The Idea
An object is barely floating in a bottle. In this project, you control whether it floats or sinks, just by applying pressure on the side of the bottle.
What You Need
- 2-liter plastic soda bottle with a reclosable cap
- water
- 1 medicine dropper
- a small object such as a paper clip to fine-tune the weight of the medicine dropper
Method
- Partially fill the medicine dropper with just enough water to establish neutral buoyancy, which is a condition where the medicine dropper does not sink, but also does not float in water. Test and adjust until the right amount of water is in the medicine dropper. Use the paper clip to increase the weight of the medicine dropper.
- Put the medicine dropper in the soda bottle.
- Fill the bottle to the top with water.
- Cap the bottle securely.
- Observe what happens as you apply pressure to the sides of the bottle. What happens as you relieve the pressure?
Expected Results
When the side of the bottle is squeezed, the diver descends.
When the pressure is released, the diver returns to the surface.
Why It Works
Pressure on the side of the bottle is transmitted to the medicine dropper. The pressure reduces the volume of the medicine dropper. The buoyant force depends on the size of the medicine dropper. A smaller-volume medicine dropper has a smaller buoyant force and sinks.
With the pressure reduced, the volume of the medicine dropper increases, leading to greater buoyant force. Greater buoyant force causes the medicine dropper to rise.
Neutral buoyancy occurs when the force of gravity equals the buoyant force resulting in equilibrium in the vertical direction. This is shown for a SCUBA diver in Figure 39-2. A diver would add weight to a belt as you did above with the paper clip to fine-tune the balance of forces.
Other Things to Try
Once you get the hang of this, you can put a "treasure" in the form of a small weight with a hook at the bottom of your bottle. You can then have your diver go down and try to recover the treasure.
The Point
Buoyancy depends on the volume of a submersed object. Pressure exerted externally reduces the volume of a submersed object, which, in turn, reduces the buoyant force.
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