We can express the rate at which the old man throws apples by
The rate is measured in apples per second across the fence, and
is the concentration of apples on the man's side of the fence in
apples per square foot of ground. The rate constant, ,
has units of square feet per second:
The value of
expresses the agility of the old man, and his speed in covering
the territory on his side of the fence.
The rate at which the boy throws apples back across the fence is
is the concentration of apples in the boy's yard, and
is the rate constant, or agility constant, which tells how fast
the boy gets around on his side of the fence, in square feet per
second. Since we have assumed that the boy is livelier than the
is greater than .