215:

Question 22:

You are given four salt solutions W, X, Y and Z of different concentrations. W is a 20% salt solution, X is a 35% salt solution, Y is a 10% salt solution whereas Z is a 50% salt solution. When a solid suspended from the hook of a spring balance is fully submerged in all these salt solutions, one by one, then the spring balance will show the minimum reading when the solid is immersed in :
(1) solution W
(2) solution X
(3) solution Y
(4) solution Z

Answer:

When a body is immersed in a liquid it feels an upward force, which is known as buoyant force. The buoyant force depends upon the weight of the liquid displaced by the body. 

In the given case, the same body is being submerged in all the solutions, so the volume of water displaced will be the same for all. But all the solutions have different densities. More the density of the solution, more the weight of the liquid displaced. As we can see from the given data that solution Z is more concentrated, so has more density as compared to other solutions. Weight of the liquid displaced will be maximum when the body will be submerged in the solution Z. So, the body will feel the maximum upward force in solution Z and will show the minimum reading on the spring balance. 

Hence, the correct answer is option 4.

Page No 215:

Question 23:

An aluminium ball is fully immersed in distilled water, well water, sea water and inland lake water, one by one. The aluminium ball will appear to suffer the maximum loss in weight when immersed in :
(1) distilled water
(2) sea water
(3) well water
(4) lake water

Answer:

When a body is immersed in a liquid it feels an upward force, which is known as buoyant force. The buoyant force depends upon the weight of the liquid displaced by the body.

In the given case, an aluminum ball is immersed in different liquids of the different densities. More the density of the liquid, more the weight of the liquid displaced by the ball. When the weight of the liquid displaced will be maximum, the ball will feel the maximum upward force, hence maximum loss in weight. Seawater is known to have the maximum density among all types of waters, so when the ball will be immersed in sea water, it will experience the maximum weight loss. 

Hence, the correct answer is option 2.



Page No 216:

Question 24:

When an object is fully submerged in strong salty water, it undergoes an apparent :
(1) loss in mass
(2) loss in volume
(3) loss in density
(4) loss in weight

Answer:

When the object will be fully submerged in strongly salty water it will feel an upward force, which is known as the buoyant force. Due to the presence of this upward buoyant force, the object will undergo an apparent loss in weight. 

Hence, the correct answer is option 4.

Page No 216:

Question 25:

If a body tied to a spring balance is fully immersed in a liquid, the apparent loss in its weight :
(1) is more in a denser liquid
(2) is less in a denser liquid
(3) does not depend on density of liquid
(4) is equal to apparent gain in weight of liquid

Answer:

When a body is immersed in a liquid it feels an upward force, which is known as buoyant force. The buoyant force depends upon the weight of the liquid displaced by the body. Due to the presence of this upward buoyant force, the body undergoes in an apparent weight loss. More the density of the liquid more will be the weight of the liquid displaced by the body, so more apparent weight loss. 

Hence, the correct answer is option 1.

Page No 216:

Question 26:

In an experiment on  determining the velocity of a pulse propagating through a stretched string, the stopwatch should be started and stopped at instants corresponding to the ones shown in :



(1) Fig. 1 and Fig. 2
(2) Fig. 1 and Fig. 3
(3) Fig. 2 and Fig. 1
(4) Fig. 2 and Fig. 3

Answer:

As given in the figure, a wave pulse starts traveling from end A towards the end B. To calculate the velocity of the given wave, the stopwatch should be started at the instant corresponding to the figure A and should be stopped at the instant when the pulse reaches the other end, which corresponds to the figure 3. By dividing the distance travelled by the pulse from end A to end B with the time reading obtained by the stopwatch, we can easily determine the velocity of the given wave. 

Hence, the correct answer is option 2.

Page No 216:

Question 27:

When a fresh egg is put into a beaker filled with water, it sinks. When the same egg is put in a strong salty water, then it floats. Which of the following is the incorrect statement in this context ?
(1) salt water enters into egg by osmosis and makes it lighter
(2) salt water exerts more buoyant force
(3) salt water is denser than tap water
(4) upthrust exerted by a liquid depends on its density

Answer:

The density of an egg is higher than the density of the water, so a normal egg sinks when it's put in the water. But if the same egg is put in strongly salty water, it floats. Because the density of the salty water is more than the egg, so it exerts more buoyant force on the egg which balances its weight.

Hence, the correct answer is option 2.

Page No 216:

Question 28:

When two balls, one of iron and the other of aluminium, are completely immersed in strong salty water, they undergo an equal loss in weight. This shows that iron and aluminium balls have :
(1) the same densities
(2) the same masses
(3) the same volumes
(4) the same weights

 

Answer:

When a body is immersed in a liquid, it undergoes an apparent weight loss due to an upward buoyant force acting on the body. The buoyant force depends only on the density of the liquid and the weight of the liquid displaced by the body.

In the given case, as both the bodies undergo an equal weight loss, that means they displace the same amount of liquid when immersed. For that, both the ball must have the same volume.

Hence, the correct answer is option 3.

Page No 216:

Question 29:

When two solids are put in two pans of a beam balance, they exactly balance each other in air. When the two solids are completely immersed in water alongwith  pans of balance, then they no longer balance each other. Which of the following is the incorrect statement about these balls ?
(1) they have equal masses in air
(2) they have equal weights in air
(3) they have equal volumes in air
(4) they have unequal densities

Answer:

As both the solid balance each other on a beam balance in the air, so they have the same mass and weight. When they are immersed in water along with beam balance, they no longer balance each-other because both the solids experience different buoyant force. The buoyant force depends on the volume of the body immersed, that means the solids do not have equal volumes.

Hence, the correct answer is option 3.

Page No 216:

Question 30:

The temperature-time graph obtained when hot water is allowed to cool resembles the graph given in one of the following figures. The correct figure is :



(1) A
(2) B
(3) C
(4) D

Answer:

When hot water cools down, its temperature falls at a continuous rate but initially, its temperature falls at a higher rate and later the rate of fall of temperature starts decreasing. The only graph matching with this description is given in option 4.

Hence, the correct answer is option 4.



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