# 16.2 Mathematics of waves  (Page 4/11)

 Page 4 / 11

## Velocity and acceleration of the medium

As seen in [link] , the wave speed is constant and represents the speed of the wave as it propagates through the medium, not the speed of the particles that make up the medium. The particles of the medium oscillate around an equilibrium position as the wave propagates through the medium. In the case of the transverse wave propagating in the x -direction, the particles oscillate up and down in the y -direction, perpendicular to the motion of the wave. The velocity of the particles of the medium is not constant, which means there is an acceleration. The velocity of the medium, which is perpendicular to the wave velocity in a transverse wave, can be found by taking the partial derivative of the position equation with respect to time. The partial derivative is found by taking the derivative of the function, treating all variables as constants, except for the variable in question. In the case of the partial derivative with respect to time t , the position x is treated as a constant. Although this may sound strange if you haven’t seen it before, the object of this exercise is to find the transverse velocity at a point, so in this sense, the x -position is not changing. We have

$\begin{array}{ccc}\hfill y\left(x,t\right)& =\hfill & A\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)\hfill \\ \hfill {v}_{y}\left(x,t\right)& =\hfill & \frac{\partial y\left(x,t\right)}{\partial t}=\frac{\partial }{\partial t}\left(A\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)\right)\hfill \\ & =\hfill & \text{−}A\omega \phantom{\rule{0.2em}{0ex}}\text{cos}\left(kx-\omega t+\varphi \right)\hfill \\ & =\hfill & \text{−}{v}_{y\phantom{\rule{0.2em}{0ex}}\text{max}}\phantom{\rule{0.2em}{0ex}}\text{cos}\left(kx-\omega t+\varphi \right).\hfill \end{array}$

The magnitude of the maximum velocity of the medium is $|{v}_{{y}_{\text{max}}}|=A\omega$ . This may look familiar from the Oscillations and a mass on a spring.

We can find the acceleration of the medium by taking the partial derivative of the velocity equation with respect to time,

$\begin{array}{cc}\hfill {a}_{y}\left(x,t\right)& =\frac{\partial {v}_{y}}{\partial t}=\frac{\partial }{\partial t}\left(\text{−}A\omega \phantom{\rule{0.2em}{0ex}}\text{cos}\left(kx-\omega t+\varphi \right)\right)\hfill \\ & =\text{−}A{\omega }^{2}\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)\hfill \\ & =\text{−}{a}_{y\phantom{\rule{0.2em}{0ex}}\text{max}}\text{sin}\left(kx-\omega t+\varphi \right).\hfill \end{array}$

The magnitude of the maximum acceleration is $|{a}_{{y}_{\text{max}}}|=A{\omega }^{2}.$ The particles of the medium, or the mass elements, oscillate in simple harmonic motion for a mechanical wave.

## The linear wave equation

We have just determined the velocity of the medium at a position x by taking the partial derivative, with respect to time, of the position y . For a transverse wave, this velocity is perpendicular to the direction of propagation of the wave. We found the acceleration by taking the partial derivative, with respect to time, of the velocity, which is the second time derivative of the position:

${a}_{y}\left(x,t\right)=\frac{{\partial }^{2}y\left(x.t\right)}{\partial {t}^{2}}=\frac{{\partial }^{2}}{\partial {t}^{2}}\left(A\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)\right)=\text{−}A{\omega }^{2}\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right).$

Now consider the partial derivatives with respect to the other variable, the position x , holding the time constant. The first derivative is the slope of the wave at a point x at a time t ,

$\text{slope}=\frac{\partial y\left(x,t\right)}{\partial x}=\frac{\partial }{\partial x}\left(A\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)\right)=Ak\phantom{\rule{0.2em}{0ex}}\text{cos}\left(kx-\omega t+\varphi \right).$

The second partial derivative expresses how the slope of the wave changes with respect to position—in other words, the curvature of the wave, where

$\text{curvature}=\frac{{\partial }^{2}y\left(x,t\right)}{\partial {x}^{2}}=\frac{{\partial }^{2}}{{\partial }^{2}x}\left(A\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)\right)=\text{−}A{k}^{2}\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right).$

The ratio of the acceleration and the curvature leads to a very important relationship in physics known as the linear wave equation    . Taking the ratio and using the equation $v=\omega \text{/}k$ yields the linear wave equation (also known simply as the wave equation or the equation of a vibrating string),

$\begin{array}{cc}\hfill \frac{\frac{{\partial }^{2}y\left(x,t\right)}{\partial {t}^{2}}}{\frac{{\partial }^{2}y\left(x,t\right)}{\partial {x}^{2}}}& =\frac{\text{−}A{\omega }^{2}\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)}{\text{−}A{k}^{2}\phantom{\rule{0.2em}{0ex}}\text{sin}\left(kx-\omega t+\varphi \right)}\hfill \\ & =\frac{{\omega }^{2}}{{k}^{2}}={v}^{2},\hfill \end{array}$

definition of inertia
the reluctance of a body to start moving when it is at rest and to stop moving when it is in motion
charles
An inherent property by virtue of which the body remains in its pure state or initial state
Kushal
why current is not a vector quantity , whereas it have magnitude as well as direction.
why
daniel
the flow of current is not current
fitzgerald
bcoz it doesn't satisfy the algabric laws of vectors
Shiekh
The Electric current can be defined as the dot product of the current density and the differential cross-sectional area vector : ... So the electric current is a scalar quantity . Scalars are related to tensors by the fact that a scalar is a tensor of order or rank zero .
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what is binomial theorem
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What is the formula to calculat parallel forces that acts in opposite direction?
position, velocity and acceleration of vector
hi
peter
hi
daniel
hi
Vedisha
*a plane flies with a velocity of 1000km/hr in a direction North60degree east.find it effective velocity in the easterly and northerly direction.*
imam
hello
Lydia
hello Lydia.
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What is momentum
isijola
hello
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I need the solving for this question
philip
is the eye the same like the camera
I can't understand
Suraia
Josh
I think the question is that ,,, the working principal of eye and camera same or not?
Sardar
yes i think is same as the camera
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samsfavor
why is the "_" sign used for a wave to the right instead of to the left?
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classical mechanics?
Victor
principle of superposition?
principle of superposition allows us to find the electric field on a charge by finding the x and y components
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Two Masses,m and 2m,approach each along a path at right angles to each other .After collision,they stick together and move off at 2m/s at angle 37° to the original direction of the mass m. What where the initial speeds of the two particles
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2m & m initial velocity 1.8m/s & 4.8m/s respectively,apply conservation of linear momentum in two perpendicular directions.
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A body on circular orbit makes an angular displacement given by teta(t)=2(t)+5(t)+5.if time t is in seconds calculate the angular velocity at t=2s
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2+5+0=7sec differentiate above equation w.r.t time, as angular velocity is rate of change of angular displacement.
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Ok i got a question I'm not asking how gravity works. I would like to know why gravity works. like why is gravity the way it is. What is the true nature of gravity?
gravity pulls towards a mass...like every object is pulled towards earth
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An automobile traveling with an initial velocity of 25m/s is accelerated to 35m/s in 6s,the wheel of the automobile is 80cm in diameter. find * The angular acceleration
(10/6) ÷0.4=4.167 per sec
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what is the formula for pressure?
force/area
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force is newtom
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and area is meter squared
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so in SI units pressure is N/m^2
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In customary United States units pressure is lb/in^2. pound per square inch
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who is Newton?
scientist
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a scientist
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that discovered law of motion
Peter
ok
John
but who is Isaac newton?
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a postmodernist would say that he did not discover them, he made them up and they're not actually a reality in itself, but a mere construct by which we decided to observe the word around us
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how?
Qhoshe
Besides his work on universal gravitation (gravity), Newton developed the 3 laws of motion which form the basic principles of modern physics. His discovery of calculus led the way to more powerful methods of solving mathematical problems. His work in optics included the study of white light and
Daniel
and the color spectrum
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