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Name: Jon G.
Status: other
Age: 50s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
What are the physical factors which would slow down and stop a speargun shaft underwater?


Replies:
John,

I suspect it would all reduce to the hydrodynamics of the moving spear -- the size and shape of the point and the overall length of the shaft. These characteristics would bear upon the frictional (drag) forces experienced by the spear as it moved through the water.

Regards,
ProfHoff


John,

The major factor slowing down a speargun shaft underwater is the fact that the shaft must push water molecules out of the way. This factor, sometimes called resistance or drag, is truly due to shaft molecules crashing into water molecules. You cannot use the standard equation (Drag force proportional to v^2) because the shape of the spear has a strong effect. A narrow pointed object doesn't have to push molecules aside as quickly as a blunt object.

Another smaller factor is drag at the back of the shaft. As the shaft moves forward, there is less pressure just behind it: the water molecules haven't had time to settle in. This also provides a small backward pull, very much like suction. How the end of the spear is designed can greatly affect the strength of this force. Again, a blunt end is pulled harder than a narrow point.

One more factor is how well water bonds with the spear material. Water molecules stick to many things, including other water molecules. A material that water runs of of very easily will not disturb molecules alongside the shaft. A material that "grabs onto" water molecules will be slowed by binding with the water.

These are the only three significant factors that come to mind.

Dr. Ken Mellendorf
Illinois Central College



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