position at the end of the equal time intervals can be
photographed under stroboscopic lighting.
A series of tests was performed using different pucks.
Conditions at impact were found to be essentially elastic;
typical values of the coefficient of restitution being 0.93.
By analysing the photographic recordings, local angles of
incidence were calculated. However, a value for the coefficient
of friction was required before these results could be
compared with the theoretical predictions.
The coefficient of friction was found by conducting a series
of tests in the entire gross-slip regime where the simple
and more complex theory are indistinguishable. Under these
conditions ~ is given by the expression
= (VXl - Vx 2)
2Vzl (i + R 2)
k2
where Vx I and Vx 2 are the initial and final local tangential
velocities respectively. A mean value of 0.12 was obtained
for ~ and this was used for the results plotted in Figure i.
Discussion
The results show a broad quantitative agreement between
experiment and the prediction from elastic theory. Negative
values of ¢2 are abundantly represented, indicating that
tangential elasticity is significant in impact when ~i < 4X"
This is visually demonstrated in Figure 3, where the impacting
region of the puck is identified by a diamond shaped marker
and the impact block by a broad band of reflective tape. The
angle of reflection can be assessed by aligning the reflection
path of the marker with the jet lattice.
The theoretical analysis is independent of the magnitude of
the approach velocity and this is borne out by experimental
results where the initial velocity was varied by a factor of
3. The coefficient of friction remains remarkably consistent,
being within ~ 5~ of the mean value for the whole test series.