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Projects in High-Speed Photography

 

Conclusions -- Part II

 

From a comparison of the two series of photos, one with the rear nail in place and the other without, it is apparent that the collision of the band with the rear nail does not influence when the front of the band will break contact with the forward nail. Granted, the collision does  produce transverse pulses which travel toward the forward nail. However, the longitudinal relaxation of the band proceeds much faster and initiates the contact-breaking event.  Moreover, the transverse pulses are created even when the rear nail is absent. These pulses are generated as the tension in the band decreases, slowing the longitudinal relaxation of the forward half. In effect, the band runs into itself from the rear.

 

One may ask why front of the band breaks contact at all. The tension forces on this portion are transverse to the direction that the band must move in order to break contact. The diagram below shows this.

The situation may be exaggerated by replacing the nail with a square block as shown below.

The forces as shown cannot pull the forward face of the rubber band free. This brings to mind an experiment performed several years earlier by Travis Williams. Travis fired rubber bands from a gun with a flat face like the one shown above. He took multiple-image photographs which showed the band from the instant of release until it was far beyond the end of the gun. A section clipped from one the photos is shown below.

 

contact.JPG (9192 bytes)

 

This photo clip shows the square end of the gun and two images of the band, one yellow and the other blue. (These images are produced with separate flash units covered with yellow and blue filters.) The yellow image, which was taken first, shows the band still in contact with the forward face of the gun. The blue image, which was taken 2.5 ms later, shows that the part of the band (labeled C) in the center of the gun's face  is actually the last to break contact with the gun. This part of the band is pulled forward by the surrounding portions of the band. These portions are being pushed forward by the sides of the band. One might expect that similar mechanisms would be at work as the band breaks contact with the curved surface of a nail. The effect, however, would not be nearly as pronounced.  The reader is invited to examine the photographs closely to see if she/he agrees.

 

A complete multi-flash photo, similar to the one from which the above clip was taken, is shown below. There were a total of 8 flashes. Explaining the shapes assumed by the band after it leaves the gun makes for some interesting discussion.

 

tw-5t-23c.JPG (7343 bytes)

 


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