This document discusses measuring the speed of objects electronically using light gates or ultrasonic sensors. Light gates use a beam of light between a transmitter and receiver that is broken when an object passes through, triggering a timer. Ultrasonic sensors reflect sound waves off moving objects and calculate position from pulse echo times. Electronic timers provide more accurate measurements than manual stopwatches by eliminating errors from human reaction times.

1. 9.3-Measuring
speed electronically
Physics
Cambridge VIII
Muhammad Abdul Mageid

2. Learning
Outcomes
After this topic you should be able to :
- Measure the speed of an object using electronic timing gates
- Explain why electronic timers can produce more accurate
timing measurements
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Expert tips
- Using electronic timers removes the errors in measurement
caused by reaction times.
- This improves accuracy

3. Improving our measurement
of speed
To measure the speed of an object
accurately both the distance travelled and
time taken must be measured accurately.
Our reaction times can be as much as half a
second when starting or stopping a
stopwatch. This will make the time
measurement inaccurates. This timing
problem is even greater when attempting to
measure small periods of time
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4. 1
Light Gates
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5. Practical Activity Light gates
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6. A light gate ⊙ Is an electronic device that can trigger a
timer when an object moves past it.
⊙ A beam of light passes between a
transmitter and receiver . When an
object moves through this beam the
time taken to pass through is measured
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7. A toy car passes
through the light
gate breaking
the beam
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8. To measure
the speed of
a toy car :
- Measure the length of a piece of rectangular card
accurately (distance travelled)
- Attach the card to the top of the car (make sure that the
card is vertical)
- Make the car pass through the beam
- As the card on the car breaks the beam an electronic
stopwatch is started
- When the card has passed completely through the beam
the stopwatch is stopped
- Use the time recorded and the length of the card
(distance travelled) to calculate the speed of the car
passing through the light gate
- EXAMPLE: Length of card = 10 cm , time taken = 0.2 s
𝒔𝒑𝒆𝒆𝒅 =
𝟏𝟎 𝒄𝒎
𝟎. 𝟐 𝒔
= 𝟓𝟎 𝒄𝒎/𝒔
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9. 2
Measuring
speed using
waves
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10. the speed of an
object can also
be measured by:
- Reflecting ultrasound waves off the object as it moves.
- An ultrasonic sensor sends out an ultrasonic pulse that
reflects off the object and travels back to the transmitter
- The transmitter can then calculate the position of the
object
- More ultrasound pulses are sent and the movement of
the object is measured over a period of time
- This time and distance information allows the speed of
the object to be monitored
- Even more accurate measurements of speed can be
made using reflections of radio waves from an object
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11. Question 1
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An ultrasonic transmitter times how long it takes for
echoes to arrive from a moving object.
The sound waves travel at 340 m/s
⊙ A) The first pulse sent produces an echo
0.001 s after the signal was sent . How far away is
the object ?
⊙ B) The first pulse sent produces an echo 0.003 s
after the signal was sent . How far away is the object
now?
⊙ C) The two pulses were sent 0.5 s apart from each
other . Use the answers from parts A and B to
calculate the speed ?

12. Question 2
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What advantages do electronic timing systems have
when compared to manual timing systems ?

13. Thanks!
Any questions?
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