Question

In a certain tropical forest, litter (mainly dead vegetation such as leaves and vines) forms on the ground at the rate of 10 grams per square centimeter per year. At the same time, however, the litter is decomposing at the rate of 80% per year. Let f(t) be the amount of litter (in grams per square centimeter) present at time t. Find a differential equation satisfied by f(t).

Answer 1

df(t)/dt = 10 - 0.8f(t)

Explanation:

The net rate of change, df(t)/dt = rate in - rate out

The rate in = rate litter forms on ground = 10 g/cm²/yr

Since f(t) is the amount of litter present at time, t, in g/cm² the rate out = rate of decomposition = the percentage rate × f(t) = 80% per year × f(t) = 0.8f(t) g/cm²/yr

Since df(t)/dt = rate in - rate out

df(t)/dt = 10 - 0.8f(t)

So the desired differential equation is

df(t)/dt = 10 - 0.8f(t)