Introduction

Dominoes can be taped to a long flat base, such as a ruler, in a way that let's them be reset easily by tipping the ruler. An engineering challenge can then be created to make the speed of the domino fall propagate the fastest, the slowest, or even to travel from one end to the other reliably with the fewest dominoes. Go to the end of this page to see how this activity correlates with the engineering practices in the next generation science standards, NGSS.

Material

• Dominoes, a dozen
• A ruler with one flat side, or an equivalent piece of wood or plastic.
• Masking tape, 3/4" wide
• A second ruler
• A stopwatch
• Scissors (optional)

Assembly

Once you have completed a design, make a hinge out of masking tape and attach the first domino to the flat side of a ruler. Cutting the masking tape with scissors makes a better hinge than tearing it.

The hinge will allow the domino to fall over in one direction. Make a line of hinged dominoes so that when one falls over it will knock over its neighbor. Once you have the hinge in place put second piece of masking tape across the hinge at a right angle to help hold it down. For an example, see the image at the beginning of this activity.

To Do and Notice

Push over the first domino so that it falls and hits the second which knocks over the third etc. Observe the propagating wave of falling dominoes
To reset the dominoes simply lift the far end of the ruler and they will all return to their original position ready to be knocked down again. There is something inherently satisfying about knocking over dominoes
Measure the length that the falling dominoes cover, and measure the time it takes for them to cover this length. Calculate the speed of propagation of the falling dominoes
Speed = length/time. Don't forget to include units in all your measurements and calculations.

What’s Going On?

A falling domino converts gravitational potential energy change to kinetic energy. If the dominoes are spaced correctly one falling domino will deliver enough energy to the second domino to make it fall.
You will discover that the speed of propagation depends on the spacing of the dominoes By adjusting the spacing, the speed can be varied. Engineering challenges might be to make the fastest speed of domino fall, the slowest speed or even to make the dominoes fall closest to a target speed. If you use dominoes made of different density material or dominoes with different thicknesses this will also change the speed.

Math Root

Measure the domino speed several times and average your results.
How reliable is your design? What fraction of the time does it work correctly?

Going Further

You can also use this activity to ask the builders, "what is moving from one end of the ruler to the other?" Each domino merely falls over, no domino moves from one end to the other. Many people say that energy moves from one end to the other, this is correct, but we might ask "what kind of energy?" To which the answer is kinetic energy, the energy of motion. So at the base what moves is motion itself, each domino falls but the falling of the dominoes moves from one end to the other.
I usually summarize this by saying, "Motion moves".

Etc

Once you have built this project you can use it to model a nerve impulse. (practice number 2)

The first domino will not fall until it is pushed beyond a critical angle.
A nerve impulse will not be triggered until the nerve is excited beyond its firing threshold. (Nerves also occasionally fire spontaneously, just as the domino sometimes fall over without triggering, as when someone jiggles the table.)

Once the pulse of falling dominoes begins to propagate, it moves at a constant speed independent of the size of the starting push.
The speed of propagation of a nerve impulse is independent of the size of the triggering signal.

The dominoes cannot fall again until they are reset.
After a nerve impulse has propagated down the axon, there is a "refractory period" during which the nerve cannot fire again.

Engineering practices in the Next Generation Science Standards can be investigated using falling dominoes.

1. Defining Problems : Choose an engineering challenge using falling dominoes, for example to make the speed of propagation of the falling dominoes the fastest.
2. Developing and using models: The final project will have the dominoes taped to a ruler, but investigations can take place with dominoes set up on a flat surface.
3. Planning and Carrying out investigations: Use this rapidly variable model from practice number 2 to investigate how the spacing of the dominoes affects the speed.
4. Analyze and interpret data: Measure the time it takes the falling dominoes to cover a measured distance using a ruler and a stopwatch.
5. Using Mathematics and computational thinking: Use the measurements of practice 4 to calculate the speed of propagation of the falling dominoes
6. Design Solutions: Tape the dominoes to the ruler using the information you have obtained.
7. Engaging in Argument from evidence: Run the experiment and discuss how the predicted result agreed with or disagreed with your expectations.
8. Evaluate and Communicate information: Compare your solution with other engineering teams, and write a final report on your solution.

Engineering involves constraints, one constraint required in this activity is that the first domino must be mounted at the end of the ruler. A second constraint is that the final domino must project beyond the end of the ruler after it has fallen.