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Name: Raj
Status: student
Grade: 9-12
Country: India
Date: Spring 2013


Question:
When I drop a stone/or any object in water, why are the waves circular in shape. The part that really confuses me is that even if I drop any symmetric or unsymmetrical object, the waves are always concentric circles and so symmetric. I am really wondering why this happens?

Replies:
It depends on how the size of the object compares with the wavelength of the waves. If you drop a long board into the water, you will get non-circular ripples.

Tim Mooney


Raj, The wave begins when and where the object first makes contact with the water. If you drop a very long and narrow object, such as a rod, into the water horizontally, the wave will not be circular near the rod. As you look further from the rod, the wave looks more circular. This relates to how the object ?looks? from that distance. Up close, the rod is extremely long and extremely narrow. The waves are not circular up close. Far away, the object looks smaller, closer to a point-like object. Far away, the waves look almost circular. If distance to all points of the object are ?about the same? relative to the average distance, then the waves will be close to circular.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


A very perceptive observation. A detailed answer is not so easy because what happens in the first few seconds decays into a circular pattern. I will give you a some references to look up for more details. When you drop an object into water (assuming the water is sufficiently deep so that the wave is free from the bottom of the water). The water, at a given distance from the point of contact, actually moves “up and down”. This motion moves outward is the transfer of momentum along the radius of the water wave. If you observe the motion in the first few fractions of a second there probably is a “shape effect”, but this is a transient initial condition. What confuses people is that it appears that the water (molecules) are moving away from the point of impact. Careful observation however shows that the water moves “up and down”. That motion is transferred to the next “up and down” motion. It is this transfer of momentum that gives rise to the apparent circular motion of the wave. The key is that the medium (water) does not move along the surface, it moves “up and down” transferring its momentum to the next increment of water, which responds by moving “up” or “down” from the previous motion “down” or “up”. This fulfills the conservation of momentum and energy. Unfortunately, there is not a really “simple” answer to your inquiry that I can find.

You can find more info at: http://hyperphysics.phy-astr.gsu.edu/hbase/waves/watwav2.html : http://wiki.answers.com/Q/How_do_water_particles_move_within_a_wave

Vince Calder


Hi Raj,

The reason the waves end up approximately round is because when you drop an object, you are creating a situation where you have an outward-moving wave front starting from a single point. The size variations of the object become insignificant as the wave front moves away from the object because the wave shape is based on the time it travels away from the dropped object. The waves spread out at the same speed -- so after a few seconds, the length traveled may be several meters. If you drop a large object, and wait a small period of time, they will not appear round.... but if you drop a small object, regardless of shape, and wait long enough, the waves will appear to be round. In contrast, if you used a long straight board, and pushed waves away from you, the wave front is not starting from a single point, and you will create linear waves (although they will also appear round at the corners).

Hope this helps, Burr Zimmerman


The waves radiate in all directions from their source. Once they have traveled a distance that is much greater than the size of the source, they just look circular. But close to the source, it is easier to see the shape of the source. Richard E. Barrans Jr., Ph.D., M.Ed.


Hi Raj, Thanks for the question. When you drop objects into water, the waves tend to be circles for two reasons. First, the speed of the waves in water is uniform. If the wave speed was different, then there would not be a symmetrical pattern. Second, the object tends to create a uniform displacement of water. The displacement of water is what starts the motion of the water wave.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell





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