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Reaction time

FSN Sport Science – Episode 3 – Reaction Time – Steve Finley

In this episode Sports Science tests Ben Roethlisberger’s passing, Luc Robitaille’s defense, Jerry Rice and Chad Johnson off the line, Steve Finley’s ability to read pitches, NBA players.


REACTION TIME lab adapted from  http://physics.nmsu.edu/research/lab110g/html/AREACTION.html

It takes every person a time to react to any event.

For example, there is a small delay before you apply your brakes, after you see the stop lights of a car just in front of you go on.

This is called your reaction time.

We’ll measure our own reaction time, through an indirect measurement.

We can’t trust our own sense and hands to use a stop-watch quickly enough to get accurate, direct time measurements.

So we’ll measure something else, accurately.

Then we’ll use physics equations to work backwards, and find the time from that.

Review a kinematic equation:
What distance (d) will an object fall, due to the acceleration of gravity (g), in a given amount of time (t) ?

equation falling object

Using algebra, solve for ‘t’

d = distance dropped

g = 10 m/s2  (acceleration of gravity, on Earth.)

t = time of fall

In this experiment, you will measure the distance that an object (a ruler) falls

You’ll then calculate the time it fell, with this equation.

1. Hewitt, P.G. Conceptual Physics, 7th Ed., (Harper Collins College Publishers, 1993) pp 29-32.

2. Crummet, W. P. and Western, A. B., University Physics (Models and applications), advance copy, (Wm. C. Brown, Publishers, Dubuque, IA, 1994) pp 52-59.

3. Ostdick, V. S. and Bord, D. J., Inquiry Into Physics, 2nd Ed., (West Publishing Company, St. Paul, MN, 1991) pp 28, 34-37.

4. Serway, R., Principles of Physics, (Saunders College Publishing NY, 1992) pp 41-49.

5. Tipler, P.A., Physics, (Worth Publishers Inc., NY, 1976) pp 33-36.

6. Young, H.D., Physics, 8th Ed., (Addison Wesley, NY, 1992) pp 39, 44.


Another person

A ruler (longer than 25 cm)


Because it is impossible for you to accurately measure your reaction to something that you do yourself, you will have to work with somebody else.

Have this other person hold a ruler, lightly, between the fingers at the top.

Place your thumb and forefinger on either side of the ruler near the bottom (where the mark is). Do not, however, touch the ruler.

Have the other person release that ruler at an unpredictable time.

When you see the ruler start to fall (the event) pinch your fingers together, catching it. (Your reaction).

During the time you were reacting, the ruler was falling.

Use equation (2) to calculate the time the ruler falls. That time is your reaction time.

You will get a more accurate measure of reaction time if you do this several times and take an average of your results.

Do a few practice runs before starting to keep records. Be sure that your helper releases the ruler cleanly and at a time that is not predictable. You will get best results if you concentrate on seeing the ruler start to fall. If you let your mind wander, you will get an unusually high value of reaction time. Do not try to anticipate the drop of the ruler. If you do, you will get unusually low values.

Do not discard a value just because it is unusually high or low. If there is a reason to discard it, such as release anticipation, ruler sticking to fingers, or ruler not being cleanly caught, that trial can be discarded. However to discard data just because you do not like the results is contrary to good practice. If you have data points which you suspect (but cannot prove) are bad, the correct procedure is to take so many other data points that the unusual ones do not affect the final average.


If you want to do more experiments similar to this one, there are lots of variations that you can try.

Does the size of the ruler have an effect? Borrow a meter stick and repeat the experiments.

Do you get the same result in the morning when you are fresh as in the evening when you are tired?

You studied your eye-hand reaction time. Can you devise an experiment to test you ear-hand reaction time?

Try having your partner yell “bang” as the start button of your stop watch is pushed, then you push the stop button as soon as possible.

Outline a computer program that will measure eye-hand and ear-hand reaction time. What other reaction times might be measured?

If you try to find your reaction time under different conditions as indicated above, you will need to record an uncertainty in your results. The standard deviation (see Appendix 4) is a good measure of uncertainty to use here.

Learning standards

2016 Massachusetts Science and Technology/Engineering Standards
HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)
PS2.A Forces and motion. How can one predict an object’s continued motion, changes in motion, or stability?

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)
Introductory Physics. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.

1.1 Compare and contrast vector quantities (e.g., displacement, velocity, acceleration force, linear momentum) and scalar quantities (e.g., distance, speed, energy, mass, work)

1.2 Distinguish between displacement, distance, velocity, speed, and acceleration. Solve problems involving displacement, distance, velocity, speed, and constant acceleration.

1.3 Create and interpret graphs of 1-dimensional motion, such as position vs. time, distance vs. time, speed vs. time, velocity vs. time, and acceleration vs. time where acceleration is constant.

Physical Setting/Physics Core Curriculum (New York)
Key Idea 5: Energy and matter interact through forces that result in changes in motion

5.1 Explain and predict different patterns of motion of objects (e.g., linear and uniform
circular motion, velocity and acceleration, momentum and inertia).

i. construct and interpret graphs of position, velocity, or acceleration versus time
ii. determine and interpret slopes and areas of motion graphs
iii. determine the acceleration due to gravity near the surface of Earth

5.1e An object in free fall accelerates due to the force of gravity.* Friction and other
forces cause the actual motion of a falling object to deviate from its theoretical motion.


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