Physics MCQs for NEET — Practice Questions with Answers

(b) The volume of liquid displaced by floating ice ${\mathrm{V}}_{\mathrm{D}}=\frac{\mathrm{M}}{{\mathrm{\sigma }}_{\mathrm{L}}}$

Volume of water formed by melting ice, ${\mathrm{V}}_F=\frac{\mathrm{M}}{{\mathrm{\sigma }}_W}$

If ${\mathrm{\sigma }}_L>{\mathrm{\sigma }}_{\mathrm{W}}$ , then, $\frac{\mathrm{M}}{{\mathrm{\sigma }}_L}<\frac{\mathrm{M}}{{\mathrm{\sigma }}_W} i.e. {\mathrm{V}}_{\mathrm{D}}<{\mathrm{V}}_{\mathrm{F}}$

i.e. volume of liquid displaced by floating ice will be lesser than water formed and so the level if liquid will rise.

At height h the rate of discharge of liquid through hole of bottom = $\sqrt{2gh}\times a$

Let in time dt level of water in tank decreases by dh.

Equating volume

$$\begin{gathered} Adh=dt\sqrt{2gh}\times a \\ \Rightarrow dt=\frac{A}{a\sqrt{2g}}\frac{dh}{\sqrt{h}} \end{gathered}$$

Now on inegrating

$$\begin{gathered} \int dt=\frac{A}{a\sqrt{2g}} {\int }_{H_2}^{H_1}\frac{dh}{h} \\ \Rightarrow t=\frac{A}{a}\sqrt{\frac{2}{g}}\left[\sqrt{H_1}-\sqrt{H_2}\right] \end{gathered}$$

(b) 

We know that the liquids make the solid substances wet when they get into physical contact with each other. This depends on the contact angle between the liquid and solid surfaces. When the liquid does not wet the sides of a solid object, it means that the angle of contact is more than $90°$ but less than $180°.$

Convex meniscus is formed because of cohesion and adhesion.

(d) Surface tension decreases with increase in temperature.

(b)

Cohesive force is the intermolecular attraction between like molecules, whereas adhesive force is the intermolecular force between unlike molecules. In the given case cohesive force between mercury molecules is more than adhesive force between mercury molecules and glass rod, hence mercury does not stick to rod.

(b)

Cohesive force is greater than the adhesive force.

(d)

(d) Because surface tension of water > surface tension of oil

(d) Sphere has the minimum surface area for the given volume of the liquid.