NAEST 2019 Screening Test: Waves on Water Surface
By Jitender Singh
In screening round of National Anveshika Experimental Skills Test ( NAEST 2019 ), a question was asked on the waves formed on the water surface when rain-drops fall on it.
Question from NAEST 2019
The raindrops falling on the water surface produce a disturbance on it. The disturbance (water waves) moves outwards in a circular shape. This phenomenon is so common that we take it for granted. Why the disturbance takes a circular shape? What happens when two water waves meet each other? A question in NAEST 2019 explores these ideas.
Question: Observe the waves and bubbles on the water surface created by the falling raindrops. Choose the correct option(s)
- The disturbance pattern formed on the water surface is circular because isotropic properties of water (on the surface) makes the disturbance travels with the same speed in all directions.
- The waves generated by the falling raindrop on the water surface are longitudinal.
- When two waves meet, they add together momentarily and then continue to move forward without getting affected by the other.
- The air pressure inside the bubble is less than the atmospheric pressure.
Solution: Option (A) is correct. The disturbance pattern formed on the water surface is circular because isotropic properties of water (on the surface) makes the disturbance travels with the same speed in all directions.
The waves on the water surface cannot be characterised as transverse or longitudinal. The waves shown in the video are look to be transverse because water particle moves up and down whereas disturbance moves radially outward.
The water waves superpose with each other. When two waves meet, their amplitudes add together momentarily and then they continue to move forward without getting affected by the other. This is visible in the video.
The air pressure inside the bubble is more than the atmospheric pressure. The excess pressure inside the bubble balances the force of surface tension.
Question from NAEST 2018
Question: Carefully observe the waves on a slinky. Select the correct option(s)
- The hump moves from one end to the other.
- Few coils of the slinky move from one end to the other.
- The wave is longitudinal.
- The wave is transverse.
Solution: The hump (disturbance) moves from one end of the slinky to the other.
The displacement of spring particles (coils) is in a direction perpendicular to the direction of wave motion. Thus, waves on the slinky are transverse.
