Gravity-Defying Water Tricks (8/3/16)
With the help of some objects found in your own house (and a little bit of science), you can use water to perform some fun experiments that appear to defy gravity!
- Zipper-seal plastic sandwich bag
- Sharpened pencils (color pencils work well, too!)
- A towel
- Ask an assistant to hold open a plastic bag with a zipper seal, while you fill it halfway with regular water.
- Tightly close the zipper seal.
- With one hand, hold the plastic bag from the top, or ask your assistant to hold it for you.
- Using the tip of a sharpened pencil, push the pencil into the zipper-seal bag just above the water line.
- Aim the pencil downward and push the tip of the pencil out through the other side of the bag BENEATH the water line!
- Be sure to not push the pencil all of the way through the plastic lining. It should stick out a bit on either side of the bag!
- Watch for any drips or sprays of water. Is the bag leaking?
- Repeat steps 4 through 7 with more pencils until your bag of water looks like a pencil-porcupine!
- Holding the bag over a sink or bathtub, slowly pull out just one of the pencils. Does the bag still hold water with an open hole in its side?
HOW DOES IT WORK:
Most zipper-seal bags are made from a polymer called Low-Density Polyethylene (or LDPE). It is affordable to make and easy to work with, it is very lightweight, waterproof, durable, and it is quite flexible. For these reasons, LDPE is one of the most common materials used for packaging.
The polymers that make up LDPE are long molecule chains that work together to create a waterproof barrier. When the sharpened tip of your pencil breaks through the plastic bag, the chains of LDPE polymers are pushed apart, but not broken. The flexible polymer chains squeeze tightly to the sides of the pencil to create a temporary watertight seal.
Once the pencil is removed, the polymers do not fully seal back together, and the water leaks out all over the place!
YOU CAN ALSO TRY:
- Repeat the experiment using more or less water. Does the amount of water in the bag make a difference? What would happen if you completely filled the zipper-seal bag with water?
- Repeat the experiment using different sizes or thicknesses of plastic bags. Freezer bags are made from thicker pieces of LDPE. Does the thickness of the bag make a difference? **It might be tricky to push a pencil through a gallon-sized bag, but give it a try!**
- Try using different types of pencils. Usually, color pencils are round, with smooth sides. On the other hand, writing pencils are usually hexagonal, with flat surfaces on their sides. Which type of pencil works better for this experiment? Why?
Bonus Experiment! Anti-Gravity Water Glass!
- A deck of playing cards or an index card made from heavy cardstock
- Drinking glass that is narrower than the paper card
- Kitchen sink
- A towel
- It is recommended that you practice this experiment outside, or over a large sink to minimize the many spills that you will probably make!
- Before you begin this experiment, double check that your playing card or index card is big enough to completely cover the mouth of the drinking glass. There shouldn’t be any gaps around the edge of the card!
- Fill the drinking glass almost to the top with regular water. It shouldn’t be totally full of water.
- Place the card over the mouth of the glass, making sure that it completely covers the glass!
- Lift the glass with one hand, and hold the card tightly against the rim of the glass with your other hand.
- Carefully turn the glass upside down, making sure to hold the card tightly against the glass rim at all times.
- Slowly take your hand away from the card, and the water should stay safely (and dryly) sealed inside of the upside-down drinking glass!
- Put your hand back on the card before turning the drinking glass to its upright position again!
- If you want to repeat the experiment, you may want to use a different card! If the card gets too wet, it can actually break apart when you turn the glass over!
HOW DOES IT WORK:
Even though you remove your hand from the card, there is still an invisible object pressing it against the mouth of the drinking glass…AIR!
Molecules of air in the atmosphere push against everything around them. This pushing force is called air pressure. Scientists can measure the exact amount of force created by the air around us (at sea level, air pressures is about 15 pounds of force for every square inch of an object’s surface area!).
At first, when you flip the drinking glass upside down, the pressure of the air pocket trapped inside of the glass is the same as the air pressure around the glass. You may notice, however, that a small amount of water will actually squeeze out between the glass and the card. This tiny change in water level causes the air pocket inside of the cup to expand a tiny amount (causing the air pressure inside of the cup to slightly drop).
When you remove your hand from the card, the outside air pressure is stronger than the air pressure inside of the cup, and the card is actually held in place by the air!
If the watertight seal between the card and the glass is broken at all, then air will rush into the glass and you’ll definitely be in need of a dry towel!
YOU CAN ALSO TRY:
- Repeat the experiment with a different container. You could try using a water bottle, a tea cup, even a flower vase! Does the size of the mouth of the container make a difference? **For really wide-mouthed containers, you can use a Styrofoam plate instead of the paper card!**
- Repeat the experiment; but before you flip the glass upright, try to tilt the glass slightly. How far can you tilt the glass before the seal breaks around the card?
- Repeat the experiment using a plastic or paper cup. Will the water still stay inside of the cup? Using a thumb tack, poke a small hole in the bottom of the plastic cup. Will the activity still work if air is allowed to sneak into the cup? Why or why not?