- TERMINOLOGY
- SIMPLE HARMONIC MOTION
- MASS ON A SPRING
- SIMPLE PENDULUM
- ENERGY IN SHM
- FORCED VIBRATIONS AND RESONANCE

An oscillation (or vibration) involves a particular
*cycle* of motion being repeated over and over.

- O is the rest or
*equilibrium*position - The
*amplitude*is the size of the maximum displacement = OA or OB (*not*A to B) - One
*cycle*is one complete oscillation: E.g. (O to A to B to O) or (A to B to A) - The
*period*(T) is the time for one cycle (usually in seconds) - The
*frequency*(f) is the number of cycles per second (‘cycles per second’ is called hertz (Hz))

If 5 cycles of a vibration take one second, then one cycle takes 1/5 = 0.2 seconds. In general:

We could determine the period of a pendulum or mass on a spring by timing a single cycle. However, timing just one cycle would be inaccurate. Rather, we would time, say, 20 cycles and divide by 20.

E.g.. For a particular pendulum, 20 cycles takes 15 sec** \ **20T = 15 s, \ period, T =
15/20 = 0.75 sec.

**SIMPLE HARMONIC
MOTION (‘SHM’) **(return
to start of page)

In the diagram below, point N is the projection of point P onto
the line JK. Line PN is always at right angles (90^{0}) to JK.

P moves
uniformly round the circle of radius A. As P goes round and round, the point N
moves up and down the line JK.

Point N is repeating the
same basic motion, or cycle, over and over. Thus, we have a close link between
the circular motion of point P and oscillations along a straight line of point
N.

**Motion of N**

**Displacement, x**

**Velocity, v**

It can be shown, from the expression for x, that v is related to time by:

And v is related to position, i.e. distance x from the centre,
by:

We can also show, from the expression for v, that the acceleration a is related to time t by:

*An object is said to move with simple harmonic motion if
its acceleration is given by an equation of the above form.*

The above equation indicates that:

- when x is positive (N is above the centre) a is negative
- when x is negative (N is below the centre) a is positive

Thus, **the conditions for simple harmonic motion **can be
expressed as:

*A body undergoes simple harmonic motion if its acceleration is always directed towards a fixed point and is directly proportional to its distance from that point*

**Graphical representations**

**Variations with time**

As already stated:

The following represents graphs for a-x and v-x superimposed:

A coin lies on a surface which is vibrating vertically with SHM
of amplitude 5cm. At what frequency will the coin begin to lose contact with the
surface, and start to 'chatter'. (g = 10 ms^{-2})

Amplitude = 5cm = 0.05m

The coin will lose contact with the surface if the surface accelerates downwards at a greater rate than the coin, i.e. if:

**MASS ON A SPRING **(return
to start of page)

*l* = the extension produced when the mass m is
attached to the spring. If the mass is pulled a little further down and
released, it will oscillate about its equilibrium position. At a distance x from
equilibrium, the mass experiences a ‘restoring force’ F, i.e. one trying to
force it back to the equilibrium position.

The constant k is called the ‘*spring constant*’ of the
spring (Þ k = F/x, unit Nm^{-1})

We see from F = kx that the bigger the value of k the bigger
the force F needed to get a given extension x. Thus, k is a measure of the
*stiffness* of the spring, i.e. a measure of how hard it is to stretch (or
compress) it.

The expression F = kx gives the *magnitude* of the
restoring force.

- when the mass is below the equilibrium position F points upwards
- when the mass is above the equilibrium position F points downwards

We can take the relative directions of F and x into account by writing:

F and a have the same direction as each other, and are always opposite to x:

Now, in the above equation for acceleration a, (k/m) is a
positive constant for a given spring and mass, so the equation is of the
form:

Thus, *a mass oscillating on a spring satisfies
the conditions for SHM*. The mass oscillates just like the point N in the
earlier analysis.

The power ‘½’ representing ‘square-root’ of the content of the brackets.

In the position shown in (b) for the mass-spring system above, the mass is at its equilibrium position. In this position:

Where,* l *= original extension produced when the mass m
is hung on the spring.

**SIMPLE PENDULUM **(return
to start of page)

This consists of a small bob on a thread of a fixed length.

* For example, find sin10

To indicate the relative directions of F and x, we write:

Thus, *the pendulum bob moves with simple harmonic
motion*.

This equation is identical to that for a mass on a spring, but
notice that the meaning of* l* is different in each case.

**Experiment**

A simple ‘at-home’ experiment can be performed to use the above to measure g.

- Make a simple pendulum, say, 1 m long
- Time, say 20, oscillations (using small amplitude oscillations)
- Hence, calculate the period T
- Hence, determine a value for g using the above equation (the
accepted value is 9.8ms
^{-2})

Consider a mass m oscillating between J and K with SHM of amplitude A. Point O is the equilibrium position.

At a displacement x:

**Total energy**

In a *free oscillation* (i.e. one which neither gains nor
loses energy) the *total *energy (= kinetic + potential) is constant, i.e.
the same at every point in the oscillation. So if we know it at *one*
point, we know it at *all* points.

In the above oscillation, at the middle, PE = 0 and KE = its
maximum, \ KE at middle = total energy

Thus, total energy = KE when x = 0, so from the above equation:

Notice that, while KE and PE both depend upon x, the total energy does not, since its value is the same at every point, i.e. for any x value.

**Graphical representation**

**Example**

Show that PE = KE when x = A/Ö2, as indicated above.

**Energy in a spring**

In both an oscillating pendulum and a mass-spring system there is a continuous exchange between PE and KE. If the oscillations are 'free', then the sum of these is constant, and is given by:

For a mass-spring system, we can express this in terms of the spring
constant k.

This equals the energy when the spring is stretched to its
maximum, i.e. when the extension equals the amplitude, A.

Now, the strain energy stored in a spring when it is stretched by an amount A by a force F is given by:

**Damping**

A free vibration is one which continues with constant
amplitude, neither gaining nor losing energy.

For such
an oscillation a displacement (x) against time (t) curve might look like:

An oscillation that loses its energy is said to be
*damped*.

**a) Light damping**

Ping-Pong ball pendulum - the air provides light damping

This is said to occur when the time for the displacement to become zero is a minimum:

Damping is made use of in a number of devices. In a car, for example, the suspension makes the ride smoother when passing over bumps, but it has to be damped to prevent the car bouncing too much after a bump has been passed.

**FORCED
VIBRATIONS AND RESONANCE **(return
to start of page)

A system that can vibrate, such as a pendulum or a mass on a spring, has a natural period, this being the period with which it vibrates if it is set in motion and then left to oscillate. It also has a corresponding natural frequency. Some systems have a number of natural periods/frequencies.

When you push a child's swing you automatically push it, i.e.
apply a *forcing vibration*, in time with its own natural period, and this
allows the amplitude of the swing to build up. This is an example of
*resonance.*

*Resonance is said to occur when a system is forced to vibrate at its own natural frequency, and this can produce large amplitude vibrations*

Pendulum A is set in motion.

*We see that:*

The vibration carries along the top string (which should be a little loose). All the other three pendulums are affected – but pendulum C picks up the biggest amplitude vibration.

*We infer that:*

Pendulum A applies a forcing frequency to all the other three.

However, pendulum A and C are the *same* length, and so
have the *same* natural period and the *same* natural frequency. So
resonance occurs between A and C, producing a large vibration of C.

B and D have different lengths and therefore different natural frequencies to A and so resonance does not occur between these and A.

**Graphical representation**

Maximum amplitude occurs when:

Note: Resonance is a potentially dangerous phenomenon. In 1940
the Tacoma Narrows Suspension Bridge collapsed, just a few months after its
opening. When subject to a quite moderate wind it was set in vibratory motion,
and the amplitude of vibration built up till the bridge collapsed. Engineers
have to be aware of possible resonance occurring when structures are
designed.

A Level Physics - Copyright © A
C Haynes 1999 & 2004