Class 11th

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(c), (d) For liquids coefficient of viscosity $\eta \propto \frac{1}{\sqrt[]{T}}$

i.e with increase in temperature $\eta$ decreases.

For gases coefficient of viscosity $\eta \propto \sqrt[]{T}$

i.e with increase in temperature $\eta$ decreases.

This is a multiple choice answer as classified in NCERT Exemplar

(a), (b) When the coin falls into the water, weight of the (block + coin) system decreases, which was balanced by the upthrust force earlier. As weight of the system decreases, hence upthrust force will also decrease which is only possible when l decreases.

As l decreases volume of water displaced by the block decreases, hence h decreases. As the coin falls into water, it displaces some volume of water which is very less. Hence, we neglect volume of the coin.

This is a multiple choice answer as classified in NCERT Exemplar

(b), (c) Pressure is defined as the ratio of magnitude of component of the force normal to the area and the area under consideration.

As magnitude of component is considered, hence, it will not have any direction. So, Pressure is a scalar quantity.

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(b), (d) Consider the diagram where two molecules of a liquid are shown. One is well inside the liquid and other is on the surface. The molecule (A) which is well inside experiences equal forces from all directions, hence net force on it will be zero.

And molecules on the liquid surface have some extra energy as it surrounded by Only lower half side of liquid molecules.

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(c) According to the question, the observed meniscus is of convex figure shape. Which is only possible when angle of contact is obtuse. Hence, the combination will be of mercury-glass (140°)

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(a) d₁= diameter at 1^st point is 2.5

d₂= diameter at 2^nd  point is 3.75

Applying equation of continuity for cross section A₁ and A₂

A₁V₁=A₂V₂

^{$\frac{V_1}{V_2}=\frac{A_2}{A_1}=\frac{\pi r_2^2}{\pi r_1^2}=\left(\frac{r_2}{r_1}\right)$ 2}

= ${\left(\frac{\frac{3.75}{2}}{\frac{2.5}{2}}\right)}^2$ = ${\left(\frac{3.75}{2.5}\right)}^2=\frac{9}{4}$

This is a multiple choice answer as classified in NCERT Exemplar

(b) As we know for a streamline flow of a liquid velocity of each particle at a particular cross-section is constant, because Av = constant (law of continuity) between two cross-section of a tube of flow.

This is a multiple choice answer as classified in NCERT Exemplar

(d) In a streamline flow at any given point, the velocity of each passing fluid particles remains constant. If we consider a cross-sectional area, then a point on the area cannot have different velocities at the same time, hence two streamlines of flow cannot cross each other.

This is a multiple choice answer as classified in NCERT Exemplar

(c) When the pebble is falling through the viscous oil the viscous force is

F=6 $\pi \eta rv$

where r is radius of the pebble, v is instantaneous speed, his coefficient of viscosity. As the force is variable, hence acceleration is also variable so v-t graph will not be straight line. First velocity increases and then becomes constant known as terminal velocity.