Question Discussion & Solution

Concept:

• Force: Force is a push or pull on an object. A force can cause an object to accelerate, slow down, remain in place, or change shape.

Force exerted = mass x acceleration

F = m × a

Acceleration = (Final velocity(v) - Initial velocity(u))/ time

\(a = \frac{{v - u}}{t}\)

Where F is the force exerted on the body, m is mass of the body, a is the acceleration of the body, v is final velocity, u is initial velocity and t s time.

Calculation:

Given,

Mass of body = 6 kg.

Change in velocity = (18 - 6) = 12 m/s

Time= 3 sec

Acceleration = 12/ 3 = 4m/s² 

Force = mass × acceleration

Force = 6 × 4 = 24 Newton.

Hence the correct option is 24 Newton

The pattern obtained when the transparent sheet is folded is,

Hence, option 4 is the correct answer.

The first two statements give information about payal, monika and Sonam. Information about kunal and monika cannot be determined.

CONCEPT:

• Ultrasonic waves are the sound waves with frequencies higher than the upper audible limit of human hearing.
  • They have a short wavelength.
• Echo: If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo.
  • Echoes are heard due to the phenomenon of Reflection of sound waves.
• SONAR: It a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, communicate with or detect objects on or under the surface of the water
  • SONAR stands for ‘Sound Navigation and Ranging.’
  • SONAR is based on the principle of echo.
  • Sonar is often used as a measurement of acoustic location and echo characteristics of the target object in the water.
  • It sends ultrasonic waves towards the bottom area of the sea and after reflection, the sound comes to the ship. We measure the time taken by the sound wave and then we get the depth. 

EXPLANATION:​

• From the above discussion, we can say that the SONAR uses the principle of reflection of sound waves(Echo) to detect and locate objects. So option 2 is correct.
• Reverberation: The persistence of sound in a closed enclosure, due to continuous reflections at the walls or the floor or the ceiling of the enclosure, even after the source has stopped producing sound is known as 'reverberation'.

The Correct Answer is Two.

 Key Points

• The water molecule is composed of two hydrogen atoms, each linked by a single chemical bond to an oxygen atom.
• Most hydrogen atoms have a nucleus consisting solely of a proton. 
• Water is a tasteless and odorless liquid at room temperature.
• It has the important ability to dissolve much other substance.

 Additional Information

• Two isotopic forms, deuterium and tritium, in which the atomic nuclei also contain one and two neutrons, respectively, are found to a small degree in water.
• Deuterium oxide (D2O) is called heavy water
• It is used as a neutron moderator in some nuclear reactors.