Automated measurement of speed | Exit Quiz | Oak National Academy

Match each of the following measuring devices to its description.

• light gate

• uses changes in frequency of reflected waves to measure speed

• speed camera

• sends signals to a timer when a passing object cuts its infrared beam

• motion sensor

• takes photos of road vehicles a short time apart

• radar gun

• sends out sound pulses and the reflections from an object are detected

Answer explanation

A radar gun and motion sensor can directly measure and display the speed of an object. A speed camera takes photos which can be used to work out speed. A light gate measures times which can be used to work out speed.

A student measures the time taken for a trolley to travel down a ramp and get these measurements: 0.19 s, 0.18 s, 0.26 s, 0.17 s. Which of the following mean times should the student record?

• 0.18 s

• 0.20 s

• 0.27 s

• 0.72 s

• 0.80 s

Answer explanation

The third measurement, 0.26 s, is quite different from the others. It is anomalous and should not be used in the calculation of the mean. Mean time = (0.19 + 0.18 + 0.17) ÷ 3 = 0.18 s.

A trolley with a card attached travels through a light gate, which measures the time for the card to pass through. Which of the following statements is correct if the trolley moves faster?

• The measured time is shorter.

• The measured time is longer.

• The measured time is the same.

Answer explanation

The measured time is shorter because at higher speeds the card takes less time to pass through the gate.

A pair of speed cameras take photos of a car 0.50 s apart. The photos show that the car travelled 12 m during this time. The speed of the car is ___ m/s.

Answer explanation

speed = distance ÷ time = 12 ÷ 0.50 = 24 m/s

A motion sensor sends out sound pulses. The pulses reflect off an object and return to the sensor after 0.0040 s. How far is the object from the sensor? The speed of sound in air is 330 m/s.

• 66 cm

• 132 cm

• 165 cm

• 264 cm

Answer explanation

Speed = distance ÷ time, and so distance = speed × time = 330 × 0.0040 = 1.32 m. This is the distance to the object and back, so the distance between the object and sensor is 1.32 ÷ 2 = 0.66 m, or 66 cm.

A trolley has a card of length 15 cm attached to it. The trolley passes through a light gate and the card cuts off the beam for 0.20 s. Which of the following is the speed of the trolley in cm/s?

• 0.75 cm/s

• 1.3 cm/s

• 3 cm/s

• 30 cm/s

• 75 cm/s

Answer explanation

speed = distance ÷ time = 15 ÷ 0.20 = 75 cm/s