CONTENT

• Definition
• Speed
• Amplitude
• Displacement
• Acceleration
• Period
• Frequency

DEFINITION

This is the periodic motion  of  a body or particle  along a straight  line  such that the acceleration of  the body  is directed  towards  a fixed  point .

A particle undergoing simple harmonic motion will move to and fro in a straight line under the influence of a force. This influential force is called a restoring force as it always directs the particle back to its equilibrium position.

Examples of simple harmonic motions are

i.  loaded test tube  in a liquid

ii Mass  on a string iii   The simple pendulum

Angular velocity (ω)   is defined by

ω = angle turned  through  by the body/ Time taken

ω = θ /t (rad /sec)

θ = ωt

This is similar to the relation distance = uniform  velocity  x time (s= =vt )  for motion  in a straight line

As the angle is changing with time so is the arc  length 

S = z p

 Changing with time. By definition θ in  radians = s/r  and  hence

S= rθ

A = r = radius of the circle

s/t = rθ /t = s/r /t

s/t = s/t  x 1/r = r θ /t

v =r ω

The linear velocity  v at any point ,Q  whose distance from C the central point is x is given by

V = ω √ A² – X²

The minimum velocity ,Vm corresponds to the point at X = 0 that is the velocity at the central  point or centre  of motion .

Hence, Vm =ω A

Thus the  maximum  velocity   of the SHM  occurs at the centre of the motion  (X=0)  while the  minimum  velocity  occurs at the  extreme  position of motion  (x=A ).

RELATIONSHIP BETWEEN LINEAR ACCELERATION AND ANGULAR VELOCITY

X = A COS θ

θ = ωt

X = A  cos ω t

dx/dt = -ωA sin ω t

dv/dt =-ω² A cos ω t

=-ω²X

The negative sign indicates that the acceleration is always inwards towards C while the displacement is measured outwards from C.

• Energy of simple harmonic motion
• Forced vibration and  resonance

ENERGY OF SIMPLE HARMONIC MOTION

Since force and displacement are involved, it follows that work and energy are involved in simple harmonic motion.

At any  instant of the motion , the  system  may  contain  some energy  as kinetic energy (KE ) or potential energy(PE) .The total  energy (KE + PE ) for a body performing SHM is  always  conserved  although  it may  change  form  between PE and KE .

When  a mass  is suspended  from the end  of a spring stretched vertically  downwards  and released , it oscillates  in a  simple  harmonic  motion .During  this motion , the force tending to  restore the  spring  to its elastic restoring  force  is simply the  elastic restoring force which is given  by

                F= – ky

K  is the force  constant of the spring 

FORCED VIBRATION AND RESONANCE

Vibrations resulting from the action of an external periodic force on an oscillating body are called forced vibrations.  Every vibrating object possesses a natural frequency (f_o) of vibration. This is the frequency with which the object will oscillate when it is left undisturbed after being set into vibration. The principle of the sounding board of a piano or the diaphragm of a loudspeaker is based on the phenomenon of forced vibrations.

Whenever the frequency of vibrating body acting on a system coincides with the natural frequency of the system, then the system is set into vibration with relatively large amplitude. This phenomenon is called resonance.

How To Solve Simple Harmonic Motion Problems In Physics

CLASSWORK

1. What is simple harmonic motion? Give four examples to illustrate simple harmonic motion
2. A particle moves round a circle of radius 10cm with a constant velocity of 20m/s, calculate the angular velocity
3. A particle undergoes simple harmonic motion with an amplitude of 5cm and an angular velocity of 10π rads^-1, calculate: (i) the maximum velocity (ii) the velocity when it is 2cm from the equilibrium position (iii) the maximum acceleration of the particle (iv) the period of oscillation
4. Define the following terms: frequency, period, amplitude of simple harmonic motion. What is the relation between period and frequency?

ASSIGNMENT

SECTION A

1. The period of oscillation of a simple pendulum is 2.0s. Calculate the period if the length of the pendulum is doubled (a) 1.0s (b) 1.4s (c) 2.8s (d) 4.0s
2. The period of a body performing simple harmonic motion is 2.0s. If the amplitude of the motion is 3.5cm, calculate the maximum speed of the body (π=22/7) (a) 22.0cms^-1 (b) 11.0cms^-1 (c) 7.0cms^-1 (d) 1.8cms^-1
3. A pendulum bob, executing simple harmonic motion has 2cm and 12Hz ass amplitude and frequency respectively. Calculate the period of the motion (a) 2.00s (b) 0.83s (c) 0.08s (d) 0.06s
4. What is the angular speed of a body vibrating at 50 cycles per seconds (a) 200πrads^-1 (b) 400 πrads^-1 (c) 100 πrads^-1 (d) 50 πrads^-1
5. The motion of a body is simple harmonic if the: (a) acceleration is always directed towards a fixed point (b) path of motion is a straight line (c) acceleration is directed towards a fixed point and proportional to its distance from the point (d) acceleration is proportional to the square of the distance from a fixed point
6. Which of the following assumptions is made in a simple pendulum experiment? The (a) suspending string is inextensible (b) bob has a finite size (c) bob has a definite mass (d) initial angle of oscillation must be large
7. A simple pendulum has a period of 17.0s.When the length is shortened by 1.5m, its period is 8.5s. Calculate the original length of the pendulum (a) 1.5m (b) 2.0m (c) 3.0m (d) 4.0m
8. The period of oscillatory motion is defined as the (a) average of the time used in completing different numbers of oscillations (b) time to complete a number of oscillations (c) time to complete one oscillation (d) time taken to move from one extreme position to another
9. Which of the following correctly gives the relationship between linear speed V and angular speed ω of a body moving uniformly in a circle of radius r? (a) v= ωr (b) v= ω²r (c) v= ωr² (d) v²= ωr