How we did it:
- plastic bottle - small, with cap
- push pin
- paper clips (2)
- rubber bands (3)
- popsicle stick
- pennies (2)
- hot glue / hot glue gun
- container lid
- heat gun - or candle
Using the push pin, make a hole in the center of the bottom of the bottle.
Straighten one paper clip and push it through the hole. Then, using the pencil, bend the end of the paper clip inside the bottle so that it forms a hook (you may need an adult to assist you). Secure the end of the paper clip on the outside of the bottle.
Note: To secure the end of the paper clip, we made a second hole in the bottom of the bottle and pushed the paper clip back into the bottle.
Draw a four blade propeller onto a piece of paper and cut it out. This will be your template.
Using your template, trace the propeller onto the yogurt container lid.
Cut out the propeller.
Use the heat gun to make the plastic more pliable. Then, bend each blade of the propeller so that the pointy side is slightly raised.
Using the push pin, make a hole in the center of the propeller and the center of the bottle cap.
Straighten the second paper clip except for one hook. Then, push it through the hole in the cap so that the hook is on the underside of the cap.
Hook one end of a rubber band onto it and hook the other end of the rubber band onto the hook inside the bottle. Then, close the cap.
Slide the propeller onto the paper clip so that it is lies flush with the top of the bottle. Then, bend the paper clip to secure it in place.
Hot glue the pennies to either end of the popsicle stick.
Using the last 2 rubber bands, strap the popsicle stick to the bottle near the bottom. This will be your submarine’s fins.
Now you are ready to experiment with your submarine! Fill the basin with water, wind up your submarine, place it in the water, and let it go! Allowing more or less water into the bottle will determine whether your submarine floats to the top of the tub or sinks to the bottom. Try to get it to float in the middle of the tub!
What's going on?
Submarines travel underwater thanks to two basic principles. First, the submarine needs to be able to control its buoyancy, or how much it floats (or sinks) in the water. Submarines can change their buoyancy using ballast tanks, which can be filled with air and water in different ratios. If there's a lot of water in the ballast tanks, the submarine will sink, while if there's a lot of air in the ballast tanks, the submarine will rise. Try playing around with the amount of water versus air in your water bottle submarine — a certain amount should give your sub the right buoyancy to float just beneath the surface of the water.
The second principle of submarine travel rests on Newton's third law, which states that for every action, there is a reaction. In the case of submarines, a propellor with angled blades spins, pushing water behind the submarine (the action). In response to this backwards movement of water, the submarine is pushed forwards (the reaction). The propellor on your submarine is spun by a twisted rubber band. The more that the rubber band is twisted, the more the rubber band will try to untwist and relax into its original shape. When you let go, this untwisting spins the propellor, pushing water back and driving the submarine forwards.
To stay upright while moving around underwater, submarines also use a variety of fins, like the one on your water bottle submarine. Real submarines can move their fins around, which, along with the ballast tanks and propellor, can enable motion in nearly any direction.