Waves and Motion in the Natural World
In 2010, biologists at Monash University measured a peregrine falcon diving at 389 km/h, using the same F = ma physics as a Formula 1 car's aerodynamics.
Bats navigate in complete darkness using echolocation, emitting high-pitched sounds and listening for echoes. What property of sound waves makes this possible, and why would low-frequency sounds be less useful for this purpose?
Astronomers can determine what a distant star is made of, how hot it is, and whether it is moving toward or away from us, all without leaving Earth. What property of light makes this incredible information possible?
● Know
- How waves and motion operate in natural systems
- Examples of waves in geology, ecology, astronomy, and biology
- How organisms use waves for communication and navigation
● Understand
- How wave principles apply across different natural phenomena
- Why understanding natural waves matters for science and society
- The connections between wave physics and living systems
● Can do
- Identify wave phenomena in natural contexts
- Explain how organisms use waves for survival
- Connect wave science to real-world natural events
Watch a peregrine falcon pull out of a 389 km/h dive: its hollow bones flex slightly to absorb the 25 g deceleration forces, its swept-wing shape reduces drag, and its talons lock closed on impact in under 5 milliseconds. Animals are masters of applied physics, every wing shape, limb length, and muscle arrangement is an evolutionary solution to force and motion problems that engineers still study for inspiration.
Cheetahs: The fastest land animal (top speed ~100 km/h). Their flexible spine acts like a spring, storing and releasing elastic energy. Large leg muscles generate enormous horizontal force. Non-retractable claws provide traction like cleats. A long tail acts as a rudder for high-speed turns. But they overheat quickly and can only sustain top speed for about 20-30 seconds.
Mountain goats: Specialized for steep, rocky terrain. Their hooves have rough, rubbery pads and sharp edges that grip rock. Their centre of mass is low and positioned over their feet for stability. They use small, precise movements rather than explosive power.
Kangaroos: Their long Achilles tendons store elastic energy with each hop, making locomotion highly efficient at speed. At 20 km/h, a kangaroo uses less energy per kilometre than a quadruped of similar size.
A kangaroo hopping mechanics illustrates elastic energy storage beautifully. As the kangaroo lands, its leg tendons stretch, storing gravitational potential energy as elastic potential energy. As it pushes off, the tendons recoil, returning most of this energy and reducing the muscular work required. This is like a pogo stick: the spring does most of the work, and the rider only needs to add enough energy to compensate for losses. At slow speeds, kangaroos use an awkward pentapedal gait (tail plus four limbs), but above 15 km/h, hopping becomes more efficient than running. This speed-dependent gait transition is unique to macropods and is a remarkable example of evolutionary optimisation.
Australian biomechanics research: The University of Queensland locomotion laboratory studies kangaroo and wallaby hopping to understand elastic energy storage in tendons. This research has applications for prosthetic design and robotics. Australian researchers also study emu locomotion (energy-efficient walking), crocodile swimming (using tail undulation), and spider web vibration sensing. Australia unique fauna provides extraordinary examples of evolutionary engineering that inspire biomimetic technologies.
Animals move efficiently because they are strong. This is only partly true. Efficiency comes from structure and mechanics, not just strength. Kangaroos are not particularly muscular compared to other mammals their size, but their tendon anatomy makes them extraordinarily efficient hoppers. Cheetahs are not the strongest cats (lions and tigers are stronger), but their lightweight build, flexible spine, and large stride length make them fastest. Evolution optimises for specific ecological niches, and strength is only one of many variables. Biomechanics matters as much as brute force.
Match each animal adaptation to the physics principle it exploits.
Tsunamis are not ordinary waves. They are generated by sudden vertical displacement of the seafloor during earthquakes, landslides, or volcanic eruptions. Unlike wind waves (which affect only the surface), tsunamis involve the entire water column from surface to seafloor.
Deep water behaviour: In the deep ocean (depth ~4 km), tsunami speed is approximately v = √(gd) where g is gravity and d is water depth. This gives speeds around 200 m/s (720 km/h) - as fast as a jet airliner. In deep water, the wavelength is enormous (hundreds of kilometres) and the amplitude is small (typically less than 1 metre). Ships at sea may not even notice a passing tsunami.
Shallow water behaviour: As the wave approaches shore, depth decreases, so speed decreases. Wave energy is conserved, so as speed drops, the energy concentrates into greater amplitude. The wave slows, steepens, and grows dramatically - sometimes reaching heights of 30 metres or more.
The 2004 Indian Ocean tsunami was triggered by a magnitude 9.1 earthquake off Sumatra. In deep water, the wave travelled at about 800 km/h with amplitude around 0.5 m - barely noticeable to ships. But as it approached coastlines, it slowed and grew. In Banda Aceh, Indonesia, the wave reached 24 metres high and penetrated several kilometres inland, killing over 170,000 people. The physics is straightforward: wave speed decreases in shallow water (v = √(gd)), and conservation of energy causes the amplitude to increase. But the human cost was catastrophic. The event led to the establishment of Indian Ocean tsunami warning systems using seismic sensors and ocean-bottom pressure gauges.
Australian tsunami research: Geoscience Australia models tsunami risk for the Australian coast, identifying communities vulnerable to tsunamis from earthquakes in the Pacific Ring of Fire, the Indonesian subduction zone, and local sources. The Joint Australian Tsunami Warning Centre (JATWC) monitors seismic activity 24/7 and issues warnings when tsunamis are likely. Australian researchers use computer models based on shallow-water wave equations to predict tsunami arrival times and inundation extents. These models have been validated against historical events and inform evacuation planning for coastal communities.
Tsunamis are just big surf waves. This is false. Ordinary surf waves are generated by wind, affect only the surface layer, have periods of 5-15 seconds, and break as they approach shore. Tsunamis are generated by seafloor displacement, involve the entire water column, have periods of 10 minutes to 2 hours, and do not break like surf - they surge inland like a rapidly rising tide. A tsunami is more like a sudden high tide that keeps coming than a breaking wave. This difference matters for warnings and survival: you cannot surf a tsunami, and the danger persists for hours as multiple waves arrive.
Seismic waves are our primary tool for understanding Earth deep interior. We cannot drill deeper than about 12 km, but seismic waves from earthquakes travel through the entire planet, carrying information about the materials they pass through.
P-waves (Primary): Longitudinal waves that travel through both solids and liquids. Fastest seismic waves. Speed depends on the compressibility and density of the material.
S-waves (Secondary): Transverse waves that only travel through solids. Slower than P-waves. Their absence in certain regions proves the existence of a liquid outer core.
Wave behaviour at boundaries: When seismic waves encounter boundaries between materials with different properties, they reflect, refract, and convert between types. By analysing arrival times and wave types at seismometers worldwide, geophysicists map Earth internal structure.
In 1906, Richard Dixon Oldham noticed that P-waves from distant earthquakes arrived later than expected and S-waves from very distant earthquakes did not arrive at all. He correctly inferred that Earth has a core that affects seismic waves. In 1936, Inge Lehmann used P-wave data to propose that Earth core has a solid inner core surrounded by a liquid outer core. This was confirmed decades later by detailed seismology. Today, we know the inner core is solid iron-nickel (radius ~1,220 km), the outer core is liquid iron (thickness ~2,260 km), the mantle is solid but ductile rock (thickness ~2,900 km), and the crust is thin and rigid (thickness 5-70 km). All this knowledge comes from analysing seismic waves.
Australian seismology: Geoscience Australia operates the National Seismograph Network, which detected the 2011 Christchurch earthquake, the 2004 Sumatra earthquake, and thousands of smaller events. Australian seismologists contribute to the International Seismological Centre and study the structure beneath Australia unique ancient crust. The Warramunga Seismic and Infrasound Research Station in the Northern Territory is one of the most sensitive seismic stations in the world, located in a geologically quiet area with low background noise. It monitors nuclear test ban compliance and studies deep Earth structure.
We know Earth interior from drilling deep holes. This is false. The deepest hole ever drilled (Kola Superdeep Borehole in Russia) reached only 12.2 km, which is about 0.2% of Earth radius. We cannot drill to the core because pressure and temperature become prohibitive. All our knowledge of Earth deep interior comes from indirect evidence, primarily seismic waves, supplemented by meteorite composition, magnetic field measurements, and laboratory experiments on materials at high pressure and temperature. Seismology is our "telescope" for seeing inside Earth.
Click each stage to trace how seismic waves reveal Earth structure.
Earthquake
Sudden release of energy creates seismic waves.
Wave travel
P-waves and S-waves travel through Earth at different speeds.
Shadow zones
S-waves do not pass through liquid outer core, creating shadow zones.
Inference
Scientists infer Earth layered structure from wave behaviour.
Light from stars and galaxies carries information across vast distances:
- Spectroscopy splits light into its component wavelengths, revealing what stars are made of, their temperature, and their motion.
- The Doppler effect causes light from moving stars to shift in wavelength: light from stars moving away is red-shifted; light from stars moving toward us is blue-shifted.
- This red shift of distant galaxies provided key evidence that the universe is expanding.
- Cosmic background radiation is faint microwave radiation left over from the Big Bang, detected in all directions.
Australian example: Australian astronomers use telescopes like the Australia Telescope National Facility and contribute to international projects like the Square Kilometre Array to study radio waves from space, revealing the structure and history of the universe.
The light from a distant galaxy is red-shifted compared to what we would expect. What does this indicate?
Wrong: "Tsunamis are just big ocean waves caused by strong winds." No, tsunamis are caused by underwater earthquakes, landslides, or volcanic eruptions, not wind. They have much longer wavelengths and travel much faster across oceans than wind-generated waves.
Right: Tsunamis are caused by sudden large-scale displacement of water, typically from underwater earthquakes, submarine landslides, or volcanic eruptions. Unlike wind waves (which only disturb the surface), a tsunami involves the movement of the entire water column. They can travel at over 800 km/h across open ocean and are virtually undetectable at sea, becoming devastating only as they approach shallow coastlines.
Wrong: "Bats are blind and rely entirely on echolocation because they cannot see." No, most bats can see, and many have good night vision. Echolocation complements their vision, giving them a detailed sound-map of their surroundings.
Right: Most bats have functional eyes and can see reasonably well. Echolocation uses high-frequency sound waves that bounce off objects and return to the bat, providing precise information about location, size, and movement of prey, particularly useful in complete darkness. It is an additional sense that works alongside vision, not a replacement for it.
Wrong: "The Doppler effect only applies to sound waves." No, the Doppler effect applies to all waves, including light. It is used by astronomers to measure the motion of stars and galaxies and provided evidence for the expanding universe.
Right: The Doppler effect is a property of all waves: when a wave source moves relative to an observer, the observed frequency (and wavelength) changes. For light, this produces red shift (source moving away) or blue shift (source approaching). Astronomers use this to measure galaxy velocities, which provided the key evidence that the universe is expanding.
Australian Natural Wave Phenomena
The Victorian earthquake of 2021: A magnitude 5.9 earthquake near Mansfield, Victoria, was felt across southeastern Australia, reminding us that Australia is not immune to seismic activity. Geoscience Australia's seismic monitoring network recorded P-waves and S-waves that helped pinpoint the epicentre and understand the fault mechanism.
Humpback whale migration: Each year, humpback whales migrate along Australia's east and west coasts, travelling from Antarctic feeding grounds to tropical breeding waters. They communicate using complex songs, low-frequency sound waves that can travel hundreds of kilometres through the ocean, allowing pods to stay connected across vast distances.
The Aurora Australis: The Southern Lights occur when charged particles from the Sun (solar wind) interact with Earth's magnetic field and atmosphere, creating spectacular waves of coloured light. Tasmania and southern Victoria offer some of the best views of this electromagnetic phenomenon in Australia.
✍ Copy Into Your Books
▾Seismic Waves
- P-waves: compression waves, travel through solids and liquids
- S-waves: shear waves, only through solids
- Surface waves: cause most earthquake damage
Tsunamis
- Caused by underwater earthquakes, landslides, eruptions
- Long wavelength (100-200 km), fast in deep ocean
- Grow dramatically in shallow water
Waves in Nature
- Echolocation: reflected sound for navigation (bats, dolphins)
- Photosynthesis: plants capture light energy
- Doppler effect: wavelength shift reveals motion
Natural Waves Analysis
Biology and Waves
At the start of this lesson you were shown the Parkes radio telescope detecting the first fast radio burst from deep space in 2007, a signal lasting just milliseconds that had travelled for billions of years, and how waves in nature span scales from a bat's echolocation click to the edge of the observable universe.
Now that you've worked through the lesson, how has your thinking shifted? Can you explain that hook idea more precisely using what you've learned today?
Q1. 1. Describe how seismic waves revealed that Earth has a liquid outer core. Explain which waves provide this evidence and why. 4 MARKS
Q2. 2. Explain how the Doppler effect is used in two different scientific contexts: one involving sound waves and one involving light waves. 4 MARKS
Q3. 3. Analyse how at least two different organisms have evolved to use waves for survival. Discuss the wave type, how it is produced or detected, and the survival advantage it provides. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now identify three natural phenomena involving waves and explain the science behind each?
- How does understanding natural waves help scientists predict events like earthquakes and tsunamis?
Model answers (click to reveal)
Answers
▾MCQ 1
AP-waves are compression waves that can travel through both solids and liquids. S-waves are shear waves that only travel through solids.
MCQ 2
BTsunamis are primarily caused by underwater earthquakes, landslides, or volcanic eruptions. They are not caused by wind (which creates normal ocean waves) or tides.
MCQ 3
CBoth bats and dolphins use echolocation. Bats emit ultrasound pulses in air and listen for echoes to navigate and hunt. Dolphins use clicks in water for similar purposes.
MCQ 4
BThe red shift of light from distant galaxies (wavelengths stretched to longer, redder wavelengths) indicates these galaxies are moving away from us. This is key evidence that the universe is expanding.
MCQ 5
AChlorophyll absorbs red and blue light most efficiently for photosynthesis but reflects green light. This reflected green light is what we see, making plants appear green.
Short Answer 1
Model answer: Seismic waves revealed Earth's liquid outer core through the behaviour of S-waves and P-waves. S-waves are shear waves that can only travel through solids, not liquids. Scientists observed that S-waves do not pass through the region beyond 103 degrees from an earthquake's epicentre, creating an S-wave shadow zone. This indicates the presence of a liquid layer that blocks S-waves. P-waves, which can travel through both solids and liquids, are refracted (bent) at the core-mantle boundary, creating a P-wave shadow zone between 103 and 143 degrees. The existence of both shadow zones and the way P-waves bend provides strong evidence for a liquid outer core surrounding a solid inner core.
Short Answer 2
Model answer: The Doppler effect is used with sound waves in police speed detection. A radar gun emits radio waves toward a moving vehicle; the waves reflect back at a different frequency depending on the vehicle's speed. The frequency shift is measured and converted to speed. With light waves, astronomers use the Doppler effect to study star and galaxy motion. Light from a star moving away from Earth is shifted toward longer (redder) wavelengths, red shift. Light from a star moving toward Earth is blue-shifted. Edwin Hubble observed that distant galaxies show red shift, and the greater the distance, the greater the red shift, providing evidence that the universe is expanding.
Short Answer 3
Model answer: Bats have evolved ultrasound echolocation for survival in darkness. They emit high-frequency sound pulses (typically 20-200 kHz) from their larynx or nose and listen for echoes with specialised ears. The time delay and intensity of returning echoes create a detailed sound-map of their surroundings, allowing them to detect tiny insects and avoid obstacles while flying at night. This gives bats a massive survival advantage: they can hunt in complete darkness when visual predators cannot. Humpback whales use low-frequency infrasound (as low as 20 Hz) for long-distance communication. These low-frequency sound waves travel hundreds of kilometres through ocean water with little energy loss. Whales use songs to coordinate migration, find mates, and maintain social bonds across vast ocean distances. This wave-based communication is essential for the survival of these highly social, migratory animals.