Tag: physics

Questions Related to physics

A man fires a shot and hears an echo from a cliff after $2sec$. He walks $85m$ towards the cliff and the seconds shot is now heard after $1.5sec$. What is the distance o the man from the cliff when the first echo was heard.

physics study of sound determination of speed of sound by echo problems on sound sound needs a medium

  1. $170m$

  2. $340m$

  3. $510m$

  4. $680m$

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Correct Option: A

A boy clapped his hands near a cliff and heard the echo after 4s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 ms$^{-1}$?

physics study of sound determination of speed of sound by echo problems on sound sound needs a medium

  1. 2 m

  2. 1384 m

  3. 692 m

  4. 2768 m

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Correct Option: C
Explanation:

Given, Speed of sound, $v = 346 ms^{-1}$ Time taken for hearing the echo, $t = 4 s$
Distance travelled by the sound $ = v \times t = 346 m/s \times 4 s = 1384 m$
In 4 s sound has to travel twice the distance between the cliff and the person.
Hence, the distance between the cliff and the person $= 1384 m/2 = 692 m$

A sound wave of wavelength $\lambda$ travels towards the right horizontally with a velocity V. It strikes and reflects from a vertical plane surface, travelling at a speed v towards the left. The number of positive crests striking during a time interval of three seconds on the wall is :

physics study of sound determination of speed of sound by echo problems on sound sound needs a medium

  1. $3(V+v)/\lambda$

  2. $3(V-v)/\lambda$

  3. $(V+v)/3\lambda$

  4. $(V-v)/3\lambda$

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Correct Option: A

A person is in front of a fort wall.  The person can hear an echo of sound produced by him when minimum distance between person and wall is 16.75 m. The velocity of sound in air is:

physics study of sound determination of speed of sound by echo problems on sound sound needs a medium

  1. 330 m/s

  2. 335 m/s

  3. 337.5 m/s

  4. 345 m/s

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Correct Option: B
Explanation:

Minimum time for echo is $0.15$
So, distance travelled for echo is $=2\times 16.75=$ speed of sound $\times$ minimum time for echo
$\Rightarrow 2\times 16.75=\vartheta _{s}\times 0.1$

$\Rightarrow \vartheta _{s}=\dfrac{2\times 16.75}{0.1}$$=335 m/s$

A boy shouts while standing in front of a hill. He hears an echo after 6 sec. If the speed of sound  in air is 340 m/s. What is the distance of the hill from the position of the body?

physics study of sound determination of speed of sound by echo problems on sound sound needs a medium

  1. 519 m

  2. 1020 m

  3. 2040 m

  4. none

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Correct Option: B
Explanation:

Let distance of hill from the position of the body = d
Echo is heard after 6 sec
$d = v \dfrac{t}{2}$
$= \dfrac{(340 \times 6)}{2}$
$= 1020 m$

A sonar echo takes 8.8 s to return from a shack. How far away is the shack?

Given speed of sound in sea-water = 1533 m/s

physics study of sound determination of speed of sound by echo problems on sound sound needs a medium

  1. 348 m

  2. 5720 m

  3. 6745 m

  4. 13490 m

Show answer
Correct Option: D
Explanation:

Let $d$ be the distance of shack from the sonar . Therefore  distance covered by sound to come back to sonar after reflection from shack ,  $=d$ ,

given , speed of sound in sea water $v=1533m/s  ,  t=8.8s$ ,
we have , distance=speed$\times time$ ,
               $d=v\times t=1533\times8.8$ ,
or            $d=13490.4m$

A couple produces motion in a straight line.

physics forces - vectors and moments the turning effect of a force moment of force or torque turning effect of force

  1. True

  2. False

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Correct Option: B
Explanation:

a couple produces rotational motion of a body . 

so the answer is B.

While opening a tap with two fingers, the forces applied are  

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. equal in magnitude

  2. parallel to each other

  3. opposite in direction

  4. All of the above

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Correct Option: D
Explanation:

While opening a tap with two fingers, a couple should be applied which is defined as a pair of parallel equal forces acting in opposite direction about two different axis.

On applying a constant torque on a body-

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. linear velocity increases

  2. angular velocity changes

  3. it will rotate with constant angular velocity

  4. it will move with constant velocity

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Correct Option: B

_____ is applied to unwind a screw.

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. Single force

  2. Couple

  3. Parallel force

  4. None of these

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Correct Option: B
Explanation:

Two equal and opposite forces are applied about different axis, which is termed as couple, to unwind a screw.