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Reaction Time Ruler

How fast can you react?

In this activity, the students participate in a simple ruler drop experiment and learn about the body’s response behind it.

When your friend drops the timer in the experiment, you see it start to move. A nerve signal travels from your eye to your brain then to your finger muscles. Your finger muscles move to catch the timer. The whole process takes between 150 and 220 milliseconds.

The neural pathway involved in a reaction time experiment involves a series of neural processes. This experiment does not test a simple reflex. Rather, this activity is designed to measure the response time to something that you see. 

Catching a dropped ruler begins with the eye watching the ruler in anticipation of it falling. After the ruler is dropped, the eye sends a message to the visual cortex, which perceives that the ruler has fallen. The visual cortex sends a message to the motor cortex to initiate catching the ruler. The motor cortex sends a message to the spinal cord, which then sends a message to the muscle in the hand/fingers. The final process is the contraction of the muscles as the hand grasps the ruler. All of these processes involve individual neurons that transmit electrochemical messages to other neurons. 

A person’s reaction time depends on a couple of things that can be improved and a couple that cannot.

Practice does make perfect because you can create a “muscle memory” that means you do not have to think so much to catch the ruler. You can take the time it takes to decide things out of the equation. Much of the time it takes you to react to the ruler dropping is the time it takes electrical signals to travel along your nerves. Moving at about 100 metres per second, a signal telling a finger to move has to travel from your brain down your spinal cord and into your arm. Signals for muscle control generally move faster than other ones. (Pain signals for example, move very slowly, often less than one metre per second). But these signals are “involuntary” which means that no matter how hard you try, you cannot control how quickly they occur.

The distance the reaction timer travels before you catch it has been converted to time using the equation d=1/2at² where a is the acceleration due to gravity.

This is a recommended pre-visit activity to Science World.


  • Describe how the nervous system responds to a stimulus.


Key Questions

  • How fast is your reaction time?
  • What had to happen in your body for you to catch the ruler?
  • How can reaction time be improved?
  • Does your reaction time improve with practice?
  • Why was the ruler caught in the middle (after a lag period) rather than at the end (instantaneously)? What causes this hesitation?

What To Do


  1. Photocopy and cut out the reaction timer (double-check the size is accurate).
  2. Glue or tape it to a piece of stiff cardboard or ruler (unless printed onto card).


  1. Have students form partners for the activity.Each pair should decide who is number 1 and who is 2.
  2. Give each pair a ruler.
  3. Instruct student number 1 to hold the ruler near the end (highest number) and let it hang down. Student number 2 should put their hand at the bottom of the ruler and be ready to grab the ruler (however, they should not be touching the ruler).
  4. Student number 1 will drop the ruler sometime within the next 5 seconds and student number 2 must try to catch the ruler as fast as they can after it is dropped.
  5. Record the level (inches or centimeters) at which they catch the ruler (see table below for conversion into reaction time). Test student number 2 3-5 times (varying the time of dropping the ruler within the 5 second “drop-zone” so they cannot guess when the ruler will fall). Repeat and get student number 2 to switch hands.
  6. Swap positions so that student number 1 can test their reaction time.

Conversion Table (modified from Neuroscience for Kids):

Distance Time
2 in (~5 cm) 0.10 sec (100 ms)
4 in (~10 cm) 0.14 sec (140 ms)
6 in (~15 cm) 0.17 sec (170 ms)
8 in (~20 cm) 0.20 sec (200 ms)
10 in (~25.5 cm) 0.23 sec (230 ms)
12 in (~30.5 cm) 0.25 sec (250 ms)
17 in (~43 cm) 0.30 sec (300 ms)
24 in (~61 cm) 0.35 sec (350 ms)
31 in (~79 cm) 0.40 sec (400 ms)
39 in (~99 cm) 0.45 sec (450 ms)
48 in (~123 cm) 0.50 sec (500 ms)
69 in (~175 cm) 0.60 sec (600 ms)
  1. Explain that in order to catch the ruler a lot of messages have to be passed along different nerves:
  • The eye sees the ruler drop.
  • The eye sends a message to the visual cortex in the brain.
  • The visual cortex sends a message to the motor cortex in the brain.
  • The motor cortex sends a message to the spinal cord.
  • The spinal cord sends a message to the hand/finger muscle.
  • The finger muscle contracts to catch the ruler.

This happens almost instantaneously. How fast it actually happens is called the reaction time.

When comparing hands, students will usually find that their dominant hand is faster. Because the dominant hand is used more often every day, the neurons that carry messages between that hand and the brain are faster at transmitting electro-chemical signals. They are communicating along well-worn pathways. By running the same messages along the same pathway repeatedly, students can improve their motor skills.  The phrase “practice makes perfect” is scientifically accurate.


  • There are a lot of interesting questions you can ask about reaction time. Pick one or ask one of your own and design an experiment to find out the answer.
    • How did we know where the marks should go? Can you make a longer timer? Do you need to?
    • Do students who play sports or musical instruments have faster reaction times?
    • How does your reaction time change if you use your peripheral vision?
  • Make the experiment more interesting by substituting candy bars (or another long snack) for the rulers. The students with the quickest reaction time get to eat the candy bar.

Other Resources

University of Washington | Faculty of Education | Neuroscience for Kids