Kinematics Technology and Society: Speedometers, Radar, GPS and Speed Control

Physics on patrol

The moment a car passes a police radar, a baseball leaves a pitcher's hand, or your phone shows your running pace, a machine is doing kinematics: measuring a distance or a time and turning it into a speed. This lesson looks at how these technologies work, and then at the harder question: how should we use them?

Speedometers: counting turns

A car's speedometer does not measure speed directly. A sensor counts how fast a wheel or the transmission is turning. The car's computer multiplies the number of turns per second by the distance travelled in one turn (the tire's circumference, πd):

v = (πd) × (turns per second)

That means the computer assumes a certain tire size. Put on noticeably bigger tires and each turn carries the car farther than the computer thinks, so the speedometer reads low.

Radar guns: the Doppler effect

A police radar gun (and the radar guns that clock a baseball pitch) sends out a beam of microwaves with a precise frequency. The waves bounce off the moving object and return. If the object is approaching, the reflected waves come back squeezed together, at a slightly higher frequency; if it is moving away, they come back stretched out. This change is the Doppler effect. The gun's electronics measure the frequency shift, which is proportional to the speed, and display the speed directly.

A radar gun measures only the part of the velocity along the beam. If a car moves at an angle to the beam, the reading is the component v cos θ, which is less than the true speed. (You will study the Doppler effect with sound in the Waves and Sound unit.)

Lidar: timing light

A lidar speed gun fires very short pulses of infrared laser light and times how long each takes to bounce back. Light travels at 3.00 × 10⁸ m/s, so the distance to the car is

d = (speed of light × round-trip time) ÷ 2

(divided by 2 because the pulse goes there and back). The gun measures many distances in a fraction of a second, and the speed is simply Δd ÷ Δt, which is the slope of a position-time graph. Because a laser beam is very narrow, lidar can pick out one car in traffic.

Accelerometers and GPS

• Accelerometers are tiny sensors (often a microscopic mass on springs etched into a silicon chip) that detect acceleration. Your phone uses one to tell which way is up and to count steps. Cars use them to detect the huge, sudden acceleration of a collision and fire the airbags within milliseconds.
• GPS: as you learned in Grade 9, a receiver measures how long signals take to arrive from at least four satellites, converts each time into a distance, and finds its position by trilateration. By finding its position many times per second, a GPS receiver can calculate velocity (Δd ÷ Δt) too. That is how a phone shows your speed on a bike ride or your car's navigation shows when you will arrive.

Weighing the impact

Technologies that measure motion bring real benefits and real costs. Assessing them means looking at safety, the environment, the economy, privacy and fairness together.

Automated speed enforcement (photo radar). For several years Ontario law allowed municipalities to use speed cameras in school zones and community safety zones. After strong public and political debate, the province banned municipal speed cameras in November 2025, while some other provinces and many cities elsewhere still use them. Possible benefits: fewer drivers speeding where children walk, fewer serious collisions, less need for police to stop cars. Possible costs and concerns: privacy (cameras photograph vehicles), whether tickets are fair when the owner was not driving, and concerns that cameras are used to raise money. Because the rules can change, check the current law in your community.

GPS tracking. Benefits: navigation, finding people in emergencies, efficient delivery routes that save fuel, tracking public transit. Concerns: phones, apps and vehicles can record where you go, and that data can be shared, sold or misused.

Truck speed limiters. Since 2009, Ontario has required most large commercial trucks to have their speed limiters set to no more than 105 km/h. Benefits: lower speeds mean shorter stopping distances (remember braking distance grows with v²) and lower fuel use and greenhouse-gas emissions. Concerns: some truckers argue that speed differences between trucks and faster cars cause risky passing.

Careers and contributors

Kinematics is a daily tool in many careers (A2.1):

• Traffic and transportation engineers (university engineering degree, then licensing as a Professional Engineer) design intersections, set signal timing and speed limits using stopping-distance calculations.
• Collision reconstructionists (often specially trained police officers or engineers) use skid marks, video and vehicle data to work out speeds and timings.
• Geomatics technicians and surveyors (college diploma or university degree) use GPS to map land and build infrastructure.
• Aerospace engineers plan launch and landing trajectories.
• Kinesiologists and sports biomechanists (university) analyse athletes' motion from video to improve performance and prevent injury.

A Canadian contribution (A2.2): Willard Boyle, born in Amherst, Nova Scotia, co-invented the charge-coupled device (CCD), an electronic light sensor, with George Smith in 1969. CCDs made digital imaging practical, from digital cameras to the Hubble Space Telescope, and the two shared half of the 2009 Nobel Prize in Physics (the other half went to Charles Kao for work on optical fibres). Digital video, which you used to analyse motion in this unit, grew out of this kind of sensor.