Circle of reference in Simple Harmonic Motion

Circle of reference is a graphical representation which facilitate the understanding of various SHM relationships. Consider the figure given below in which a point Q moves anti-clockwise around a circle of radius A with a constant angular velocity ω (in rad.s-1).
In this blog many times i had written short notes or questions related to concept of SHM and this article is about yet another important concept of SHM that is Circle of Refrence.
Here in the figure
OQ→position of Q relative to O
θ→angle OQ vector makes with positive x-axis
This vector whose horizontal component represents the actual motion is called a
phasor

Let P be the point representing the projection of point Q (known as reference point) on the horizontal diameter of the circle (known as reference circle). As the reference point revolvs , the point P moves back and forth along the horizontal diameter always keeping below or above the point Q. The motion of this point P is comparable to that of a body moving under the influence of elstic restoring force and executing SHM in the absence of frictional forces.
From figure given above , displacement of point P at any given time t is the distance OP or x and from the figure
x=Acosθ
The angular velocity of circular motion is
ω = angle swept timetaken = θ t
so,
θ=ωt
and
x=Acosωt                                               (1)
Figure given below shows the velocity in SHM using circle of reference.
Velociy of the reference point Q would be the component of Q's velocity parallel to the x-axis since P is always directed below or above the reference point. So,
vP=-vsinθ = -ωAsinωt                            (2)
since v=ωA is the tangential velocity of motion of reference point Q. The negative sign is introduced because direction of velocity is towards the left. When Q is below horizontal diameter , the velocity of P is towards the right, but since sinθ is negative at such points, the minus sign is still needed.
Figure given below shows the acceleration in SHM using circle of reference.
The acceleration of Q on reference circle is directed radially inwards towards O as can be seen in the figure and its magnitude is equal to
a = v 2 A = ω 2 A
From figure x-component of acceleration is acceleration of point P. So,
aP=-acosθ= -ω2Acosθ                           (3)
The minus sign is introduced because the acceleration is towards the left. When Q is to the left of the center, the acceleration of P is towards the right but , since cosθ is negative at such points , the minus sign is still needed.
To prove that motion of point P is simple harmonic, we now use equation 1 in equation 3 then we have
aP=-ω2x
since ω is a constant , the acceleration of point P at any instant is equal to negative constant times displacement xat that instant and this is just essential feature of SHM and this proves that motion of point P is indeed SIMPLE HARMONIC.

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Last minute preparation for board exams


  • There are less then two weeks remaining before your board exams start and it is the high time students should concentrate and revise what they have studied over the past year.
  • Students should manage their time so that they can divide their time for revising different subjects.
  • Students should avoid being stressed and they should be very disciplined and should manage their time well for the  remaining  days before the exams.
  • Students should avoid late night studies and get a sound sleep during the night. this way their body clock would not go erratic and students will remain relaxed and stress free.
  • Students should not study anything new at this point of time and must revise the previously studied topics as studying new topics might confuse them.
  • It is advisable to students that they should avoid hangout on social networking websites as their each and every second before exams is precious.

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