7
UNIT 5
Questions 14 – 19
In some areas of the world, marine birds such as kelp gulls feed on black mussels which have been dislodged
from rocks by the action of waves and deposited on the beaches. These birds have learned to break open the
shells to get at the fleshy contents by carrying the mussels to heights and dropping them onto hard surfaces,
such as rocks or wet beach sand.
Experimental evidence indicates that the minimum drop height required to fracture a black mussel shell
depends on its size, and also on the nature of the surface onto which it is dropped. Moreover the speed of a
black mussel on impact with the ground can be related to the mussel’s drop height and its shell length.
The graphs in Figures 1 to 4 show relationships between the size, impact speed, and drop height of black
mussels, and have been based on the results of extensive mussel dropping experiments that attempted to
simulate real conditions.
Assume that all black mussels referred to in the following questions are described by these relationships.
Figure 1 Relationship between Impact Speed and Drop Height of Black Mussels of various lengths
(indicated in millimetres against each curve)
140
120
100
80
60
40
20
0
40
60
80
100
120
140
black mussel mass (grams)
shell length (millimetres)
70
60
50
40
30
20
10
0
40
60
80
100
120
140
shell area (square centimetres)
shell length (millimetres)
Figure 2 Relationship between Mass
Figure 3 Relationship between Shell
and Shell Length of Black
Area and Shell Length of
Mussels
Black Mussels
12
10
8
6
4
2
0
2
4
6
8
impact
speed
(metres per
second)
drop height (metres)
120 millimetres
100 millimetres
80 millimetres
60 millimetres