Tag: physics

Questions Related to physics

One thousand small water droplets of equal size combine to form a big drop. The ratio of the final surface energy to the initial surface energy is: (Surface tension of water = 70 dyne/cm) 

surface tension physics surface energy of a liquid work done in stretching a liquid surface: surface energy of a liquid properties of matter

  1. 10:1

  2. 1000:1

  3. 1:10

  4. 1:1000

Show answer
Correct Option: C
Explanation:

Let the radius of smaller radius be $r$ and bigger be $R$.

Since volume remains constant, thus  $\dfrac{4\pi R^3}{3}=1000\times \dfrac{4\pi r^3}{3}$
We get  $R=10r$
Ratio of final surface energy to initial is 
$\dfrac{W _f}{W _i}=\dfrac{T\times 4\pi R^2}{T\times 1000\times 4\pi r^2}$
$\dfrac{W _f}{W _i}=\dfrac{(10r)^2}{1000r^2}=1:10$

A metal plane having an area of $0.04\ m^{2}$ is placed on a horizontal wooden surface. Oil of coefficient of viscosity $2\ N/ s/m^{2}$ is introduced between the plate and the surface till the thickness of the oil layer is $0.5$ in. The horizontal force needed to drag the plate along the surface with a velocity of $5\ cm/s$ is  

surface tension physics surface energy of a liquid work done in stretching a liquid surface: surface energy of a liquid properties of matter

  1. $80\ N$

  2. $8\ N$

  3. $60\ N$

  4. $6\ N$

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Correct Option: A

Two spherical soap bubbles of a radii $r _1$ and $r _2$ in vacuum coalesce under isothermal conditions. The resulting bubble has the radius $R$ such that

surface tension physics surface energy of a liquid work done in stretching a liquid surface: surface energy of a liquid properties of matter

  1. $R=r _1+r _2$

  2. $R^2 ={ r _1^2 + r _2^2}$

  3. $R=\dfrac{r _1+r _2}{r _2}$

  4. none of these

Show answer
Correct Option: B

What is the change in surface energy, when a mercury drop of radius $R$ splits up into $1000$ droplets of radius $r$?

surface tension physics surface energy of a liquid work done in stretching a liquid surface: surface energy of a liquid properties of matter

  1. $8\pi R^2T$

  2. $16\pi R^2T$

  3. $24\pi R^2T$

  4. $36\pi R^2T$

Show answer
Correct Option: D
Explanation:

Let Surface Tension be T.

Initial surface = $4\Pi { R }^{ 2 }$
Final Surface area = $1000\times 4\Pi { r }^{ 2 }$
& Initial Volume = final volume
$\dfrac { 4 }{ 3 } \Pi { R }^{ 3 }=1000\times \dfrac { 4 }{ 3 } \Pi { r }^{ 3 }$
R = 10r
So,
    Final surface area = $1000\times 4\Pi \times \dfrac { { R }^{ 2 } }{ 100 } =40\Pi { R }^{ 2 }$
So, Workdone = Change in Surface energy
                         = $T\Delta A=(40-4)\Pi { R }^{ 2 }T$
       $\boxed { Change\quad in\quad Surface\quad energy=36\Pi { r }^{ 2 }T } $.

Potential energy of a molecule on the surface of a liquid is as compared to another molecule inside of the liquid is

surface tension physics surface energy of a liquid work done in stretching a liquid surface: surface energy of a liquid properties of matter

  1. More

  2. Less

  3. Both a and b

  4. None of these

Show answer
Correct Option: A
Explanation:

Due to surface Tension, Potential energy of a molecule at free surface is more than the molecules inside the liquid.

Assertion (A): Steel is more elastic than rubber.
Reason (R) : Under a given deforming force, steel is deformed less than rubber.


A) Both Assertion and Reason are true and the reason is correct explanation of the assertion
B)Both Assertion and Reason are true, but reason is not correct explanation of the assertion
C) Assertion is true, but the reason is false
D) Assertion is false, but the reason is true

physics hooke's law

  1. A

  2. B

  3. C

  4. D

Show answer
Correct Option: A
Explanation:

More elastic the material, higher the Young's Modulus.
Young's Modulus of Steel = 200 GPa
Young's Modulus of Rubber = 0.01-0.1 GPa
Young's Modulus of Steel is much higher than Rubber, So steel is more elastic than rubber.

Y = FL/Al
Y - Young's Modulus
L - Actual Length
A - Cross Section Area
l - Elongation

l = FL/AY
keeping F,L,A constant 
l is inversely proportional to Y
Hence, Higher the Young's Modulus lesser the deformation (for Different Materials)

Change in the shape of a body caused by the application of a force (stress) is called as :

physics hooke's law

  1. ductility

  2. deformation

  3. hardness

  4. rigidity

Show answer
Correct Option: B
Explanation:

If a force on a body changes the normal positions of the molecules of a body, resulting in a change in the configuration of the body, then the force applied is called deforming force and change in shape of the body is called deformation. 

Change in shape of a body caused by the application of stress is called:

physics hooke's law

  1. rigidity

  2. elasticity

  3. sheer

  4. deformation

Show answer
Correct Option: D
Explanation:

Deformation of a body is the change in shape of a body caused by the application of stress. According to the Hooke's law, the deformation caused in an elastic material is directly proportional to the stress applied to it.

State whether true or false :
While loading the crane with heavy bodies, it is given rest from time to time.

physics hooke's law

  1. True

  2. False

Show answer
Correct Option: A

State whether true or false :
The property of an elastic body by virtue of which its behaviour becomes less elastic under the action of repeated alternating deforming force is called elastic fatigue

physics hooke's law

  1. True

  2. False

Show answer
Correct Option: A
Explanation:

Elastic fatigue is defined as the temporary loss of elastic properties of a material by applying repeated force on the material. If there is no sufficient time between repeated force application, then a permanent deformation remain there in each time of applying force.