Build or Bust Flashcards
What waves does an Earthquake have?
seismic waves
What seismic waves does an earthquake have?
- P waves (primary)
- S waves (secondary)
- surface waves
Where is a P wave found?
its a body wave so is found all over the Earth - it is the first wave felt because it travels faster as mechanical waves are able to pass on energy more easily
Where is a S wave found?
its a body wave so if found all over the Earth but gets there second
Where are surface waves found?
only in the Earth’s crust
What type of waves are P and S waves?
P wave is a longitudinal wave
S wave is a transverse wave
Longitudinal wave
Oscillation of particles occur parallel to the direction of travel/ wave motion/ propagation/ energy transfer. Has compressions and rarefactions
Transverse wave
Oscillation of particles occur perpendicular to direction of travel/ propagation/ wave motion/ energy transfer
Young’s modulus equation
Stress / strain (measured in Nm^-2) could be Fl/Ax
What is Young’s modulus?
how much force is needed to make the 1x1x1 cube stretch by a certain amount
Find Young’s modulus of a material experiment
IV - force
DV - cross sectional area, original length, extension
Get a material measure the length of it and the cross sectional area. Use clamp, desk pulley, markers and metre ruler. Hang off edge of table, add force to hanging end and note how much a marker moves (this is the extension) Plot a graph of stress against strain, work out the gradient
What is the shadow zone?
the area waves aren’t felt
What material can’t transverse waves travel through very well?
transverse can’t travel through liquid very well (this includes S waves through the outer core)
Free oscillation
if an object is set vibrating it will oscillate at its natural frequency
Seismometer
earthquake is driver so we want the seismometer to have the same natural frequency
Amplitude
furthest distance from equilibrium (maximum point away from zero point)
How to measure the time period of a pendulum/ spring
- displace pendulum (only a small displacement)
- measure the time for 20 oscillations
- full oscillation is time between passing the same point in the same direction (always measure from the mean position)
- divide by 20 for time period of one oscillation
How can we reduce error when measuring period of pendulum?
- to reduce parallax error use a Fiducial marker - 2 posts, put pendulum between them
- increase overall oscillations measuring 20 instead of 10 or 30 instead of 20 this reduces uncertainty
How can you make a wine glass break?
Hit the wine glass once, it will oscillate at its natural frequency, if you match this frequency and hold it the glass will break. This is because maximum energy transfer is occurring so it won’t be able to withstand oscillations so will crack and then break
If the driver frequency equals the natural frequency…
…resonance will occur
What happens to the amplitude when resonance occurs?
the amplitude increases dramatically
Natural frequency
The frequency an object oscillates at when hit into motion. When driven at this frequency there will be violent movement. Also maximum amplitude occurs when maximum energy transfer happens because the natural frequency of the driver is the same as the driven
Resonance
- when an object oscillates at its natural frequency
- if driven at this frequency amplitude will get bigger due to maximum energy transfer
Simple harmonic motion
Pull pendulum to one side, force pulling back to mean position. The further from mean position, bigger force pulling it back to equilibrium.
Force is proportional to displacement (from original position)
Displacement is proportional to acceleration
Object undergoing SHM
- acceleration proportional to displacement from equilibrium position
- acceleration in opposite direction to displacement
Equations for SHM
F = -kx a = -kx
Where is the force directed in SHM?
force is always directed towards the mean position
In SHM the period is…
independent of the amplitude
Energy when pendulum at equilibrium
Potential energy = 0
Kinetic energy = max
Energy when pendulum at max amplitude
Potential energy = max
Kinetic energy = 0
2 springs in parallel =
double stiffness
The total amount of energy in the system of a simple harmonic oscillator is…
…constant - the kinetic and potential energy values change but the total energy doesn’t
How can sound waves moving through a material be modelled?
as simple harmonic motion
Stiffer material =
greater wave velocity through it
Why does sound travel faster through hotter air?
because it has a lower density
brittle
no plastic deformation before it cracks
Fixed points
melting point of ice
boiling point of water
triple point of water
Specific Heat capacity
the amount of energy required to raise 1kg by 1K (or degrees celsius)
What happens to the oscillations when damping occurs?
oscillations reduce over time
What causes damping?
normally due to friction or resistance
Latent heat
- energy required to change the state of 1kg (or g) of an object - change the intermolecular bonds (increase potential energy of particles or energy given out)
- energy involved in a phase change
Conduction
‘spreading’ through the material by atomic and molecular vibrations and motion of free electrons
Convection
heating of a fluid, expands when heated, becomes less dense so rises, cooler denser fluid takes its place creating a convection current
Radiation
energy lost by electromagnetic radiation, hotter object, shorter wavelength of emitted radiation
What is needed to produce a phase change?
energy
What is a phase change?
a change in state e.g. solid –> liquid or liquid –> gas
Phase change materials
objects change state at certain temperatures
Internal energy
sum of potential and kinetic energy in a group of particles
What happens to the internal energy during a phase change?
during a phase change the internal energy will change
TMD
tuned-mass damper
What is an effective way to reduce resonance within a building?
to have a TMD
What does a TMD do?
add massive pendulum or spring mass system that has the same natural frequency as the building
-If the building is driven to oscillate at same frequency, both structure and TMD will resonate. The TMD will oscillate in antiphase so amplitude of oscillations reduced
Damping
-the amplitude of oscillations in a system that is not subject to an external driving force will decrease over time due to resistive forces within the system
What is damping a consequence of in mechanical systems?
friction or viscous drag
What do dampers do?
dissipate the energy
Light damping
- amount of energy lost within each oscillation is quite small
- system oscillates several times before rest (swing door, oscillates several times before shutting)
Critical damping
-something moves back to its equilibrium position as fast as possible but never quite beyond it
Heavy damping
-the system takes a long time to return to equilibrium position then stays there without oscillating (door with stiff hinges, closes slowly)
Heavier the damping…
…the quicker energy is lost from that system
Damped system
absorb energy and vibrate when driven but resonate vibrations have smaller ‘spread’ over wide frequency range
Damping can make an oscillating system come to rest more quickly, it can also…
…prevent it from resonating quite so violently when driven at its natural frequency
