Waves in Slow Motion
Have you ever noticed seagulls bobbing up and down on the ocean while surfers paddle forward to catch a wave? Maybe you remember floating on a lake, going up and down as a wave passed by, or the feeling of being swept ashore by a breaking wave. Water waves are fascinating. They come in all sizes, from a tiny ripple to monster waves that are ten meters high. You have seen them, but do you know what drives them, how they move across oceans and seas?
In this activity, you will bring the ocean home and make waves in a bottle. Sharpen your observation skills and find out why some waves are slow and others are fast.
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
Wind causes most of the waves on seas and oceans. This is how it works. Wind causes water to pile up above the undisturbed water level. The water under the crest (or peak) of the wave feels the extra water pushing down on it. The extra weight then pushes water out from under the peak of the wave. The moving water overshoots and creates a peak in a new place, and so the wave moves on.
Oceans and seas are layers of water under a layer of air, the atmosphere. Because air is much less dense than water (it weighs about one thousand times less than the same quantity of water), the air has little influence on the wave. So in order to understand how waves work, you will add a layer of oil on top of the water. Oil is less dense than water, but denser than air. Curious about how this affects the wave on the water surface?
Warning: You might spill water or oil while preparing your bottles. Choose your work space accordingly.
Extra: Open the bottles, put a little piece of wax inside both bottles and close them again. Because wax is denser than oil, but less dense than water, it will float on water but sink in oil. You might need to wait a little while for the wax to sink through the oil until it reaches its place on top of the water. Think of the wax as a seagull, a boat or a surfer floating on top of the water. Create a wave like you did before, but now, watch how the wax moves. Does it move back and forth or rather, up and down with the wave? Can you explain your observations?
Extra: If you have a stopwatch, you can measure the period of the wave. The period is the time it takes for one wave to pass by, from crest to trough and back to crest. Looking back at the observations you made earlier, do you think the period is the same for waves with air on top and waves with oil on top? Use your stopwatch to time how long it takes for your wax object to start at the top of a crest, go down into a trough and back up to the top of a crest. This time measured is exactly the period of the wave. Is the period the same for waves with air on top and waves with oil on top?
Extra: The frequency of a wave is how many waves pass by in a second. Looking back at the observations you made earlier, do you think the frequency is the same for waves with air on top and waves with oil on top? Can you find a way to measure or calculate the frequency of the waves you created?
Observations and Results
Did you notice that water with oil on top produces surprisingly slow waves?
A wave moves because the extra weight of the liquid in the wave's peak pushes water from under the peak to places where the water is shallower. This happens in both bottles.
For the bottle with water and air, both the water and the air push on the deeper layers of water. Air is about one thousand times less dense than water, so when a wave passes by, deeper layers of water mainly feel the weight of all the water piled up in the wave. The difference in weight felt under the crest and under the trough (or lowest point) is the wave’s driving force. In this scenario, the big difference in weight results in a fast-moving wave.
Similarly, for the bottle with water and oil, water and oil push on the deeper layers of water. Just like air, oil is less dense than water (about two thirds as dense), so an amount of oil weighs less than the same amount of water. Thus, as water piles up in a wave, more water and less oil will weigh down on a spot underneath the wave peak. At the shallowest point of a wave, however, more oil and less water weighs down on the water below. As a result, the difference in weight of the liquids at a point under the crest and a point under the trough (or lowest point of the wave), is much smaller in this scenario, so the driving force of the wave is much smaller. It still forces water to move from underneath the wave peak. In addition, the water and the heavier-than-air oil both have to be moved, so the liquids flow much more slowly than when air flows on top of water. What you observe is a wave on the water surface that travels much more slowly.
In the extra activity, you probably observed that the wax mainly moved up and down while the wave moved forward and backward. This is because in a wave, it is the water underneath the wave that moves horizontally; the water on the surface moves up and down.
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Sabine De Brabandere, PhD, Science Buddies
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Wave, pressure, density
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