Tag: physics

Questions Related to physics

A dog while barking delivers about $1 mW$  of power. If this power is uniformly distributed over a hemispherical area, the sound level at a distance of $5 m$ is (given  10 log$ _{10}$ 6.37 $=$0.8 )

intensity and loudness waves physics

  1. $50 dB$

  2. $76 dB$

  3. $68 dB$

  4. $48 dB$

Show answer
Correct Option: C
Explanation:

$I = \dfrac{Power}{Surface Area}$

$I = 6.37 \times 10^{-6} Wm^{-2}$

$I _{o} = 10^{-12} Wm^{-2}$

$SL = 10 log (\dfrac{I}{I _{o}})$

$SL = 10 log (\dfrac{6.37 \times 10^{-6}}{10^{-12}})$

$SL = 10 log (6.37) + 10 log(10^{6})SL = 8 + 60 = 68\ dB$

When a sound wave enters the ear, it sets the eardrum into oscillation, which  in turn causes oscillation of 3 tiny bones in the middle ear called ossicles. This oscillation is finally transmitted to the fluid filled in inner portion of the ear termed as inner ear, the motion of the fluid disturbs hair calls within the inner ear which transmit nerve impulses to the brain with information that a sound is present. The three bones present in the middle ear are named as hammer, anvil and stirrup. Out of these the stirrup is the smallest one and this only connects the middle  ear to inner ear as shown in the figure below. The area of stirrup and its extent of connection with the inner ear limits the sensitivity of the human ear. Consider a person's eat whose moving part of the eardrum has an area of about 43 mm$^{2}$ and the area of stirrup is about 3.2 mm$^{2}$. The mass of ossicles is negligible. As a result, force  exerted by sound wave in air on eardrum and ossicles is same as the force exerted by ossicles on the inner ear. Consider a sound wave having maximum pressure fluctuation of $3\times10^{-2}$ Pa from its normal equilibrium pressure value which is wqual to $10^{5}$ Pa. Frequency of sound wave is 1200 Hz. 
Data: Velocity of sound wave in air is  332 m/s. Velocity of sound wave in fluid (present in inner ear) is 1500 m/s. Bulk modulus of air is $1.42\times10^{5}$ Pa. Bulk modulus of fluid is $2.18\times10^{9}$ Pa. 


If then person is using an hearing aid, which increase the sound intensity level by 30 dB, then by what factor the intensity of given sound wave change as perceived by inner ear? 

intensity and loudness waves physics

  1. 1000

  2. 100

  3. 10,000

  4. None of these

Show answer
Correct Option: A
Explanation:

The intensity of sound in decibel is given by: 


$I _{dB}=10log(\dfrac{I}{I _0})$

Hence, $30=10log(\dfrac{I}{I _0})$

$\implies I=10^3I _0=1000I _0$

$\implies \dfrac{I}{I _0}=1000$

A bird is singing on a tree and a man is hearing at a distance $'r'$ from the bird. Calculate the displacement of the man towards the bird so that the loudness heard by man increases by $20\;dB$.
[Assume that the motion of man is along the line joining the bird and the man]

intensity and loudness waves physics

  1. $\displaystyle\frac{9r}{10}$

  2. $\displaystyle\frac{r}{10}$

  3. $\displaystyle\frac{3r}{5}$

  4. $\displaystyle\frac{4r}{5}$

Show answer
Correct Option: A
Explanation:

Loudness $\beta=10\;log _{10}\displaystyle\frac{I}{I _0}$

$\therefore\;\beta _2-\beta _1=10\;log _{10}\displaystyle\frac{I _2}{I _1}$

$\because\;I=\displaystyle\frac{P}{4\pi r^2}\;\therefore\;\displaystyle\frac{I _2}{I _1}=\displaystyle\frac{r _1^2}{r _2^2}$

$\therefore\;(\beta+20)-\beta=10\;log _{10}\displaystyle\frac{r^2}{r _2^2}=20\;log _{10}\displaystyle\frac{r}{r _2}$

$\Rightarrow \displaystyle\frac{r}{r _2}=10\Rightarrow r _2=0.1\;r$

$\therefore\;shift=r-0.1\;r=0.9\;r$.

What is the relation between thrust and pressure?

physics floatation pressure in liquids and gases pressure dependence on force and area concept of pressure

  1. Pressure directly proportional to thrust

  2. Pressure indirectly proportional to thrust

  3. No relation

  4. none

Show answer
Correct Option: A
Explanation:
Hence pressure=$P=\dfrac{Thrust}{Area}$
$\implies Pressure\propto Thrust$

A force of N acts on a body of $5kg$ mass. Find the acceleration of body? 

physics force and types of force introduction to force what is force? concept of force and its unit

  1. $a=\dfrac{N}{50} m/{s}^{2}$

  2. $a=\dfrac{N}{15} m/{s}^{2}$

  3. $a=\dfrac{N}{5} m/{s}^{2}$

  4. $a=\dfrac{N}{10} m/{s}^{2}$

Show answer
Correct Option: C
Explanation:

$F=ma$

$N=5a$
$\dfrac { N }{ 5 } =a$
$a=\dfrac{N}{5} m/{s}^{2}$

A stone is suspended by a thread from a rigid support, another thread of same quality is attached at the bottom of done if the lower thread is pulled suddenly then

physics force and types of force introduction to force what is force? concept of force and its unit

  1. Upper thread break first

  2. Lower thread break first

  3. Both will break at same time

  4. None of these two threads will break

Show answer
Correct Option: B

Fill in the blanks .
At least _____________ object must interact for a force to come into play. 

physics force and types of force introduction to force what is force? concept of force and its unit

  1. 1

  2. 2

  3. 3

  4. 4

Show answer
Correct Option: B
Explanation:

Forces are used in our daily life actions like pushing, pulling, lifting, twisting etc. So, a force is a pull or push acting on an object which tends to change the state of rest/ motion of an object. An interaction of one object with another object results in a force between the two objects.

For example, Weightlifter uses muscular force to lift the weights.

Hence, At least 2 object must interact for a force to come into play. 

Which one of the following statements regarding Newton's first law of motion is incorrect?

physics force and types of force introduction to force what is force? concept of force and its unit

  1. It is an independent statement

  2. it defines an inertial frame of reference

  3. It was first enunciated by Galileo

  4. It is a special case of Newton's second law.

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Correct Option: A

Which statement defines force?

physics force and types of force introduction to force what is force? concept of force and its unit

  1. When a force acts on a body that is free to move, the force is the product of the mass of the body and its acceleration

  2. When a force acts on a body that is free to move, the force is the rate of change of momentum of the body

  3. When a force acts on a body that is free to move, the force is the work done by the force divided by the distance moved by the body

  4. When a force acts on a lever and causes a moment, the force is the moment divided by the perpendicular distance of the force from the pivot

Show answer
Correct Option: B
Explanation:

we know that 

$F= ma $ ..........(1)
and $a = \dfrac{dv}{dt}$
So from above eqn . 
$\implies F = \dfrac{mdv}{dt}$
$\implies F = \dfrac{dmv}{dt}$
$\implies F = \dfrac{dp}{dt}$
So, force = rate of change of momentum

Force exerted is :

physics force and types of force introduction to force what is force? concept of force and its unit

  1. Effort

  2. Load

  3. Fulcrum

  4. None of these

Show answer
Correct Option: A
Explanation:

Effort is defined as a force exerted by a machine or in a process.