Ruler-Drop Reaction Times Before and After Screen Use
At a school health fair, Class 10 students measured reaction time with the ruler-drop test. A partner held a 30 cm ruler vertically between the student's thumb and forefinger and released it without warning; the student caught it as fast as possible. The fall distance was read at the top of the thumb and converted to time using t = √(2d/g), with g = 9.8 m/s². Each student did five trials and the median was recorded.
| Student | At rest (cm) | Immediately after 30 min of continuous mobile gaming (cm) | After 10 min of quiet rest (cm) |
|---|---|---|---|
| A | 12 | 18 | 14 |
| B | 16 | 21 | 17 |
| C | 11 | 12 | 11 |
The teacher also noted two other things that morning. When a book was dropped behind the class with a loud bang, every student blinked before anyone could say what had happened. Student B, who had spent two minutes spinning on a swivel chair, could not walk in a straight line for several minutes afterwards.
1. Calculate the reaction times of Student A at rest and after gaming, and the percentage change. Then state, with reasoning, whether Student C was affected.
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Convert every distance to time first; comparing centimetres directly is wrong because the ruler accelerates, so d ∝ t².
- At rest, d = 0.12 m: t = √(2 × 0.12 / 9.8) = √0.0245 = 0.157 s.
- After gaming, d = 0.18 m: t = √(2 × 0.18 / 9.8) = √0.0367 = 0.192 s.
- Change = (0.192 − 0.157) ÷ 0.157 × 100 = 22%.
So A's reaction time rose from about 157 ms to 192 ms, roughly a fifth slower. For C, 11 cm gives 0.150 s and 12 cm gives 0.156 s, a change of only about 4%, which is within ordinary trial-to-trial variation, so C was essentially unaffected. A 6 cm increase in fall distance sounds large but is only a 22% change in time; equally, C's 1 cm change is only 6 ms and should not be presented as a real effect without knowing the spread of the five trials.
2. Explain why A and B changed more than C, naming the parts of the nervous system that turn the decision to catch into a coordinated movement. State one weakness of the experiment and one improvement.
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The pathway is: retina → optic nerve → visual cortex (occipital lobe) → association areas (judgement that the ruler is falling) → motor cortex (frontal lobe) → cerebellum (timing and smooth coordination) → spinal cord and motor neurons → muscles of the forearm. Fatigue and divided attention slow processing mainly at synapses, where neurotransmitter release and reception take time; a tired or distracted student needs longer to convert the visual signal into a motor command. C's near-zero change could mean C is practised at the task, was less absorbed in the game, or simply had a noisier baseline.
The design has a clear weakness: every student rested first, so order, practice and growing boredom are confounded with the gaming. There was also one test per condition and no control of sleep, caffeine, screen brightness or room noise. Improvement: randomise the order and counterbalance it (half the class games first), standardise screen distance and time of day, and repeat on several days so a median with a range can be reported.
3. The blink happened before anyone could describe the bang. Explain this with a reflex arc, and explain why B could not walk straight after spinning.
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Blinking to a sudden sound is a reflex: sound → cochlea → auditory nerve → relay centres in the brainstem → facial nerve → orbicularis oculi muscle. The route is short and does not wait for the cortex to identify the sound, so the protective response happens first and conscious description follows. This is why reflex action is faster and why it can occur even when attention is elsewhere; a reflex arc needs a receptor, sensory neuron, integrating centre, motor neuron and effector, but not a conscious decision.
B's unsteady walk is a different matter. Spinning sets the fluid in the semicircular canals moving; when B stops, the fluid keeps moving, so the vestibular apparatus reports turning while the eyes and joint receptors report standing still. The cerebellum, which compares intended movement with actual movement and issues corrections, receives conflicting information, so balance and coordinated stepping fail. This is not muscle weakness or a spinal reflex problem; it is a mismatch of sensory information, and it settles as the fluid slows.