Math Problem Statement

Consider a system model given by

2 fraction numerator d squared x left parenthesis t right parenthesis over denominator d t squared end fraction plus 3 fraction numerator d x left parenthesis t right parenthesis over denominator d t end fraction plus x left parenthesis t right parenthesis equals delta left parenthesis t right parenthesis x left parenthesis 0 to the power of minus right parenthesis equals 0 right enclose fraction numerator d x left parenthesis t right parenthesis over denominator d t end fraction end enclose subscript t equals 0 to the power of minus end subscript equals 1

What are the initial conditions at x(0+) and dx/dt (0+)?

x(0+)=-1 and dx/dt (0+)=0

x(0+)=0 and dx/dt (0+)=1.5

x(0+)=1 and dx/dt (0+)=1

x(0+)=1 and dx/dt (0+)=0

x(0+)=0 and dx/dt (0+)=1

x(0+)=0 and dx/dt (0+)=0

Solution

We are given a system model:

2d2x(t)dt2+3dx(t)dt+x(t)=δ(t)2 \frac{d^2 x(t)}{dt^2} + 3 \frac{dx(t)}{dt} + x(t) = \delta(t) with the initial conditions: x(0)=0,dx(t)dtt=0=1.x(0^-) = 0, \quad \left. \frac{dx(t)}{dt} \right|_{t=0^-} = 1.

Step 1: Analyze the Differential Equation

The system is a second-order linear differential equation with an impulse input δ(t)\delta(t). The impulse input suggests that there will be a discontinuity or jump in the derivative of x(t)x(t) at t=0t=0.

Step 2: Apply Laplace Transform

To handle the discontinuity, we apply the Laplace transform to both sides of the equation. Using the fact that:

L{d2x(t)dt2}=s2X(s)sx(0)dx(t)dtt=0,\mathcal{L}\left\{ \frac{d^2x(t)}{dt^2} \right\} = s^2 X(s) - s x(0^-) - \left. \frac{dx(t)}{dt} \right|_{t=0^-}, L{dx(t)dt}=sX(s)x(0),\mathcal{L}\left\{ \frac{dx(t)}{dt} \right\} = s X(s) - x(0^-), L{x(t)}=X(s),\mathcal{L}\left\{ x(t) \right\} = X(s), and L{δ(t)}=1\mathcal{L}\{\delta(t)\} = 1, we transform the differential equation:

2(s2X(s)sx(0)dx(t)dtt=0)+3(sX(s)x(0))+X(s)=1.2(s^2 X(s) - s x(0^-) - \left. \frac{dx(t)}{dt} \right|_{t=0^-}) + 3(s X(s) - x(0^-)) + X(s) = 1.

Step 3: Substitute Initial Conditions

We are given x(0)=0x(0^-) = 0 and dx(t)dtt=0=1\left. \frac{dx(t)}{dt} \right|_{t=0^-} = 1. Substituting these into the Laplace-transformed equation:

2(s2X(s)1)+3(sX(s))+X(s)=1.2(s^2 X(s) - 1) + 3(s X(s)) + X(s) = 1.

Simplifying:

2s2X(s)2+3sX(s)+X(s)=1,2s^2 X(s) - 2 + 3s X(s) + X(s) = 1, (2s2+3s+1)X(s)=3,(2s^2 + 3s + 1) X(s) = 3, X(s)=32s2+3s+1.X(s) = \frac{3}{2s^2 + 3s + 1}.

Step 4: Inverse Laplace Transform

Now, we take the inverse Laplace transform to find x(t)x(t). The Laplace transform of the system suggests a step response for a second-order system with an impulse input. To determine the initial conditions at t=0+t = 0^+, we use the fact that the system responds to the impulse input.

Since x(t)x(t) was initially at rest (x(0)=0x(0^-) = 0), and there was a discontinuity in dx(t)dt\frac{dx(t)}{dt}, we know:

x(0+)=x(0)=0.x(0^+) = x(0^-) = 0.

To find dx(t)dtt=0+\left. \frac{dx(t)}{dt} \right|_{t=0^+}, we analyze the jump condition caused by the impulse input. The impulse δ(t)\delta(t) forces an immediate change in the derivative. Given that dx(t)dtt=0=1\left. \frac{dx(t)}{dt} \right|_{t=0^-} = 1, and considering the dynamics of the system, we expect the initial derivative to remain the same:

dx(t)dtt=0+=1.\left. \frac{dx(t)}{dt} \right|_{t=0^+} = 1.

Final Answer:

The correct initial conditions are:

x(0+)=0,dx(t)dtt=0+=1.x(0^+) = 0, \quad \left. \frac{dx(t)}{dt} \right|_{t=0^+} = 1.

Thus, the correct choice is: x(0+)=0 and dxdt(0+)=1.\boxed{x(0^+) = 0 \text{ and } \frac{dx}{dt}(0^+) = 1.}


Would you like further details on the steps, or do you have any questions? Here are some follow-up questions that might interest you:

  1. How does the Laplace transform handle discontinuities in time-domain functions?
  2. Why does an impulse input result in a jump in the system’s derivative?
  3. What role do initial conditions play in solving differential equations using Laplace transforms?
  4. How can we interpret second-order system behavior from its differential equation?
  5. What methods can be used to find inverse Laplace transforms of rational functions?

Tip: Always remember that the Laplace transform is a powerful tool for solving differential equations, especially when dealing with initial conditions and discontinuous inputs like impulses.

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Math Problem Analysis

Mathematical Concepts

Differential Equations
Laplace Transform
System Dynamics

Formulas

2 d²x(t)/dt² + 3 dx(t)/dt + x(t) = δ(t)
Laplace Transform of derivatives: L{d²x(t)/dt²} = s²X(s) - sx(0⁻) - (dx/dt)(0⁻)
Inverse Laplace Transform

Theorems

Laplace Transform for solving linear differential equations
Initial Value Theorem

Suitable Grade Level

Undergraduate (Engineering/Physics)