The Science of Tsunamis

[deleted-815738]

My daughter completed "The Science Behind Tsunamis: Study the Effect of Water Depth on Wave Velocity," experiment from the Science Buddies site. She is getting feedback that the experiment had two variables, not only the depth of water, but the height from which the board was dropped. The procedure did not suggest moving the drop height when the water depth is changed. So, of course the drop height was different with each water depth. She has been told to put a note about this as being an error in the experiment. Any thoughts on this?

[LeungWilley]

Hi rmack,
This is just my 2 cents. I am not sure that I agree with the feedback.

According to the procedure, the wood block is dropped from the same spot (bottom of the wood block lined up to the marked line). So, in really simplified terms, the potential energy (m*g*h) for each drop is the same as the height (relative to the table) is the same. The point / height of impact changes with the water depth but that has little effects on the energy transfer (potential to kinetic) from the wood to the water. (Yes, you can argue that depending on the final resting position of the wood, the remaining potential energy could be different at the different water depth. However, based on the dimension of the box, this should be negligible.)

At any rate, sorry for the rambling.

Thanks!
Willey

[deleted-815738]

Thank you for the reply! I understand some of what you said. The potential energy is the same for each drop since it's being dropped from the same place each time. What I don't understand is how the point/height of impact doesn't change the energy transfer. You said it has little effect. Can you elaborate? I don't want my daughter to be hung-out-to-dry by judges at the science fair.

[LeungWilley]

Hi rmack,
Sure, let's give this a try. Picture the moment of impact at the water: as the wood block hits the water, water is displaced with a certain velocity (so energy is being transferred). The wood block continues to "sink" until it reaches the bottom of tank (we can call this the "0 potential energy location".) At this point, the potential energy is now fully transferred to the water. Please note that this is probably not a perfect 1:1 transfer.

At any rate, my thinking is that this sequence of action is the same for all three water depth that was listed in the experiment (1, 2, and 3cm). Now, while it is a possibility that buoyant force could come into play at the higher water depth, I don't think that will be the case given the minimum dimension of the block vs. the listed depths.

If you and your daughter have time, it may be very interesting to capture a slow motion video of this sequence. (Most cell phone camera should be able to capture this.)

I hope that make sense.
Good Luck!
Willey

[deleted-815738]

So is it really coming down to the fact that the energy transfer is complete at all depths, and the depth doesn't vary enough to change the outcome by a measurable amount? And secondly, if she had raised the board, each time, that would have altered the potential energy of the board (increased it), that being another variable. I thought the experiment was great, and she has a graph of data that clearly proves the change in wave speed with the change in water depth.

[LeungWilley]

Hi rmack,
Yes, I would agree with your statement with a slight modification. "The mechanism for energy transfer is the same for all depth and variation in depth is not significant enough that other forces would influence the results significantly." (Or to put it another way / To counter the original feedback about the height being a another variable, the energy transfer is not just at the point of impact but is carried through the entire sequence of displacement and velocity.)

And yes on the second point, increasing the height each time would change the potential energy.

Good Luck!
Willey