This post was written by educator Doug Levin.
It’s Saturday morning, the sun is up, and I’m sitting here with a cup of hot tea looking out onto my tidal creek.
The wind is blowing out of the west, but the tide is going out, emptying to the east. I know this through observation. The wind is pushing waves up the creek. At the same time, I see what appears to be just the top of a basketball floating out with the tide (it belongs to Chip, the son of my neighbor). Hold on a bit, I’ve got to go retrieve it for him…
…Okay, I’m back. The basketball was pretty deflated, which is why it floated so low in the water. I used my bike pump to inflate it and then I threw it back in the creek. It floated with more of the ball visible. It’ll be easier for Chip to find now. (Just kidding, I didn’t throw it back!)
Watching the ball in the creek, the waves show me the direction that the wind is blowing and the basketball the direction of water movement.
The basketball reminds me of a drogue, so now I’m thinking, “What do I have in my kitchen that I could use as a drogue?”
What’s a drogue, you ask? Well, a buoy floats and is anchored to the bottom. A drogue is an unanchored device that allows us to measure the direction and speed of water movement.
So, I’m in the kitchen and I need something that has a specific level of buoyancy. If it’s positively buoyant it will float; if it’s negatively buoyant it will sink; and if it’s neutrally buoyant, it will stay in the water column, neither sinking nor floating.
So, I’m searching my kitchen for something that is slightly positively buoyant. It should float just enough for me to see it and watch its movement. Like any good scientist, I want to test my materials. Here’s what I test and how it behaves when I put it into my kitchen sink filled with warm water:
- a dry tea bag floats: positively buoyant
- a wet tea bag sinks slightly: negatively buoyant
- a Brasso-brand metal cleaner container sinks: negatively buoyant
- a full can of dog food sinks: negatively buoyant
- just the top of full can of soda is visible: slightly positively buoyant
- a new plastic jar of peanut butter sinks: negatively buoyant (who knew?)
- just the top of a new container of dish soap is visible: slightly positively buoyant
- just the top of an orange is visible: slightly positively buoyant
The orange is a perfect drogue: It sits low in the water, it will not be affected by wind, and it will be easily seen.
I go to the creek and throw it in. With the wind blowing from the west, the orange starts moving to the east, in the opposite direction from where I’ve thrown it. My orange “drogue” confirms that the tide is going out.
I retrieve the orange for my breakfast.
You can conduct this simple drogue experiment with students in the classroom or with your own children at home.
- Fill a basin with water (1 foot deep is good) and grab 5-10 common items to test for buoyancy.
- Have students predict which items they think will be (a) positively buoyant, which items will be (b) negatively buoyant, and which items will be (c) neutrally buoyant. (With younger students you might want to call these conditions (a) float (b) sink and (c) dip).
- Ask students to identify specific characteristics of the objects they think will make them sink or float (e.g. weight, shape, material, absorbency, etc).
Interested in pursuing the idea of buoyancy and pressure? Try our “Engineering Pressure” lesson plan, which students investigate pressure and use the engineering process to design, model, test, document, evaluate, and re-design a submersible vehicle.
Doug Levin is the Deputy Director and Chief Innovation Officer for the Center for Environment and Society at Washington College in Chestertown, Maryland, and is an expert in underwater exploration technology, as well as designing fun programs that teach complex engineering concepts.