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A spring has spring constant \(k=250\) and mass \(m=\) 100 grams. (a) Write the second-order differential equation for oscillations of the mass on the spring. (b) Write the solution to the differential equation if the initial position is \(s(0)=5\) and the initial velocity is \(s^{\prime}(0)=-10.\) (c) How far down does the mass go?

Short Answer

Expert verified
(a) \( \frac{d^2s}{dt^2} + 2500s = 0 \); (b) \( s(t) = 5 \cos(50t) - 0.2 \sin(50t) \); (c) 5.004 units.

Step by step solution

01

Define the Equation of Motion

The motion of a mass attached to a spring is described by Hooke's Law, which leads to the following second-order differential equation for oscillations: \[ m \frac{d^2s}{dt^2} + ks = 0 \]where \( s(t) \) is the displacement, \( m = 0.1 \) kg (converted from grams), and \( k = 250 \) N/m.
02

Simplify the Differential Equation

Substitute the given values for the mass \( m \) and the spring constant \( k \) into the differential equation:\[ 0.1 \frac{d^2s}{dt^2} + 250s = 0 \]Simplify this:\[ \frac{d^2s}{dt^2} + 2500s = 0 \]
03

Determine the General Solution

The standard form of the differential equation \( \frac{d^2s}{dt^2} + \omega^2 s = 0 \) has solutions of the form:\[ s(t) = A \cos(\omega t) + B \sin(\omega t) \]with \( \omega = \sqrt{2500} = 50 \). Hence, the general solution is:\[ s(t) = A \cos(50t) + B \sin(50t) \]
04

Apply Initial Conditions

Use the initial conditions given: \( s(0) = 5 \) and \( s'(0) = -10 \).At \( t = 0 \), \( s(0) = A = 5 \).Now differentiate the general solution to find the velocity:\[ s'(t) = -50A \sin(50t) + 50B \cos(50t) \]At \( t = 0 \), \( s'(0) = 50B = -10 \), hence \( B = -0.2 \).Thus, the particular solution is:\[ s(t) = 5 \cos(50t) - 0.2 \sin(50t) \]
05

Determine the Maximum Displacement

The maximum displacement occurs when the derivative \( s'(t) \) is zero, or we can use the amplitude of the oscillation directly to find the maximum displacement.The amplitude of the oscillation is:\[ \sqrt{A^2 + B^2} = \sqrt{5^2 + (-0.2)^2} = \sqrt{25.04} \approx 5.004 \]Thus, the mass goes approximately 5.004 units from the mean position, given that the amplitude represents the distance from equilibrium.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Spring Oscillations
Spring oscillations describe the motion of a mass connected to a spring, which moves back and forth in a periodic manner. This movement is fundamentally governed by Hooke's Law, which connects the forces acting on the spring to its displacement. When a spring is compressed or stretched, it exerts a force to return to its natural length. The oscillatory motion observed is a result of this restorative force, leading to simple harmonic motion.

A key aspect of spring oscillations is the frequency of the oscillation. This is determined by the spring constant, denoted as \( k \), and the mass of the object, \( m \). The general differential equation for describing the motion is \( m \frac{d^2s}{dt^2} + ks = 0 \), where \( s(t) \) indicates the displacement over time. This motion equation simplifies to a standard form, revealing the nature of the oscillations.

When a specific frequency is involved, denoted as \( \omega = \sqrt{\frac{k}{m}} \), it dictates how fast the oscillations occur. In our exercise, the angular frequency is \( \omega = \sqrt{2500} = 50 \), thus impacting the periodic nature of the movement.
Initial Conditions
Initial conditions play a crucial role in solving differential equations, especially in oscillatory systems like a mass-spring system. They help tailor the general solution of the differential equation to fit the particular scenario being studied.

For our spring oscillation problem, we assume that the mass starts at a specific position and with a known velocity. These values are essential for determining the constants in the motion equation solution.
  • The initial position is given as \( s(0) = 5 \), resulting in the constant \( A = 5 \).
  • The initial velocity is \( s'(0) = -10 \), which, from the derivative of the general solution, calculates \( B \).
These initial data points allow us to find the particular solution - the one that fits this unique starting situation: \( s(t) = 5 \cos(50t) - 0.2 \sin(50t) \).

Such information essentially "anchors" the solution, making it specific to the oscillatory behavior from that point onward.
Hooke's Law
Hooke's Law is foundational in understanding spring oscillations. It describes the force required to compress or extend a spring by a distance \( x \) from its equilibrium position. Expressed mathematically, Hooke's Law states \( F = -kx \), where \( F \) is the force, \( k \) is the spring constant, and \( x \) is the displacement.

This force acts in a direction opposite to the displacement, which is why the negative sign is present. It indicates that the force aims to bring the spring back to its original position.
  • The spring constant \( k \) signifies the stiffness of the spring; a higher \( k \) value represents a stiffer spring.
  • Hooke's law is linear; a doubling of displacement results in doubling of the force required.
In dynamic systems like the one in this spring problem, Hooke's Law helps establish the basis for the differential equation governing motion, \( m \frac{d^2s}{dt^2} + ks = 0 \). This equation then models how the spring system responds to displacements over time, pivotal for analyzing and predicting the system's behavior.

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Most popular questions from this chapter

The differential equation \(d Q / d t=-0.15 Q+25\) represents the quantity of a drug in the body if the drug is metabolized at a continuous rate of \(15 \%\) per day and an IV line is delivering the drug at a constant rate of \(25 \mathrm{mg}\) per hour.

Each of the differential equations (i)-(iv) represents the position of a 1 gram mass oscillating on the end of a damped spring. Pick the differential equation representing the system which answers the question. (i) \(\quad s^{\prime \prime}+s^{\prime}+4 s=0\) (ii) \(s^{\prime \prime}+2 s^{\prime}+5 s=0\) (iii) \(s^{\prime \prime}+3 s^{\prime}+3 s=0\) (iv) \(\quad s^{\prime \prime}+0.5 s^{\prime}+2 s=0\) Which spring is the stiffest? [Hint: You need to determine what it means for a spring to be stiff. Think of an industrial strength spring and a slinky.]

For each of the differential equations find the values of \(c\) that make the general solution: (a) overdamped, (b) underdamped,(c) critically damped. $$s^{\prime \prime}+6 s^{\prime}+c s=0$$

Give an example of: Values of the spring constant \(k\), the mass \(m,\) and the damping coefficient \(a\) so that the motion is underdamped and shows damped oscillations.

A drug is administered intravenously at a constant rate of \(r\) mg/hour and is excreted at a rate proportional to the quantity present, with constant of proportionality \(\alpha>0.\) (a) Solve a differential equation for the quantity, \(Q,\) in milligrams, of the drug in the body at time \(t\) hours. Assume there is no drug in the body initially. Your answer will contain \(r\) and \(\alpha .\) Graph \(Q\) against \(t\) What is \(Q_{\infty},\) the limiting long-run value of \(Q ?\) (b) What effect does doubling \(r\) have on \(Q_{\infty} ?\) What effect does doubling \(r\) have on the time to reach half the limiting value, \(\frac{1}{2} Q_{\infty} ?\) (c) What effect does doubling \(\alpha\) have on \(Q_{\infty} ?\) On the time to reach \(\frac{1}{2} Q_{\infty} ?\)

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