Waves and Motion in Technology
In 1991, Bosch released the first mass-market airbag sensor, an accelerometer that fires within 15 milliseconds of detecting a crash deceleration above 30 g.
Think about a smartphone accelerometer and a car airbag. What do they have in common in terms of the physics they use? How might the same underlying science enable such different technologies?
A doctor can choose between X-ray, ultrasound, or MRI to examine a patient. What factors might influence which technology is chosen? Think beyond just accuracy, consider safety, cost, and the body part being examined.
● Know
- How Newton's laws apply to cars, planes and rockets
- That mobile phones, WiFi and satellites use electromagnetic waves
- That ultrasound, X-rays and MRI use wave properties for medical imaging
● Understand
- Why action-reaction pairs explain rocket propulsion
- Why electromagnetic waves are ideal for long-distance communication
- How different imaging technologies use different wave types and properties
● Can do
- Apply Newton's laws to explain transport technologies
- Compare communication technologies using wave properties
- Evaluate technologies using scientific evidence
Tap the screen of your phone sharply and it registers a sudden deceleration of roughly 50 g, detected by a silicon accelerometer the size of a fingernail that contains a tiny mass on a spring. That same sensor technology, scaled up and hardened, fires the airbag in your car within 15 milliseconds of a crash. Modern technology harnesses wave and force principles in ways that are invisible until you look closely, but the physics behind them is the same Newton and Maxwell worked out in the 1600s and 1800s.
Accelerometers: Tiny MEMS (Micro-Electro-Mechanical Systems) devices containing a proof mass on springs. When accelerated, the proof mass displaces due to inertia (Newton First Law), and capacitive sensors measure the displacement. This gives acceleration in three axes. Smartphones use accelerometers for screen rotation, step counting, gaming, and stabilising cameras.
Airbags: Crash sensors detect rapid deceleration (negative acceleration) exceeding a threshold. An electrical signal ignites a chemical propellant, rapidly generating nitrogen gas that inflates the airbag. The bag cushions the occupant, increasing the stopping distance and time, thereby reducing peak force (F = ma, but with longer Δt, peak F is lower).
ABS (Anti-lock Braking System): Wheel speed sensors detect when a wheel is about to lock. The system rapidly pulses brake pressure, maintaining traction and steering control while minimising stopping distance.
When you drop your phone, the accelerometer detects free-fall acceleration (near zero, because gravity acts on both phone and accelerometer mass equally in free-fall). Some phones use this to retract hard drive heads or lock the screen before impact. When the phone hits the ground, the accelerometer detects a massive deceleration spike (thousands of g). This data is used in drop-testing to design more durable phones. Apple and Samsung conduct thousands of drop tests, measuring acceleration profiles to optimise case design and internal component mounting.
Australian MEMS research: The University of Melbourne and RMIT University develop advanced MEMS sensors for aerospace, automotive, and medical applications. Australian company MEMSIC produces accelerometers and inertial sensors used in drones, vehicles, and industrial equipment. CSIRO Manufacturing researches next-generation sensors using quantum effects for unprecedented precision. These technologies underpin Australia growing autonomous systems industry, including self-driving mining trucks and agricultural drones.
Airbags deploy by detecting impact force on the bumper. This is false. Modern airbag systems use accelerometers mounted in the vehicle cabin, not force sensors on the bumper. The accelerometer measures the vehicle deceleration profile. A severe crash produces a characteristic rapid deceleration that the computer recognises. Some systems also use pressure sensors in the doors to detect side impacts. The deployment decision is based on acceleration pattern, not direct impact force measurement. This is why airbags can deploy even in collisions where the bumper is not directly hit.
Tap each card to flip. Mark Got it when you can recall the answer without flipping.
A modern smartphone contains an extraordinary collection of physics-based sensors:
Accelerometer: Measures linear acceleration in three axes. Detects gravity direction (when stationary) and motion-induced acceleration. Used for screen rotation, step counting, and gaming.
Gyroscope: Measures rotational velocity around three axes using the Coriolis effect on a vibrating MEMS structure. Detects how fast the phone is rotating. Essential for image stabilisation and precise orientation tracking.
Magnetometer: Measures magnetic field strength in three axes, acting as a compass. Detects Earth magnetic field direction for heading information.
GPS receiver: Detects radio signals from satellites to determine position. Works outdoors but not well indoors.
Sensor fusion: Software combines data from all sensors to determine orientation and position more accurately than any single sensor could. Accelerometers drift over time; gyroscopes accumulate error; magnetometers are affected by metal. Fusing them corrects individual weaknesses.
When you play a racing game and tilt the phone to steer, the accelerometer detects the tilt angle relative to gravity. When you rotate the phone to take a panorama, the gyroscope measures the rotation rate and integrates it to track orientation. When you use a mapping app, GPS provides position while the magnetometer tells you which direction you are facing. When you use augmented reality (AR), all three sensors work together: accelerometer and gyroscope track phone movement in real-time, while GPS anchors virtual objects to real-world locations. The physics of waves, forces, and electromagnetism all converge in your pocket.
Australian smartphone technology: While no major smartphones are manufactured in Australia, Australian researchers contribute significantly to mobile technology. CSIRO invention of Wi-Fi (based on radio wave physics) underpins wireless connectivity in every smartphone. Australian-developed video compression standards reduce data usage. The University of Sydney Centre for Excellence in Engineered Quantum Systems works on quantum sensors that could one day replace MEMS accelerometers with devices of extraordinary precision. Australian app developers create health, navigation, and entertainment apps that exploit smartphone physics sensors.
Gyroscopes in phones are spinning wheels like traditional mechanical gyroscopes. This is false. Modern phone gyroscopes are MEMS devices with no moving parts in the traditional sense. They contain tiny vibrating structures (tuning forks or resonant rings). When the device rotates, the Coriolis force causes a detectable shift in the vibration pattern. This shift is measured capacitively and converted to rotational velocity. The physics is elegant: no bearings, no spinning wheels, no wear - just vibrations and forces at the microscale. These solid-state gyroscopes are tiny (a few millimetres), cheap, and remarkably accurate.
Vehicle safety engineering applies Newton laws to protect occupants during collisions.
Crumple zones: The front and rear of modern vehicles are designed to deform in a controlled manner during impact. This increases the time over which the collision occurs. Since impulse = F × Δt = Δp (change in momentum), a longer Δt means a smaller F for the same momentum change. The crumple zone sacrifices the car to save the occupants.
Seatbelts: Apply stopping force across the pelvis and ribcage - the strongest parts of the body. Prevents occupants from hitting the windscreen or being ejected. The belt webbing stretches slightly, increasing stopping time.
Airbags: Deploy in milliseconds to cushion the head and chest. The bag deflates as the occupant compresses it, increasing stopping distance and time while distributing force over a larger area.
ABS and ESC: Prevent loss of control during emergency braking, maintaining the driver ability to steer and avoid obstacles.
In a 50 km/h head-on collision, a vehicle without crumple zones might stop in 0.1 seconds as the rigid chassis transfers force directly to occupants. With crumple zones, the stopping time might be 0.3 seconds. For a 70 kg occupant with initial momentum 972 kg·m/s, the average force without crumple zones is 972/0.1 = 9,720 N (about 1,000 kg of force - potentially fatal). With crumple zones, the average force is 972/0.3 = 3,240 N (survivable with seatbelt and airbag). This threefold reduction in force comes entirely from increasing stopping time. The physics is simple: F = Δp/Δt, but the engineering that achieves this safely is complex.
Australian vehicle safety standards: The Australian Design Rules (ADRs) mandate safety features including crumple zones, seatbelt pretensioners, airbags, ABS, and electronic stability control. ANCAP crash-testing subjects vehicles to frontal offset, side pole, and pedestrian impact tests, measuring forces on dummies. Australian vehicles must meet these standards to be sold. The Monash University Accident Research Centre (MUARC) analyses real-world crash data to inform safety policy. Their research shows that vehicles with 5-star ANCAP ratings have approximately 50% lower fatal injury risk than 1-star vehicles.
Stronger, stiffer cars are always safer. This is false. While cabin strength is important for occupant protection, an extremely stiff vehicle transfers more force to occupants in a collision. The safest design balances a rigid passenger compartment with deformable crumple zones. Formula 1 cars demonstrate this principle: the survival cell is extraordinarily rigid, but the front and rear structures are designed to absorb enormous energy in crashes. A tank-like vehicle with no deformation would subject occupants to lethal deceleration forces. Safety comes from controlled energy absorption, not brute rigidity.
Match each safety feature to the physics principle it exploits.
Scientists and engineers evaluate technologies by comparing them against clear criteria using reliable evidence. When evaluating technologies that use waves or motion principles, important criteria include:
- Accuracy and reliability: Does the technology produce consistent, correct results? For example, MRI is more accurate than X-ray for detecting soft-tissue injuries, but X-ray is faster for confirming a bone fracture.
- Safety: What are the risks to users, patients and the environment? X-rays carry a small radiation risk; ultrasound and MRI do not use ionising radiation.
- Cost and accessibility: Is the technology affordable and available where it is needed? Ultrasound machines are portable and relatively inexpensive, making them ideal for rural and remote Australia. MRI machines are large, expensive and usually only found in major hospitals.
- Environmental impact: Does the technology produce waste or consume significant resources? Rocket launches produce emissions; satellite networks require energy for operation and ground stations.
A patient in a remote Australian town needs imaging for a suspected soft-tissue knee injury. Which technology would be MOST practical to use and why?
Wrong: "Rockets push against the ground or air to move." No, rockets work in the vacuum of space by expelling exhaust gases downward (action). The exhaust gases push the rocket upward (reaction). Newton's third law explains propulsion without any medium to push against.
Right: Rockets work by expelling exhaust gases in one direction (action); the gases push back on the rocket in the opposite direction (reaction), Newton's third law. This mechanism requires no air or ground to push against, which is why rockets operate perfectly in the vacuum of space.
Wrong: "X-rays and MRI use the same type of waves." No, X-rays use high-energy electromagnetic waves that ionise tissue. MRI uses magnetic fields and low-energy radio waves. They are completely different technologies with different risks and applications.
Right: X-ray imaging uses high-energy electromagnetic waves that can pass through soft tissue but are absorbed by dense materials like bone. MRI uses strong magnetic fields and radio waves to make hydrogen atoms emit signals, producing detailed images of soft tissue. They use entirely different physics and carry different safety profiles, X-rays carry a small ionising radiation dose; MRI does not.
Wrong: "Ultrasound is dangerous because it is a form of radiation." No, ultrasound uses sound waves, which are mechanical vibrations, not electromagnetic radiation. It does not ionise cells and is considered very safe for routine medical use.
Right: Ultrasound uses high-frequency sound waves (mechanical vibrations) to create images, it is not electromagnetic radiation of any kind. Sound waves cannot ionise cells or damage DNA. Ultrasound is considered so safe it is routinely used to image developing fetuses and is the preferred method for imaging soft tissue in hospitals and remote clinics across Australia.
Technology in Australia
National Broadband Network (NBN): Australia's NBN uses a mix of fibre-optic cables, fixed wireless and satellite technology to deliver internet across the country. Satellite services (Sky Muster) are essential for remote and rural communities where cables are impractical. These satellites use microwaves to relay data between ground stations and homes across the outback.
Royal Flying Doctor Service: This iconic Australian service uses radio and satellite communication to coordinate emergency medical flights across vast distances. Electromagnetic waves enable rapid communication between remote locations and medical centres, saving lives in areas where other infrastructure is limited.
Australian Space Agency: Founded in 2018, the Australian Space Agency supports satellite development, Earth observation and space research. Satellites launched by Australian and international partners use Newton's laws for orbital motion and electromagnetic waves for data transmission back to Earth.
✍ Copy Into Your Books
▾Transport and Newton's Laws
- Cars: F = ma explains acceleration; seatbelts counteract inertia
- Planes: wings push air down, air pushes wing up (lift)
- Rockets: expel exhaust downward, rocket moves upward (action-reaction)
Communication Technologies
- Mobile phones, WiFi and satellites all use electromagnetic waves
- EM waves travel at the speed of light and do not need a medium
- Satellites use microwaves for reliable long-distance communication
Medical Imaging Comparison
- Ultrasound: sound waves, safe, no bone penetration
- X-ray: EM waves, good for bone, ionising radiation risk
- MRI: magnetic fields + radio waves, detailed soft tissue, expensive
Newton's Laws in Transport
Compare Medical Imaging
At the start of this lesson you were shown an MRI scanner at Royal Prince Alfred Hospital using magnetic fields and radio waves, no X-rays, to produce detailed images of soft tissue, and how every medical imaging technology was built on wave physics first discovered in a laboratory.
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. Explain how Newton's three laws of motion apply to the design and safety features of a modern car. Include at least one safety feature and explain which law it relates to. 4 MARKS
Q2. 2. Compare how mobile phones and satellites use electromagnetic waves for communication. In your answer, explain why electromagnetic waves are used rather than sound waves. 4 MARKS
Q3. 3. Evaluate the use of ultrasound, X-ray and MRI for medical imaging in rural Australia. Discuss at least two criteria (for example: safety, cost, accessibility, accuracy) and explain which technology best meets the needs of remote communities. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain rocket propulsion using Newton's third law?
- How would you explain WiFi to someone who thinks all waves need a medium?
Model answers (click to reveal)
Answers
▾MCQ 1
CNewton's third law explains rocket propulsion. The rocket pushes exhaust gases downward (action), and the exhaust gases push the rocket upward with an equal and opposite force (reaction). This works even in the vacuum of space because the action-reaction pair involves the rocket and its exhaust, not the rocket and the surrounding medium.
MCQ 2
BElectromagnetic waves are ideal for satellite communication because they can travel through a vacuum at the speed of light. Sound waves are mechanical waves that require a medium and cannot travel through the empty space between a satellite and Earth.
MCQ 3
AUltrasound is the most appropriate for checking a developing fetus because it uses sound waves and does not use ionising radiation. X-rays use ionising radiation and are avoided during pregnancy unless absolutely necessary. MRI is not typically the first choice for routine fetal checks.
MCQ 4
DX-ray uses high-energy electromagnetic waves that pass through soft tissue but are absorbed by bone. MRI uses strong magnetic fields and radio waves to create detailed images of soft tissues. They use completely different physical principles and have different strengths.
MCQ 5
BFor a remote community, portability and accessibility are the most important criteria. Ultrasound machines are relatively inexpensive, portable and do not require specialised facilities or shielding. MRI machines are large, expensive and immobile. X-ray machines also require shielding and trained operators.
Short Answer 1
Model answer: Newton's first law explains why seatbelts are necessary, when a car brakes, passengers continue moving forward due to inertia unless restrained. Newton's second law (F = ma) shows that a more powerful engine or lighter car produces greater acceleration. Newton's third law explains traction: tyres push backward on the road, and the road pushes the car forward. Airbags are another safety feature linked to the first law, they extend the stopping time for the head, reducing the force experienced (also connected to F = ma, since reducing acceleration reduces force).
Short Answer 2
Model answer: Mobile phones transmit signals as radio waves to nearby towers, which relay the signal through networks. Satellites use microwaves to communicate with ground stations across vast distances, including over oceans and remote areas. Electromagnetic waves are used rather than sound waves because EM waves can travel through a vacuum at the speed of light, carry large amounts of data, and are not blocked by the absence of a medium. Sound waves are mechanical waves that need a material medium and would be completely unable to travel between Earth and satellites.
Short Answer 3
Model answer: For rural Australia, cost and accessibility are critical criteria. Ultrasound is relatively inexpensive, highly portable and safe, making it ideal for remote clinics and the Royal Flying Doctor Service. X-ray is useful for diagnosing fractures but requires shielding and carries a small radiation risk. MRI provides the most detailed soft-tissue images but is extremely expensive, requires a specialised facility and is usually only available in major cities. Therefore, ultrasound best meets the needs of remote communities because it balances effectiveness, safety and accessibility. However, patients needing MRI may still need to travel to regional centres, highlighting an equity issue in healthcare access.