Concept of Force

Q.

Why is cotton is solid but it floats on water?

Q. Under isothermal condition two soap bubbles of radii $r_1\text{and}{r}_2$ coalesce to form a single bubble of radius $r$. The external pressure is $P_0$. Find the surface tension of the soap in terms of the given parameters.

1. $\frac{P_0(r^3-r_1^3-r_2^3)}{4(r_1^2+r_2^2-r^2)}$
2. $\frac{P_0(r^3-r^3-r_2^3)}{2(r_1^2+r_2^2)}$
3. $\frac{P_0(r^3-r_1^3-r_2^3)}{(r_1^2+r_2^2)}$
4. $\frac{P_0(r^3-r_1^3-r_2^3)}{(r_1^2+r_2^2-r^2)}$

Q. Two small drops of mercury each of radius $R$ coalesce to form a single large drop. The ratio of total surface energy before and after the change is :

1. $2^{\frac{1}{3}}:1$
2. $2:1$
3. $1:2$
4. $1:2^{\frac{1}{3}}$

Q.

A cubical block of wood weighing 200 g has a lead piece fastened underneath. Find the mass of the lead piece which will just allow the block to float in water. Specific gravity of wood is 0.8 and that of lead is 11.3.

Q.

The excess pressure inside a soap bubble is twice the excess pressure inside a second soap bubble. The volume of the first bubble is n times the volume of the second where n is

1. 4

2. 1

3. 2

4. 0.125

Q.

What causes potential energy in a molecule inside a liquid?

Q.

A drop of liquid of radius $R={10}^{-2}m$ having surface tension $S=\frac{0.1}{4\pi }N{m}^{-1}$ divides itself into $K$ identical drops. In this process the total change in the surface energy $\Delta U={10}^{-3}J$. If $K={10}^{\alpha }$ then the value of $\alpha$ is

Q.

If work done in increasing the size of rectangular soap film with dimensions $8cm\times 3.75cmto10cm\times 6cm$ is $2\times 10–4J$. The surface tension of film in newton per meter is

1. $6.6\times {10}^{-2}$

2. $165\times {10}^{-2}$

3. $8.25\times {10}^{-2}$

4. $3.3\times {10}^{-2}$

Q. At the middle of the mercury barometer tube, there is a little column of air with the length $l_0$ and there is vacuum at the top as shown. Under the normal atmospheric pressure and the temperature of $300\text{K}$, $l_0=10\text{cm}$. What will be the length of the air column $($in $\text{cm})$ if the temperature rises to $330\text{K}$ ?

$($Neglect expansion of the tube$)$

Q.

The radius of a soap bubble is increased from $\frac{1}{\sqrt{\pi }}$ cm to $\frac{2}{\sqrt{\pi }}$cm If the surface tension of water is 30 dynes per cm, then the work done will be

1. 180 ergs

2. 360 ergs

3. 720 ergs

4. 960 ergs

Q.

Why is waters surface tension so high?

Q.

The rising air bubble in water increases in size. Why?

Q. Why do potential energy of a gas increase when is volume increases

Q. A convex meniscus is observed when mercury is taken in a capillary tube. Choose the correct option.

1. Mercury molecules shows higher cohesive force than adhesive force with glass.
2. Mercury molecules shows less cohesive force than adhesive force with glass.
3. Both (a) and (b)
4. None of these

Q. Two spherical bubbles coalesce isothermally. $V$ is the consequent change in volume and $S$ is the total change in surface area, then

Let $P$ be atmospheric pressure outside the bubbles and $T$ be surface tension.

1. $3PV+4ST=0$
2. $4PV+3ST=0$
3. $2PV+3ST=0$
4. $3PV+2ST=0$

Q.

The surface energy of a liquid drop is u. It is splitted into 103 equal droplets. Then its surface energy becomes:

1. u

2. 100u

3. 1000u

4. 10u

Q. If $F$ represents the strength of cohesive force, then what will be the correct order for strength of cohesive force for solid, liquid and gas ?

1. $F_{solid}<F_{liquid}<F_{gas}$
2. $F_{solid}>F_{liquid}>F_{gas}$
3. $F_{solid}<F_{liquid}=F_{gas}$
4. $F_{liquid}>F_{gas}>F_{solid}$

Q. If the dimension of liquid drop and bubble of same liquid solution is same, then find the ratio of excess pressure inside liquid drop to the excess pressure inside bubble?

1. $2:1$
2. $1:2$
3. $3:2$
4. $2:3$

Q.

The potential energy of a molecule on the surface of liquid compared to one inside the liquid is

1. Zero

2. Smaller

3. The same

4. Greater

Q. Two holes are made in the side of the tank such that the jets of water flowing out of them meet at the same point on the ground. If one hole is at a height of 3 cm above the bottom, then the distance of the other hole from the top surface of water is

1. $\frac{3}{2}cm$
2. $2\times 3cm$
   
3. $\sqrt{3}cm$
   
4. $3cm$

Q. Two masses $m_1$ and $m_2$ which are connected with a light string are placed over a frictionless pulley. This set up is placed over a weighing machine as shown in figure. Three combination of masses $m_1$ and $m_2$ are used, in first case $m_1=6\text{kg}$ and $m_2=2\text{kg}$, in second case $m_1=5\text{kg}$ and $m_2=3\text{kg}$, in the third case $m_1=4\text{kg}$ and $m_2=4\text{kg}$. Masses are released from rest. If $W_1,W_2$ and $W_3$ be the reading of weighing machine in three cases, then:

1. $W_1>W_2>W_3$
2. $W_1<W_2<W_3$
3. $W_1=W_2=W_3$
4. $W_1=W_2<W_3$

Q. After the drop detaches, its surface energy is

1. $5.4\times {10}^{-6}\text{J}$
2. $1.4\times {10}^{-6}\text{J}$
3. $2.7\times {10}^{-6}\text{J}$
4. $8.1\times {10}^{-6}\text{J}$

Q. A piece of gold with a cavity present in it weighs $10\text{g}$ in air and $9\text{g}$ in water. What is the volume of the cavity?
$[$Density of gold $=19.3{\text{g/cm}}^3]$

1. $0.182{\text{cm}}^3$
2. $0.482{\text{cm}}^3$
3. $1.382{\text{cm}}^3$
4. $2.482{\text{cm}}^3$

Q. Choose the correct statement.

1. The force of attraction between the molecules of two different substance is called adhesive force.
2. A strong adhesive force causes the liquid to spread all over the surface, on which it is poured.
3. Both $\left(a\right)$ and $\left(b\right)$
4. None of these.

Q.

The surface tension of soap solution is $25\times {10}^{-3}N{m}^{-1}$. The excess pressure inside a soap bubble of diameter 1 cm is

1. 10 Pa

2. 2 Pa

3. 20 Pa

4. 5 Pa

Q. Which of the following statement is incorrect?

1. The force of attraction between the molecules of the same substance is called cohesive force.
2. Surface tension is the effect of cohesive force.
3. A strong force of cohesion forms water droplets on any surface.
4. None of these.

Q.

Define adhesive forces?

Q.

If T denotes the surface tension and R, the radius of a capillary tube, then the dimensions of T/R are the same as that of

1. pressure

2. energy

3. force

4. None of these

Q. If the dimension of liquid drop and bubble of same liquid solution is same, then find the ratio of excess pressure inside liquid drop to the excess pressure inside bubble?

1. $2:1$
2. $1:2$
3. $3:2$
4. $2:3$

Q. A concave meniscus is observed for a liquid kept in a capillary tube, then angle of contact can be

1. ${85}^{\circ }$
2. ${93}^{\circ }$
3. ${100}^{\circ }$
4. ${95}^{\circ }$