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Catching speeding drivers is easy with modern technology. Police officers can stand at the side of the road and use a laser gun to instantly measure the speed of a moving car. But how, exactly do these speed guns work?

In this lesson, students perform unit conversions in scientific notation to understand how a speed gun calculates the speed of a moving vehicle. Then they apply the Pythagorean Theorem to see how those speeds are affected the position of the officer on the side of the road. In the end, they’ll decide whether or not speed guns can be trusted, and will learn about who stands to benefit from any innaccuracies in their readings.

Students will

  • Calculate the distance between a speed gun and a moving car using the speed of light
  • Calculate the speed of a moving car based on two distance measurements and a time between measurements
  • Apply the Pythagorean Theorem to see how measurements are affected by the distance between the officer and the road
  • Apply the Triangle Inequality to see what types of inaccuracies are possible with a speed gun
  • Discuss what someone should do if they get cited for speeding

Before you begin

Students should be comfortable calculating unit rates (e.g. traveling 44 ft in 0.4 s means your speed is 44 ft ÷ 0.4 s = 110 ft/s). Unit conversions (between ft/s and mi/hr) also make an appearance, so some prior experience is helpful, though not required.

Common Core Standards

Content Standards
Mathematical Practices