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On a compact disc (CD), music is coded in a pattern of tiny pits arranged in a track that spirals outward toward the rim of the disc. As the disc spins inside a CD player, the track is scanned at a constant linear speed of v=1.25鈥尘/蝉. Because the radius of the track varies as it spirals outward, the angular speed of the disc must change as the CD is played. (See Exercise 9.20.) Let鈥檚 see what angular acceleration is required to keep constant. The equation of a spiral is r()=v0+, where r0 is the radius of the spiral at u=0and is a constant. On a CD, r0is the inner radius of the spiral track. If we take the rotation direction of the CD to be positive, must be positive so that rincreases as the disc turns and increases.

  1. When the disc rotates through a small angle d, the distance scanned along the track is ds=rd. Using the above expression for r(), integrate dsto find the total distance sscanned along the track as a function of the total angle through which the disc has rotated.
  2. Since the track is scanned at a constant linear speed v, the distance found in part (a) is equal to. Use this to find as a function of time. There will be two solutions for ; choose the positive one, and explain why this is the solution to choose.
  3. Use your expression for r()to find the angular velocity z and the angular accelerationzas functions of time. Is zconstant?
  4. On a CD, the inner radius of the track is 25.00mm, the track radius increases by 1.55碌m per revolution, and the playing time is 74.00 min . Find r0, 尾, and the total number of revolutions made during the playing time.
  5. Using your results from parts (c) and (d), make graphs of 蝇z(in rad/s) versust and伪z(in rad/s2) versus t between t=0and t=74.00 min.

Short Answer

Expert verified
  1. S=r0+22
  2. =r1+rf2+2vtz
  3. z=er02+2vt,z=v2r03+2vt3
  4. =133697.45, revolution
  5. Graph is drawn

Step by step solution

01

Identification of given data:

Here we have v=1.25鈥尘/蝉

Constant linear speed of scanning r=r0+

Theequationofaspiralr0=theradiusofthespiralatthe=0The radius of the spiral at the

is the constant

02

Angular velocity and angular acceleration

(a) Angular velocity: The pace at which an object's location angle with respect to time changes is known as its angular velocity.

z=ddt

Wherezis an angular velocity and is angle of rotation.

(b) Angular acceleration: The time rate of change of angular velocity is known as angular acceleration

z=dzdt

Where zis an angular velocity andz is angular acceleration.


03

Finding the total distance s scanned along the track as a function of the total angle θ through which the disc has rotated.

Here we havedS=rd,.

From above equation, we can determine the total distances S=f.

ds=rdds=r0+d0sds=0r0+dS-0=0r0d+0dS=r0+22

04

Step 4: Finding θ as a function of time by using s found in part (a)

In the case of uniform rectilinear motion, the distance traveled can be calculated as:

S=vt
Using the equation above, we can determine the form of function=ft.

S=vtr0+2=vt22+r0vt=0

The equation above is a quadratic equation so, its solution can be written in the following form:

=r0r02+42vt=r0r02+2vt

In the previous equation, we will consider only positive solutions. So,

=r0+r02+2vt

05

 Step 5: To find the angular velocity and angular acceleration as functions of time. 

Here we have to consider =ftcalculated in the part b), we can determine the change of the angular velocity over time z=ft, using the equation (1):

z=ddt=ddtr0+r02+2vt=1ddtr02+2vt=2v1r02+2vt12=vr02+2vt

Using the equation 2 we can calculate z=ft as

z=dzdt=ddtvr02+2vt=vddtr02+2vt12=v2v2r02+2vt32=v2r02+2vt32=v2r02+2vt3

06

Step 6: Finding r0,β and the total number of revolutions made during the playing time.

Using the parameters given from part d) we can determine the angle, Before, it is necessary to show in units ofmrad.

=1.55mrevolution=1.55106mrevolution1revoltuon2rad=0.247106鈥尘/谤补诲

Now, using the equation from part b) we can determine as follows.

=r0+r02+2vt=25103鈥尘+25103鈥尘2+2(0.247106鈥尘/谤补诲)1.25鈥尘/蝉4440鈥塻0.247106鈥尘/谤补诲=133697.45鈥塺别惫辞濒耻迟颈辞苍

07

Make graphs of ωz  (in rad/s ) versus  t and  αz (in rad/s2) versus   t between t=0  and  t=74.00 min from the result of part C and D.

Based on the laws calculated in the part under c ). And by applying the parameters from the part under d), it is possible to show graphs of the dependence of z=ft

So, we have z=vr02+2vt

So, graph of above function is which is given by,

As well as z=ft

Here we have,z=v2r02+2vt3

So, graph of above function is given as

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