# Concept of Force

## Trending Questions

**Q.**

Why is cotton is solid but it floats on water?

**Q.**Under isothermal condition two soap bubbles of radii r1 and r2 coalesce to form a single bubble of radius r. The external pressure is P0. Find the surface tension of the soap in terms of the given parameters.

- P0(r3−r31−r32)4(r21+r22−r2)
- P0(r3−r3−r32)2(r21+r22)
- P0(r3−r31−r32)(r21+r22)
- P0(r3−r31−r32)(r21+r22−r2)

**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 :

- 213:1
- 2:1
- 1:2
- 1:213

**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

4

1

2

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=0.14π Nm−1 divides itself into K identical drops. In this process the total change in the surface energy ΔU=10−3 J. If K=10α then the value of α is

**Q.**

If work done in increasing the size of rectangular soap film with dimensions $8cm\xc3\u20143.75cmto10cm\xc3\u20146cm$ is $2\xc3\u201410\xe2\u20ac\u201c4J$. The surface tension of film in newton per meter is

$6.6\xc3\u2014{10}^{-2}$

$165\xc3\u2014{10}^{-2}$

$8.25\xc3\u2014{10}^{-2}$

$3.3\xc3\u2014{10}^{-2}$

**Q.**At the middle of the mercury barometer tube, there is a little column of air with the length l0 and there is vacuum at the top as shown. Under the normal atmospheric pressure and the temperature of 300 K, l0=10 cm. What will be the length of the air column (in cm) if the temperature rises to 330 K ?

(Neglect expansion of the tube)

**Q.**

The radius of a soap bubble is increased from 1√π cm to 2√πcm If the surface tension of water is 30 dynes per cm, then the work done will be

180

*ergs*360

*ergs*720

*ergs*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.

- Mercury molecules shows higher cohesive force than adhesive force with glass.
- Mercury molecules shows less cohesive force than adhesive force with glass.
- Both (a) and (b)
- 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.

- 3PV+4 ST=0
- 4PV+3 ST=0
- 2PV+3 ST=0
- 3 PV+2 ST=0

**Q.**

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

u

100u

1000u

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 ?

- Fsolid<Fliquid<Fgas
- Fsolid>Fliquid>Fgas
- Fsolid<Fliquid=Fgas
- Fliquid>Fgas>Fsolid

**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?

- 2:1
- 1:2
- 3:2
- 2:3

**Q.**

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

Zero

Smaller

The same

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

- 32 cm
- 2×3 cm

- √3 cm

- 3 cm

**Q.**Two masses m1 and m2 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 m1 and m2 are used, in first case m1=6 kg and m2=2 kg, in second case m1=5 kg and m2=3 kg, in the third case m1=4 kg and m2=4 kg. Masses are released from rest. If W1, W2 and W3 be the reading of weighing machine in three cases, then:

- W1>W2>W3
- W1<W2<W3
- W1=W2=W3
- W1=W2<W3

**Q.**After the drop detaches, its surface energy is

- 5.4×10−6 J
- 1.4×10−6 J
- 2.7×10−6 J
- 8.1×10−6 J

**Q.**A piece of gold with a cavity present in it weighs 10 g in air and 9 g in water. What is the volume of the cavity?

[Density of gold =19.3 g/cm3]

- 0.182 cm3
- 0.482 cm3
- 1.382 cm3
- 2.482 cm3

**Q.**Choose the correct statement.

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

**Q.**

The surface tension of soap solution is 25×10−3Nm−1. The excess pressure inside a soap bubble of diameter 1 cm is

10

*Pa*2 Pa

20

*Pa*5

*Pa*

**Q.**Which of the following statement is incorrect?

- The force of attraction between the molecules of the same substance is called cohesive force.
- Surface tension is the effect of cohesive force.
- A strong force of cohesion forms water droplets on any surface.
- 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

pressure

energy

force

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?

- 2:1
- 1:2
- 3:2
- 2:3

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

- 85∘
- 93∘
- 100∘
- 95∘