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The type of rubber band used inside some baseballs and golf balls obeys Hooke's law over a wide range of elongation of the band. A segment of this material has an unstretched length \(\ell\) and a mass \(m\). When a force \(F\) is applied, the band stretches an additional length \(\Delta \ell\). (a) What is the speed (in terms of \(m, \Delta \ell\), and the spring constant \(k\) ) of transverse waves on this stretched rubber band? (b) Using your answer to (a), show that the time required for a transverse pulse to travel the length of the rubber band is proportional to \(1 / \sqrt{\Delta \ell}\) if \(\Delta \ell \ll \ell\) and is constant if \(\Delta \ell \geqslant \ell\).

Short Answer

Expert verified
(a) \( v = \sqrt{\frac{k \Delta \ell (\ell + \Delta \ell)}{m}} \). (b) \( t \propto 1/\sqrt{\Delta \ell} \) if \( \Delta \ell \ll \ell \); constant if \( \Delta \ell \geqslant \ell \).

Step by step solution

01

Understanding Hooke's Law

According to Hooke's Law, the force required to stretch or compress a spring by some distance is proportional to that distance. This can be expressed as \( F = k \Delta \ell \), where \( k \) is the spring constant and \( \Delta \ell \) is the change in length of the spring (or in this case, the rubber band).
02

Finding the Wave Speed Formula

The speed of a wave on a string or band is given by \( v = \sqrt{\frac{T}{\mu}} \), where \( T \) is the tension in the string and \( \mu \) is the linear mass density (mass per unit length). The linear mass density can be expressed as \( \mu = \frac{m}{\ell + \Delta \ell} \), considering the stretched length of the band.
03

Expressing Tension Using Hooke's Law

Use Hooke's Law to express the tension \( T \) in the band: \( T = k \Delta \ell \). The tension provided by stretching the rubber band is directly linked to the force applied and the change in length.
04

Substituting into the Wave Speed Equation

Substitute \( T = k \Delta \ell \) and \( \mu = \frac{m}{\ell + \Delta \ell} \) into the wave speed formula:\[ v = \sqrt{\frac{k \Delta \ell}{m/(\ell + \Delta \ell)}} = \sqrt{\frac{k \Delta \ell (\ell + \Delta \ell)}{m}} \]
05

Evaluating Time for Pulse Travel Over Different \( \Delta \ell \) Conditions

The time \( t \) required for a pulse to travel the length of the band is \( t = \frac{\ell + \Delta \ell}{v} = \frac{\ell + \Delta \ell}{\sqrt{\frac{k \Delta \ell (\ell + \Delta \ell)}{m}}} \). Simplify using \sqrt{a/b} = \frac{\sqrt{a}}{\sqrt{b}}.\ If \Delta \ell \ll \ell,\ then \ t \approx \frac{\ell+\Delta\ell}{\sqrt{\frac{k \ell \Delta \ell}{m}}}\ = \frac{\ell}{\sqrt{\frac{k \ell \Delta \ell}{m}}} \approx \text{Proportional to} \ 1/\sqrt{\Delta \ell}.\ If \Delta \ell \geqslant \ell,\ then \ t \approx const.

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Key Concepts

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

Hooke's Law
Hooke's Law is a principle that describes the behavior of elastic materials, such as springs or rubber bands, when they are stretched or compressed. According to this law, the force needed to change the length of an elastic object is directly proportional to the amount of stretch or compression. This relationship can be expressed with the equation:
  • \( F = k \Delta \ell \)
Here, \( F \) represents the force applied, \( k \) is the spring constant, and \( \Delta \ell \) is the change in length from the object's original state. The spring constant \( k \) is a measure of the stiffness of the material. If \( k \) is large, the material is stiff and requires more force to stretch.
Understanding Hooke's Law is crucial for solving problems involving elastic materials, as it helps link the physical properties of the material with the forces acting upon it.
Wave on a String
When a wave travels along a string, its speed is influenced by two key factors: the tension in the string and the linear mass density of the string. The speed \( v \) of a wave can be calculated using the formula:
  • \( v = \sqrt{\frac{T}{\mu}} \)
In this formula, \( T \) is the tension applied to the string, and \( \mu \) is the linear mass density, which is the mass per unit length. For a rubber band or string that is being stretched, \( \mu \) can be calculated as:
  • \( \mu = \frac{m}{\ell + \Delta \ell} \)
Where \( m \) is the mass, \( \ell \) is the original length, and \( \Delta \ell \) is the additional length due to stretching.
This concept is useful because it provides insight into how mechanical waves propagate through different materials, and can help us understand the factors that affect wave speed naturally.
Spring Constant
The spring constant \( k \) is a fundamental parameter in Hooke's Law and a critical concept when exploring the behavior of elastic materials like springs and rubber bands. It represents the stiffness of the spring or material and determines how much force is needed for a particular amount of stretch. A higher spring constant means the material is more resistant to stretching or compression.
The value of \( k \) can greatly influence how a material behaves under stress and helps define the tension \( T \) in the context of waves on a string, notably expressed as \( T = k \Delta \ell \). Understanding how \( k \) interacts with other variables like the change in length \( \Delta \ell \) is key to calculating wave properties, such as wave speed, on strings or bands subjected to forces.
Having a solid grasp of the spring constant helps in predicting and altering the dynamic responses of materials under various force conditions.

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Most popular questions from this chapter

A standing wave pattern on a string is described by $$ y(x, t)=0.040(\sin 5 \pi x)(\cos 40 \pi t) $$ where \(x\) and \(y\) are in meters and \(t\) is in seconds. For \(x \geq 0\), what is the location of the node with the (a) smallest, (b) second smallest, and (c) third smallest value of \(x ?\) (d) What is the period of the oscillatory motion of any (nonnode) point? What are the (e) speed and (f) amplitude of the two traveling waves that interfere to produce this wave? For \(t \geq 0\), what are the \((\mathrm{g})\) first, \((\mathrm{h})\) second, and (i) third time that all points on the string have zero transverse velocity?

A wave on a string is described by $$ y(x, t)=15.0 \sin (\pi x / 8-4 \pi t) $$ where \(x\) and \(y\) are in centimeters and \(t\) is in seconds. (a) What is the transverse speed for a point on the string at \(x=6.00 \mathrm{~cm}\) when \(t=0.250 \mathrm{~s} ?\) (b) What is the maximum transverse speed of any point on the string? (c) What is the magnitude of the transverse acceleration for a point on the string at \(x=6.00 \mathrm{~cm}\) when \(t=\) \(0.250 \mathrm{~s} ?(\mathrm{~d})\) What is the magnitude of the maximum transverse acceleration for any point on the string?

A wave has a speed of \(240 \mathrm{~m} / \mathrm{s}\) and a wavelength of \(3.2 \mathrm{~m}\). What are the (a) frequency and (b) period of the wave?

The equation of a transverse wave traveling along a very long string is \(y=6.0 \sin (0.020 \pi x+4.0 \pi t)\), where \(x\) and \(y\) are expressed in centimeters and \(t\) is in seconds. Determine (a) the amplitude, (b) the wavelength, (c) the frequency, (d) the speed, (e) the direction of propagation of the wave, and (f) the maximum transverse speed of a particle in the string. \((\mathrm{g})\) What is the transverse displacement at \(x=3.5 \mathrm{~cm}\) when \(t=0.26 \mathrm{~s}\) ?

A sinusoidal wave of angular frequency \(1200 \mathrm{rad} / \mathrm{s}\) and amplitude \(3.00 \mathrm{~mm}\) is sent along a cord with linear density \(2.00\) \(\mathrm{g} / \mathrm{m}\) and tension \(1200 \mathrm{~N}\). (a) What is the average rate at which energy is transported by the wave to the opposite end of the cord? (b) If, simultaneously, an identical wave travels along an adjacent, identical cord, what is the total average rate at which energy is transported to the opposite ends of the two cords by the waves? If, instead, those two waves are sent along the same cord simultaneously, what is the total average rate at which they transport energy when their phase difference is (c) 0, (d) \(0.4 \pi\) rad, and (e) \(\pi\) rad?

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