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Part Two: Oscillations, Waves, & Fluids

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Examples of oscillations & waves:

Earthquake – Tsunami

Electric guitar – Sound wave

Watch – quartz crystal

Radar speed-trap

Radio telescope

Part Two: Oscillations, Waves, & Fluids

Examples of fluidmechanics:

Flow speed vs river width

Plane flight

High-speed photo: spreading circular waves on water.

13. Oscillatory Motion

Describing Oscillatory Motion

Simple Harmonic Motion

Applications of Simple Harmonic Motion

Circular & Harmonic Motion

Energy in Simple Harmonic Motion

Damped Harmonic Motion

Driven Oscillations & Resonance

Dancers from the Bandaloop Project perform on vertical surfaces, executing graceful slow-motion jumps.

What determines the duration of these jumps?

Wilberforce Pendulum

pendulum motion: rope length & g

Disturbing a system from equilibrium results in oscillatory motion.

Absent friction, oscillation continues forever.

Examples of oscillatory motion:

Microwave oven: Heats food by oscillating H2O molecules in it.

CO2 molecules in atmosphere absorb heat by vibrating global warming.

Watch keeps time thru oscillation ( pendulum, spring-wheel, quartz crystal, …)

Earth quake induces vibrations collapse of buildings & bridges .

Oscillation

Characteristics of oscillatory motion:

- Amplitude A = max displacement from equilibrium.
- PeriodT = time for the motion to repeat itself.
- Frequencyf = # of oscillations per unit time.

same period T

same amplitude A

[ f ] = hertz (Hz) = 1 cycle / s

A, T, f do not specify an oscillation completely.

Oscillation

An oscillating ruler completes 28 cycles in 10 s & moves a total distance of 8.0 cm.

What are the amplitude, period, & frequency of this oscillatory motion?

Amplitude = 8.0 cm / 2 = 4.0 cm.

Simple Harmonic Motion (SHM):

2nd order diff. eq 2 integration const.

Ansatz:

angular frequency

A, B determined by initial conditions

( t ) 2

x 2A

C = amplitude

= phase

Note: is independent of amplitude only for SHM.

Curve moves to the right for < 0.

Oscillation

|x| = max at v = 0

|v| = max at a = 0

Two identical mass-springs are displaced different amounts from equilibrium & then released at different times.

Of the amplitudes, frequencies, periods, & phases of the subsequent motions, which are the same for both systems & which are different?

Same: frequencies, periods

Different:

amplitudes ( different displacement )

phases ( different release time )

Tuned mass damper :

Damper highly damped ,

Overall oscillation overdamped.

Taipei 101 TMD:

41 steel plates,

730 ton, d = 550 cm,

87th-92nd floor.

Also used in:

- Tall smokestacks
- Airport control towers.
- Power-plant cooling towers.
- Bridges.
- Ski lifts.

Movie

Tuned Mass Damper

The tuned mass damper in NY’s Citicorp Tower consists of a 373-Mg (vs 101’s 3500 Mg) concrete block that completes one cycle of oscillation in 6.80 s.

The oscillation amplitude in a high wind is 110 cm.

Determine the spring constant & the maximum speed & acceleration of the block.

- The Vertical Mass-Spring System
- The Torsional Oscillator
- The Pendulum
- The Physical Pendulum

Spring stretched by x1 when loaded.

mass m oscillates about the new equil. pos.

with freq

= torsional constant

Used in timepieces

Small angles oscillation:

Simple pendulum (point mass m):

Tarzan stands on a branch as a leopard threatens.

Jane is on a nearby branch of the same height, holding a 25-m-long vine attached to a point midway between her & Tarzan.

She grasps the vine & steps off with negligible velocity.

How soon can she reach Tarzan?

Time needed:

- What happens to the period of a pendulum if
- its mass is doubled,
- it’s moved to a planet whose g is ¼ that of Earth,
- its length is quadrupled?

no change

doubles

doubles

- A pendulum becomes nonlinear if its amplitude becomes too large.
- As the amplitude increases, how will its period changes?
- If you start the pendulum by striking it when it’s hanging vertically,
- will it undergo oscillatory motion no matter how hard it’s hit?

- If it’s hit hard enough,
- motion becomes rotational.

(a) sin increases slower than

smaller

longer period

Physical Pendulum = any object that’s free to swing

Small angular displacement SHM

When walking, the leg not in contact of the ground swings forward,

acting like a physical pendulum.

Approximating the leg as a uniform rod, find the period for a leg 90 cm long.

Table 10.2

Forward stride = T/2 = 0.8 s

Circular motion:

2 SHO with same A & but = 90

x = R

x = R

x = 0

Lissajous Curves

The figure shows paths traced out by two pendulums swinging with different frequencies in the x- & y- directions.

What are the ratios x : y ?

1 : 2

3: 2

Lissajous Curves

SHM:

= constant

Energy in SHM

Linear force:

parabolic potential energy:

Taylor expansion near local minimum:

Small disturbances near equilibrium points SHM

- Two different mass-springs oscillate with the same amplitude & frequency.
- If one has twice as much energy as the other, how do
- their masses & (b) their spring constants compare?
- (c) What about their maximum speeds?

- The more energetic oscillator has
- twice the mass
- twice the spring constant
- (c) Their maximum speeds are equal.

sinusoidal oscillation

Damping (frictional) force:

Damped mass-spring:

Amplitude exponential decay

Ansatz:

At t = 2m / b, amplitude drops to 1/e of max value.

(a) For

is real, motion is oscillatory ( underdamped )

(c) For

is imaginary, motion is exponential ( overdamped )

(b) For

= 0, motion is exponential ( critically damped )

Damped & Driven Harmonic Motion

A car’s suspension has m = 1200 kg & k = 58 kN / m.

Its worn-out shock absorbers provide a damping constant b = 230 kg / s.

After the car hit a pothole, how many oscillations will it make before the amplitude drops to half its initial value?

Time required is

# of oscillations:

bad shock !

External force Driven oscillator

Let

d= driving frequency

( long time )

Prob 75:

= natural frequency

Damped & Driven Harmonic Motion

Resonance:

Buildings, bridges, etc have natural freq.

If Earth quake, wind, etc sets up resonance, disasters result.

Collapse of Tacoma bridge is due to self-excitation described by the van der Pol equation.

Tacoma Bridge

Resonance in microscopic system:

- electrons in magnetron microwave oven
- Tokamak (toroidal magnetic field) fusion
- CO2 vibration: resonance at IR freq Green house effect
- Nuclear magnetic resonance (NMR) NMI for medical use.