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151.

In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as (a) If both assertion and reason are true and reason is the correct explanation of assertion. (b) If both assertion and reason are true but reason is not the correct explanation of assertion. (c ) If assertion is true but reason is false. (d) If assertion and reason are false. Q. Assertion: A large soap bubble expands while a small bubble shrinks, when they are connected to each other by a capillary tube. Reason: The excess pressure inside bubble (or drop) is inversely proportional to the radius.

Answer» Correct Answer - A
Since the excess pressure due to surface tension is inversely proportional to its radius, it follows that smaller the bubble, greater is the excess pressure. Thus, when the larger and the smaller bubbles are put in communication, air starts passing from the smaller into the large bubble because excess pressure inside the former is greater than inside the letter. As a result the smaller bubble shrinks and the larger one swells.
Discussion
152.

In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as (a) If both assertion and reason are true and reason is the correct explanation of assertion. (b) If both assertion and reason are true but reason is not the correct explanation of assertion. (c ) If assertion is true but reason is false. (d) If assertion and reason are false. Q. Assertion: At critical temperature, surface tension of a liquid becomes zero. ltbr. Reason: At this temperature intermolecular forces for liquids and gases become equal. Liquid can expand without any restriction.

Answer» Correct Answer - A
Zero surface tension means no opposition to expansion.
Discussion
153.

The breaking stress of a wire depends onA. length of the wireB. radius of the wireC. material of the wireD. shape of the cross section

Answer» Correct Answer - C
The breaking stress of a wire is independent of its geometrical properties it depends upon type of material of wire.
Discussion
154.

In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as (a) If both assertion and reason are true and reason is the correct explanation of assertion. (b) If both assertion and reason are true but reason is not the correct explanation of assertion. (c ) If assertion is true but reason is false. (d) If assertion and reason are false. Q. Assertion: The bridges are declared unsafe after long use. Reason: Elastic strength of bridges losed with time.

Answer» Correct Answer - A
When the huge traffic is passed over bridge, the strain undergoes for a long number of times every day. When the bridge is used for a long time, the elastic strength is being lost continuously. Because of it the strain in the bridge becomes very large for particular stress and hence bridge may damage.
Discussion
155.

The wattability of a surface by a liquid depends primarily onA. ViscosityB. Surface tensionC. DensityD. Angle of contact between the surface and the liquid.

Answer» Correct Answer - D
Wetability of a surface by a liquid primarily depends on angle of contact between the surface and liquid if angle of contact is acute liquids wet the solid and vice versa.
Discussion
156.

The following four wires are made of the same material. Which of these will have the largest extension when the same tension is applied?A. length`=50cm`,`diameter`=0.5mm`B. length`=100cm`, diameter`=1mm`C. length`=200cm` diameter`=2mm`D. length`=300cm`, diameter`=3mm`

Answer» Correct Answer - A
`triangle_1prop(100)/(1^2)prop100`
`triangleL_2prop(100)/(2^2)prop50`
`triangleL_3prop(300)/(3^2)prop(100)/(3)`
`triangleL_3prop(50)/((1)/(4))prop200`
`triangleLprop(L)/(A)` or `triangleLprop(L)/(D^2)`
The ratio of `(L)/(D^2)` is maximum for case (a) Hence, option (a) is correct.
Discussion
157.

The following four wires are made of the same material. Which of these will have the largest extension when the same tension is appliedA. length 50 cm and diameter 0.5 mmB. length 100 cm and diameter 1 mmC. length 200 cm and diameter 2 mmD. length 300 cm and diameter 3 mm

Answer» Correct Answer - A
`Y=(FL)/(pir^2l)`
`l=(FL)/(pir^2Y)implieslprop(L)/(r^2)`
`(L)/(r^2)` is greatest for option A.
Discussion
158.

Two wires of same diameter of the same material having the length `l` and `2l` If the force `F` is applied on each, the ratio of the work done in two wires will beA. `1:2`B. `1:4`C. `2:1`D. `1:1`

Answer» Correct Answer - A
`W=(1)/(2)Fl` `because` `Wpropl`
`(W_1)/(W_2)=(l_1)/(l_2)=(l)/(2l)=(1)/(2)`
Discussion
159.

When a pressure of 100 atmosphere is applied on a spherical ball, then its volume reduces to `0.01%`. The bulk modulus of the material of the rubber in dyne/`cm^2` isA. `10xx10^12`B. `100xx10^12`C. `1xx10^12`D. `20xx10^12`

Answer» Correct Answer - C
`K=(100)/((0.01)/(100))=10^6atm=10^11(N)/(m^2)=10^2(dyn e)/(cm^2)`
Discussion
160.

An air bubble of radius `r` in water is at a depth `h` below the water surface at some instant. If `P` is atmospheric pressure, `d` and `T` are density and surface tension of water respectivley . the pressure inside the bubble will be :A. `p + hdg - (4T)/(r)`B. `p + hdg + (2T)/(r)`C. `p + hdg - (2T)/(r)`D. `p + hdg + (4T)/(r)`

Answer» Correct Answer - B
Discussion
161.

When the load on a wire is slowly increased from `3kgwt` to `5 kg wt`, the elongation increases from `0.61` to `1.02 mm`. The work done during the extension of wire isA. `0.16J`B. `0.016J`C. `1.6J`D. `16J`

Answer» Correct Answer - B
Work done `=(1)/(2)` stretching force `xx` extension.
Net work done in increasing the length from `0.164mm` to `1.02mm` is
`W-W_1=(1)/(2)xx5xx9.8xx1.02xx10^-3-(1)/(2)xx3xx9.8xx10^-3`
`=0.016J`
So the correct choice is (b).
Discussion
162.

A wire of length `L` and radius `r` is fixed at one end. When a stretching force `F` is applied at free end, the elongation in the wire is `l`. When another wire of same material but of length `2L` and radius `2r`, also fixed at one end is stretched by a force `2F` applied at free end, then elongation in the second wire will beA. `l`B. `2l`C. `(l)/(2)`D. `(l)/(4)`

Answer» Correct Answer - A
`l=(FL)/(AY)=(FL)/(pir^2Y)` `because` `lprop(FL)/(r^2)`
`(l_2)/(l_1)=(F_2)/(F_1)xx(L_2)/(L_1)((r_1)/(r_2))^2=2xx2xx((1)/(2))^2=1`
`l_2=l_1` i.e., increment in its length will be `l`.
Discussion
163.

Choose the wrong alternative.A. Detergents should have small angles of contactB. Surface tension of liquid is independent of the area of the surfaceC. A drop of liquid under no external forces (other than surface tension) is always spherical in shape.D. None of these

Answer» Correct Answer - D
Discussion
164.

Mark the wrong statementA. Sliding of molecular layer is much easier than compression or expansion.B. Reciprocal of bulk modulus of elasticity is called compressibilityC. It is difficult to twist a long rod as compared to small rodD. Hollow shaft is much stronger than a solid rod of same length and same mass

Answer» Correct Answer - C
For twisting, angle of shear `phiprop(1)/(L)`
i.e., if `L` is more than `phi` will be small.
Discussion
165.

A uniform cube is subjected to volume compression. If each side is decreased by `1%`, then bulk strain isA. 0.01B. 0.06C. 0.02D. 0.03

Answer» Correct Answer - D
If side of the cube is L then `V=L^3implies(dV)/(V)=3(dL)/(L)`
`%` change in volume`=3xx(%`change in length)
`=3xx1%=3%` `because` Bulk strain `(triangleV)/(V)=0.03`
Discussion