Physics Photo of the Week - Sept. 27, 2024

 Physics of a splash of water

This week's photo shows details of the splash observed when a tennis ball was thrown into a body of water.  The ball was thrown from someone on shore when a slo-mo video was recorded on a smart phone.  This is the first video frame immediately after the ball landed in the water.  The immediate reaction from the water is the ejection of a crown-shaped splash that we see in the photo.

 

 


A few frames later we can see the rebound effect sending a long column of water shooting up from the splash area as a rebound effect of the water - seen in the photo at right.

Click on either of the images to view a slo-mo video of the ball approaching the water, the immediate crown effect, then the spout of water as a result of the rebound of the water as the water rushed back into the sizable cavity made by the ball when it hit the water from a height of a few meters.

Examining the slo-mo video frame-by-frame we can see that the kinetic energy of the ball excavated a "large" cavity in the water surface (an approximately spherical cavity about 2-3 times the diameter of the ball).  The cavity soon collapsed.  However, as the walls of the cavity came together at a rather high speed, the water "piled up" on itself and it was "squeezed out" of the compression zone and formed the narrow jet of water that spouted about 3/4 meter high.

After the spout subsided, the video reveals another crater being formed - probably from the collapsing spout falling back into the water, which again collapsed into another smaller rebound - that is much less organized.  The two collapses form two concentric circular waves that propagate away from the impact site. visible in the last frame of the video - see the image at left.

This final frame also features "Gibbs" - the "owner" of the ball eagerly retrieving it - and Chris, an extended family member and owner of Gibbs.  I thank Chris and Gibbs for helping create this physics video.

The same effects can be observed in lunar and planetary surfaces and astrophysical phenomena.  The rebound effect has become solidified in lunar craters that were formed by large meteors colliding with the lunar surface.  The image of the lunar surface at right -made at the College View Observatory shows an impact crater with a central peak. The impact of a meteor deposited its kinetic energy into the lunar rocks, melted some of the rocks, and formed a crater due to the shock wave of the collision propagating with the speed of sound in the lunar crust.  As the hemispherical crater partially collapsed soon after its formation, the rebound spout became "frozen", and formed a permanent central peak in the crater floor.  We don't know much about how much volume of lunar and meteoric material melted, although some craters on Earth show evidence of material was even vaporized.  It's also puzzling that if the material liquified, how it re-froze rather quickly before flowing back down to the crater's bottom.

This final image shows the galaxy M97 (featured in PPOW for May 12, 2023).  Here we see a jet of luminous material that is ejecting from the central super massive black hole lurking in this galaxy.  The collapsing of the stars into the black hole squeezes out ionized stellar material to form the jet - similar to the jet of water streaming from the collapsing crater of water made by the impact of the tennis ball.  The galaxy is so far away from Earth, that we do not see the jet moving in anyone's lifetime.  The jet is 10's of thousands of light years long!

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

To join the mailing list, please request to Donald Collins at the e-mail address below.  You will receive no more than one message every two weeks, or when a very interesting physics/astronomy event is happening.


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