Math Problem Statement

A tank is shaped as a cuboid. The base has dimensions 10 cm by 10 cm. Initially the tank is empty. Water flows into the tank at 25 cm3 per second. Water also leaks out of the tank at 4h2 cm3 per second, where h cm is the depth of the water after t seconds. Find the time taken for the water to reach a depth of 2 cm.

Solution

To solve this problem, we need to set up an equation for the depth of water in the tank as a function of time, taking into account both the water flow in and the leakage out.

Given:

  • The base dimensions of the tank are 10 cm by 10 cm.
  • Water flows in at a rate of 25cm325 \, \text{cm}^3 per second.
  • Water leaks out at a rate of 4h2cm34h^2 \, \text{cm}^3 per second, where hh is the depth of water in centimeters.
  • We need to find the time taken for the water to reach a depth of 2 cm.

Step 1: Volume Flow Equations

  • The volume of water in the tank at any time tt is given by the product of the base area and the depth h(t)h(t): V(t)=base area×h(t)=10×10×h(t)=100h(t)V(t) = \text{base area} \times h(t) = 10 \times 10 \times h(t) = 100h(t)

  • The rate at which the volume changes, dVdt\frac{dV}{dt}, is the difference between the rate of water flowing in and the rate of water leaking out. This can be expressed as: dVdt=254h2\frac{dV}{dt} = 25 - 4h^2

Step 2: Relating Volume and Depth

We know that V(t)=100h(t)V(t) = 100h(t), so dVdt=100dhdt\frac{dV}{dt} = 100 \frac{dh}{dt}. Substituting this into the volume flow equation, we get: 100dhdt=254h2100 \frac{dh}{dt} = 25 - 4h^2 Simplifying: dhdt=254h2100\frac{dh}{dt} = \frac{25 - 4h^2}{100}

Step 3: Solving the Differential Equation

We need to solve the differential equation: dhdt=254h2100\frac{dh}{dt} = \frac{25 - 4h^2}{100} Separate the variables: 100254h2dh=dt\frac{100}{25 - 4h^2} \, dh = dt Now integrate both sides. The left side is a bit tricky, so we use partial fractions or an appropriate substitution. Let's first simplify the integral: 100254h2dh=dt\int \frac{100}{25 - 4h^2} \, dh = \int dt

We can factor the denominator on the left side: 254h2=(52h)(5+2h)25 - 4h^2 = (5 - 2h)(5 + 2h) Using partial fraction decomposition, we can express 100254h2\frac{100}{25 - 4h^2} as a sum of simpler fractions. After integrating, we’ll have the solution for h(t)h(t).

Step 4: Find the Time for Depth to Reach 2 cm

Finally, after solving for h(t)h(t), we substitute h=2h = 2 and find the corresponding time tt.

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Math Problem Analysis

Mathematical Concepts

Differential Equations
Volume Flow
Rate of Change

Formulas

dV/dt = 25 - 4h^2
V(t) = 100h(t)
dh/dt = (25 - 4h^2) / 100

Theorems

Separation of Variables
Partial Fraction Decomposition

Suitable Grade Level

Grades 11-12