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In the last of the Physics Project Lab blog posts, Paul Gluck, co-author of Physics Project Lab, describes how to create and investigate the domino effect…
Many dominoes may be stacked in a row separated by a fixed distance, in all sorts of interesting formations. A slight push to the first domino in the row results in the falling of the whole stack. This is the domino effect, a term also used in figuratively in a political context.
You can use this amusing phenomenon to carry out a little project in physics. Instead of dominoes it’s preferable to use units that are uniformly smooth on both sides, say for example building blocks for kids. Chuildren’s building blocks usually come in sets of 100, 200 or 280 blocks.
The blocks are stacked in a perfect straight line, absolutely uniformly spaced. To ensure this, lay them along the extended metal strip of a builder’s ruler several meters long, fixed at both ends. A non polished wooden floor is a suitable surface, since its roughness is enough to prevent any sliding of the blocks while falling.
What is interesting to measure and correlate in your experimentation? You want to measure the speed of the pulse when the first block is given a reproducibly slight push. In other words, you must measure the total length of the stack, as well as the time between the beginning of the fall of the first block and the fall of the last one. The speed will then be the total distance divided by the time elapsed.
Domino Rally, by mikeyp2000. CC-BY-NC-2.0 via Flickr.
There are several questions you can ask and investigate. First, how does the spacing between the blocks affect the pulse speed? Second, for the same spacing, how do the pulse speeds compare between two cases: the first, with the regular blocks, and the second when you double the height of each block (by sticking two blocks on top of each other to form a single block)? Third, for large numbers of units N in the stack, does the speed depend on the number of units (say when N = 100 and when N = 200)? Finally, does the speed vary for small numbers of units in the stack, say for values between 5 and 15?
For fair comparison between the various cases, you must devise a way to give the slight initial push reproducibly. One way you can arrange this is by releasing a pendulum above the first block and releasing it from a fixed distance so that at the end of its swing the bob just touches the first block, causing it to fall.
For time measurements you need a stopwatch. Be aware that you have a reaction time between when you perceive any event and the pressing of the stopwatch – this can be anything from 0.1 to 0.3 seconds. So repeat each measurement a number of times and take the average. If you have access to two photogates in a physics lab, you can devise a more accurate way of measuring the pulse speed. Actuate the first one by the beginning of the fall of the first block, the second one by the fall of the last one. Couple the two photogates by a circuit that triggers measuring the time when the first brick starts to fall and stops measuring it when the second block falls. You can also video the whole event and analyze the clip frame-by-frame to calculate times.
Happy tinkering!
We hope you have enjoyed the Physics Project Lab series. Have you tried this experiment or any of the other experiments at home? Tell us how it went to get the chance to win a free copy of ‘Physics Project Lab’. We’ll pick our favourite descriptions on 9th January.
Over the next few weeks, Paul Gluck, co-author of Physics Project Lab, will be describing how to conduct various different Physics experiments. In his second post, Paul explains how to build your own drinking bird and study its behaviour in varying ways:
You may have seen the drinking bird toy in action. It dips its beak into a full glass of water in front of it, after which it swings to and fro for a while, returns to drink some more, and so on, seemingly forever. You can buy one on the internet for a few dollars, and perform with it a fascinating physics project.
But how does it work?
A dyed volatile liquid partially fills a tube fitted with glass bulbs at both ends. The lower end of the tube dips into the liquid in the bottom bulb, the body. The upper bulb, the head, holds a beak which serves two functions. First, it shifts the center of mass forward. Secondly, when the bird is horizontal its head dips into a beaker of liquid (usually water), so that the felt covering soaks up some of the liquid. As the moisture in the felt evaporates it cools the top bulb, and some of the vapor within it condenses, thereby reducing the vapor pressure of the internal liquid below that in the bottom bulb. As a result, liquid is forced upward into the head, moving the center of mass forward. The top-heavy bird tips forward and the beak dips into the water. As the bird tips forward, the bottom end of the tube rises above the liquid surface in the bulb; vapor can bubble up from the bottom end of the tube to the top, displacing some liquid in the head, making it flow back to the bottom. The weight of the liquid in the bulb will restore the device to the vertical position, and so on, repeating the cycle of motion. The liquid within is warmed and cooled in each cycle. The cycle is maintained as long as there is water to wet the beak.
‘A drinking bird’, image provided by Paul Gluck and used with permission
The rate of evaporation from the beak depends on the temperature and humidity of the surroundings. These parameters will influence the period of the motion. Forced convection will strongly enhance the evaporation and affect the period. Such enhancement will also be created by the air flow caused by the swinging motion of the bird.
Here are some suggestions for studying the behaviour of the swinging bird, at various degrees of sophistication.
Measure the period of motion of the bird and the evaporation rate, and relate the two to each other. You can do this also when water in the beaker is replaced by another liquid, say alcohol. To measure the evaporation rate the bird may be placed on a sensitive electronic balance, accurate to 0.001 g. A few drops of the external liquid may be applied to the felt of the head by a pipette. Measure the time variation of the mass of this liquid, and that of the period of motion, without replenishing the liquid when the bird bows into its horizontal position. Allow for the time spent in the horizontal position. Establish experimentally the time range for which the evaporation may be taken as constant.
Explore how forced convection, say from a small fan directed at the head, changes the rate of evaporation, and thereby the period of the motion.
The effects of humidity on the period may be observed as follows: build a transparent plexiglass container with a small opening. Place the bird inside. Vary the internal humidity by injecting controlled amounts of fine spray into the enclosed space. You can do this by using the atomizer of a perfume bottle.
By taking a video of the motion and analyzing it frame-by-frame using a frame grabber, measure the angle of inclination of the bird to the vertical as a function of time.
Do away altogether with the beaker of liquid in front of the bird and show that all it needs for oscillatory motion is the presence of a difference of temperature between the bottom and the top, a temperature gradient. To do this, paint the lower bulb and the tube black, and shine a flood lamp on them at controlled distances, while shielding the head, so as to create a temperature gradient between head and body. Such heating increases the vapor pressure within, causing liquid to be forced up into the head and making the toy dip, just as for the cooling of the head by evaporation. It will then be interesting to study how the time elapsed before the first swing and the period of motion are related to the effective surface being illuminated (how would you measure that?), and to the effective energy supplied to the bird which itself will depend on the lamp’s distance from the bird
There are many more topics that can be investigated. As one example, you could follow the time dependence of the head and stem temperatures in each cycle by means of tiny thermocouples, correlating these with the angular motion of the bird. Heat enters the tube and is transported to the head, and this will be reflected in a steady state temperature difference between the two. Both head and tube temperatures may vary during a cycle, and these variations can then be related to heat transfer from the surroundings and evaporation enhancement due to the convection generated by the swinging motion. But for this, and other more advanced topics, you would have to have access to a good physics laboratory, obtain guidance from a physicist, and be willing to learn some heat and thermodynamics as well as the mechanics of rotational motion, in addition to investing more time in the project.
Have you tried this experiment at home? Tell us how it went to get the chance to win a free copy of the Physics Project Lab book! We’ll pick our favourite descriptions on 9th January.
Featured image credit: ‘Drinking bird photo’ by Christopher Zurcher, CC-by-2.0, via Flickr
