# 10.1E: Introduction to Systems of Differential Equations (Exercises)

- Page ID
- 18297

## Q10.1.1

1. Tanks \(T_1\) and \(T_2\) contain 50 gallons and 100 gallons of salt solutions, respectively. A solution with 2 pounds of salt per gallon is pumped into \(T_1\) from an external source at \(1\) gal/min, and a solution with \(3\) pounds of salt per gallon is pumped into \(T_2\) from an external source at \(2\) gal/min. The solution from \(T_1\) is pumped into \(T_2\) at \(3\) gal/min, and the solution from \(T_2\) is pumped into \(T_1\) at \(4\) gal/min. \(T_1\) is drained at \(2\) gal/min and \(T_2\) is drained at \(1\) gal/min. Let \(Q_1(t)\) and \(Q_2(t)\) be the number of pounds of salt in \(T_1\) and \(T_2\), respectively, at time \(t>0\). Derive a system of differential equations for \(Q_1\) and \(Q_2\). Assume that both mixtures are well stirred.

2. Two 500 gallon tanks \(T_1\) and \(T_2\) initially contain 100 gallons each of salt solution. A solution with \(2\) pounds of salt per gallon is pumped into \(T_1\) from an external source at \(6\) gal/min, and a solution with \(1\) pound of salt per gallon is pumped into \(T_2\) from an external source at \(5\) gal/min. The solution from \(T_1\) is pumped into \(T_2\) at \(2\) gal/min, and the solution from \(T_2\) is pumped into \(T_1\) at \(1\) gal/min. Both tanks are drained at \(3\) gal/min. Let \(Q_1(t)\) and \(Q_2(t)\) be the number of pounds of salt in \(T_1\) and \(T_2\), respectively, at time \(t>0\). Derive a system of differential equations for \(Q_1\) and \(Q_2\) that’s valid until a tank is about to overflow. Assume that both mixtures are well stirred.

3. A mass \(m_1\) is suspended from a rigid support on a spring \(S_1\) with spring constant \(k_1\) and damping constant \(c_1\). A second mass \(m_2\) is suspended from the first on a spring \(S_2\) with spring constant \(k_2\) and damping constant \(c_2\), and a third mass \(m_3\) is suspended from the second on a spring \(S_3\) with spring constant \(k_3\) and damping constant \(c_3\). Let \(y_1=y_1(t)\), \(y_2=y_2(t)\), and \(y_3=y_3(t)\) be the displacements of the three masses from their equilibrium positions at time \(t\), measured positive upward. Derive a system of differential equations for \(y_1\), \(y_2\) and \(y_3\), assuming that the masses of the springs are negligible and that vertical external forces \(F_1\), \(F_2\), and \(F_3\) also act on the masses.

4. Let \({\bf X}=x\,{\bf i}+y\,{\bf j}+z\,{\bf k}\) be the position vector of an object with mass \(m\), expressed in terms of a rectangular coordinate system with origin at Earth’s center (Figure 10.1.3). Derive a system of differential equations for \(x\), \(y\), and \(z\), assuming that the object moves under Earth’s gravitational force (given by Newton’s law of gravitation, as in Example 10.1.3) and a resistive force proportional to the speed of the object. Let \(\alpha\) be the constant of proportionality.

5. Rewrite the given system as a first order system.

- \(\begin{array}{lcc} x''' = f(t,x,y,y')\\[4pt] y'' = g(t,y,y') \end{array}\)
- \(\begin{array}{lcl} u' = f(t,u,v,v',w')\\[4pt] v''=g(t,u,v,v',w) \\[4pt] w''=h(t,u,v,v',w,w')\end{array}\)
- \(y''' = f(t,y,y',y'')\)
- \(y^{(4)} = f(t,y)\)
- \(\begin{array}{lcc} x'' = f(t,x,y)\\[4pt] y'' = g(t,x,y) \end{array}\)

6. Rewrite the system Equation 10.1.14 of differential equations derived in Example 10.1.3 as a first order system.

7. Formulate a version of Euler’s method (Section 3.1) for the numerical solution of the initial value problem \[\begin{array}{rcl} y_1'&=&g_1(t,y_1,y_2),\quad y_1(t_0)=y_{10},\\ y_2'&=&g_2(t,y_1,y_2),\quad y_2(t_0)=y_{20}, \end{array}\nonumber \] on an interval \([t_0,b]\).

8. Formulate a version of the improved Euler method (Section 3.2) for the numerical solution of the initial value problem \[\begin{array}{rcl} y_1'&=&g_1(t,y_1,y_2),\quad y_1(t_0)=y_{10},\\ y_2'&=&g_2(t,y_1,y_2),\quad y_2(t_0)=y_{20}, \end{array}\nonumber \] on an interval \([t_0,b]\).