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High Bounce Approximation

 

In the high bounce approximation we imagine that the table's displacement amplitude is always small compared to the ball's maximum height. This approximation is depicted in Figure 1.6 where the ball's trajectory is perfectly symmetric about its midpoint, and therefore

  equation652

The velocity of the ball between the kth and k+1st impacts is given by

  equation656

At the k+1st impact, the velocity is tex2html_wrap_inline11663 and the time is tex2html_wrap_inline11665 , so

  equation661

Using equation (1.15) and simplifying, we get

  equation667

which is the time map in the high bounce approximation.

  
Figure 1.6: Symmetric orbit in the high bounce approximation.

To find the velocity map in this approximation we begin with the impact relation (eq. (1.7)),

  eqnarray681

where the last equality follows from the high bounce approximation, equation (1.15). The table's velocity at the k+1st impact can be written as

  eqnarray689

when the time map, equation (1.18), is used. Equations (1.19) and (1.20) give the velocity map in the high bounce approximation,

  equation698

The impact equations can be simplified somewhat by changing to the dimensionless quantities

    eqnarray703

which recasts the time map (eq. (1.18)) and the velocity map (eq. (1.21)) into the explicit mapping form

  equation712

In the special case where tex2html_wrap_inline11556 , this system of equations is known as the standard map  [4]. The subscripts of tex2html_wrap_inline11671 explicitly show the dependence of the map on the parameters tex2html_wrap_inline11245 and tex2html_wrap_inline11651 . The mapping equation (1.25) is easy to solve on a computer. Given an initial condition tex2html_wrap_inline11677 , the map explicitly generates the next impact phase and impact velocity as tex2html_wrap_inline11679 , and this in turn generates tex2html_wrap_inline11681 , etc., where, in this notation, the superscript n in tex2html_wrap_inline11685 indicates functional composition (see section 2.2). Unlike the exact model, both the phase map and the velocity map are explicit equations in the high bounce approximation.

The high bounce approximation shares many of the same qualitative properties of the exact model for the bouncing ball system, and it will serve as the starting point for several analytic calculations. However, for comparisons with experimental data, it is worthwhile to put the extra effort into numerically solving the exact equations because the high bounce model fails in at least two major ways to model the actual physical system [5]. First, the high bounce model can generate solutions that cannot possibly occur in the real system. These unphysical solutions occur for very small bounces at negative table velocities, where it is possible for the ball to be projected downward beneath the table. That is, the ball can pass through the table in this approximation. Second, this approximation cannot reproduce a large class of real solutions, called ``sticking solutions,''  which are discussed in section 1.4.3. Fundamentally, this is because the map in the high bounce approximation is invertible, whereas the exact model is not invertible. In the exact model there exist some solutions--in particular the sticking solutions--for which two or more orbits are mapped to the same identical point. Thus the map at this point does not have a unique inverse.


next up previous contents
Next: Qualitative Description of Motions Up: Bouncing Ball Previous: Parameters

Nicholas B. Tufillaro
Mon Mar 3 01:58:02 PST 1997