Sound Waves/Terminal Velocity


The sensation of hearing is produced by sound, which is a form of energy. Mechanical vibrations produce sound waves, which propagate as a longitudinal mechanical wave with alternate compressions and rarefactions. Solids, liquids, and gasses can all carry sound waves. Their speed is determined by the medium’s properties. As sound waves travel across a medium, the particles within it perform simple harmonic motion in the wave’s propagation direction. Pressure waves and harmonic waves are other names for sound waves.

The human ear is sensitive to sound waves whose frequency ranges from 20 Hz to 20,000 Hz. These waves are audible. They are generated by human vocal cords, musical instruments and loudspeakers. Sound waves with frequencies below the audible range are called infrasonic waves. For example, earthquakes or seismic waves. Sound with frequencies above the audible range is called ultrasonic waves.

Properties of Sound Waves

  1. A sound is a form of energy which produces the sensation of hearing.
  2. Sound waves cannot travel in a vacuum. They require a material medium for propagation.
  3. Sound waves travel through a medium in the form of longitudinal waves.
  4. They transport energy from one region to another.
  5. Sound waves of all frequencies travel with a constant speed in a homogenous medium, provided the temperature is constant.
  6. Sound waves travel at different speeds in different media. The speed of a medium depends on the elastic property and the density of the medium.
  7. Sound waves undergo reflection.
  8. They undergo refraction. When the sound waves travel from one medium to another, their velocity and wavelength change, but the frequency remains unaltered.
  9. Sound waves exhibit the phenomena of interference and diffraction.
  10. When sound waves are incident on a surface, the surface is set into vibration.

Terminal Velocity

When the body falls through a viscous fluid, it drags along with it the layers of the fluid in contact with it. But the layers of fluid at a large distance from the falling body remain undisturbed. Thus, the falling body produces a relative motion between different layers of the fluid. Due to this relative motion, a backward dragging force comes into play which opposes the motion of the falling body. This opposing force or dragging force increases with the increase in velocity of the body. It is found that the body after attaining a certain velocity starts moving with constant velocity in the fluid. This uniform velocity of the body while moving in a fluid is called terminal velocity. The expression of terminal velocity is given by the expression v = 2r2(ρ – σ)/9η

r is the radius of the body

ρ is the density of the material of the body

σ is the density of the viscous fluid

η is the coefficient of viscosity of the viscous fluid

The raindrop, parachute etc., fall with the terminal velocity. When a body is dropped in a viscous fluid, it is first accelerated, and then its acceleration becomes zero, and it attains a constant velocity called the terminal velocity.

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