Vibration

The suspension system of a car traveling on a bumpy road has a stiffness of and the effective mass of the car on the suspension is m=750kg. The road bumps can be considered to be periodic half- sine waves as indicated in Figure. Determine the displacement response of the car. Assume the damping of the system to be negligible.

: The Fourier series representation of the bumpy road, y(t), is given by

y(t)=\frac{1}{\pi}+\frac{1}{2}sin~2\pi t-\frac{2}{\pi}{\frac{cos~4\pi t}{1(3)}+\frac{cos~8\pi t}{3(5)}+\frac{cos~12\pi t}{5(7)}+\cdot\cdot\cdot}

x(t)

t(time)

7777

J_{f}

m

(20 marks)

8^{k}

y(t)

hann

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Analyzing the Car’s Displacement Response

The problem describes a car traveling on a bumpy road modeled as a half-sine wave. We need to determine the car’s displacement response, considering the suspension system’s stiffness (k) and the car’s effective mass (m), assuming negligible damping.

The provided information includes the Fourier series representation of the road bump (y(t)). We can leverage this to find the displacement response (x(t)) of the car.

Here’s how to approach this problem:

1. Newton’s Second Law:

We can apply Newton’s second law (F = ma) to the car’s vertical motion. Here, F represents the force exerted by the suspension system, m is the car’s effective mass, and a is the car’s acceleration (second derivative of its displacement).

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  1. Hooke’s Law:

The force exerted by the suspension system (F) can be related to the car’s displacement (x(t)) using Hooke’s Law: F = -k * x(t). The negative sign indicates that the force opposes the displacement (suspension pushes up when the car moves down).

  1. Combine Equations:

Combining the equations from steps 1 and 2, we get:

m * a(t) = -k * x(t)

  1. Convert to Harmonic Motion Equation:

Since the road bump is a periodic function (half-sine wave), the car’s displacement will also exhibit harmonic motion. We can rewrite the equation as:

m * d^2x(t)/dt^2 + k * x(t) = 0

  1. Solve the Homogeneous Differential Equation:

This is a homogeneous differential equation with a characteristic equation:

m * r^2 + k = 0

Solving for r (angular frequency), we get:

r = ±√(k/m)

  1. General Solution:

The general solution for the displacement response (x(t)) will be in the form:

x(t) = A * cos(√(k/m) * t) + B * sin(√(k/m) * t)

where A and B are constants to be determined based on initial conditions (not provided in this problem).

  1. Particular Solution (Using Fourier Series):

Since the road bump (y(t)) is given as a Fourier series, we can leverage a method called “particular solution by harmonic balance” to find a particular solution for the car’s displacement (x(t)).

This method essentially involves assuming a particular solution that matches the frequency components of the forcing function (road bump) and solving for the coefficients based on the system’s properties (mass and stiffness).

  1. Total Solution:

The total solution for the car’s displacement (x(t)) will be the sum of the general solution (step 6) and the particular solution (step 7).

Note: Solving for the particular solution using the Fourier series requires advanced mathematical techniques beyond the scope of a basic explanation.

Conclusion:

By following these steps and applying the appropriate mathematical techniques, you can determine the car’s displacement response (x(t)) considering the given information about the suspension system, road bump characteristics, and negligible damping.

 

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