COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 107
x
a
t
6
t
c
" .... _" ,/
,/
'"
'" '"
,,-- .......
,/ '" "
,/ ",
'" ' "
d
,,~--""" ...........
,/
'" "
.....
" '" "
"" '" '"
" '"
'..... ,,""
......_--
Fig. 1. Curves representing sand motion and pebble motion are plotted
as a function of time in full line and dashed line respectively. In a and b the
elongations and velocities of the linear motion and, in c and d those of the
rotation al motion are drawn. The figures are not in scale.
t
If the density of the pebble is the same as that of the medium, the linear
velo city of the pebble will be ab out equal to the weighted me an velocity
of the medium in the vicinity of the pebble. The mean motion of the sand
and the motion of the pebble are in phase. The horizontal component of the
resultant force acting on the pebble is opposite to the elongation, i. e., to
the distance from the equilibrium, of the pebble. The linear velo city shows
a phase shift of 90 degrees in advance of the linear motion and the angular
velo city is just 180 degrees behind the linear motion. Because of symmetry
of the motion in space and time, there will be no vertical net force in this
case.
If the density of the pebble is greater than that of the medium and the
frequency is not too high, the motion of the pebble will be retarded and
17 8996-63