Fun and easy science experiments for kids and adults.

Plastic bag kite

Build i simple kite from a plastic bag. An experiment about air pressure and how kites fly.

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Video

Materials

1 normal-sized plastic bag from the grocery store, or other piece of plastic that is at least 30 x 30 cm (12 x 12 in)
Two 30 cm (12 in) long, thin, wooden sticks (such as wooden plant sticks, skewers or bamboo sticks)
1 ruler
1 pen
1 pair of scissors
Tape
20 m (65 ft) of polyester sewing thread (or other lightweight and strong string)
1 piece of wood (or other object that you can wind string around)

Environment!

Use a plastic bag that would have been trash otherwise or one that is made of environmentally friendly plastic. Recycle it when done.

Step 1

Draw a mark in the middle of one wooden stick. Draw a mark a quarter in on the other wooden stick.

Step 2

Place the wooden sticks on top of the plastic bag, forming a cross. They should cross each other at the markings.

Step 3

Draw a dot where each wooden stick ends. Use a ruler to connect the dots.

Step 4

Cut along the line to get a diamond-shaped piece of plastic.

Step 5

Place the wooden sticks on the plastic again. Tape the ends of the wooden sticks to the plastic. Take a long piece of tape, place it across the wooden stick and fold the edges of the tape around to the other side of the plastic.

Step 6

Cut out an approximately 4 cm (1 1/2 in) wide and 2 m (7 ft) long ribbon from the plastic bag. Create this by cutting out shorter pieces, which you then tie together. The knots then become nice "bows" on the ribbon.

Step 7

Tie the ribbon around the wooden stick near the lower tip of the kite.

Step 8

Cut and wrap about 20 m (65 ft) of string around the piece of wood.

Step 9

Use the pen to make a small hole in the plastic, just underneath where the wooden sticks cross. Insert the free end of the string there, from underneath. Then tie the string around the cross where the two wooden sticks meet.

Step 10

Go out and test the kite. If it's not windy, you can run towards the wind and slowly release the string.

Short explanation

A kite is a sail that is lifted by the wind and is held back with a string. The kite flies by air pushing on the underside of the kite.

Long explanation

When the wind blows or when you run with the kite, a lot of air is gathered underneath the kite. This means that many air molecules are within a small area and push on each other and what is around them. Therefore, the kite is pushed away by the air.

Some lift is also generated by the air above the kite moving faster than the more stagnant air underneath the kite. Air that moves quickly puts less pressure on the surroundings. Therefore, the difference in pressure between above and underneath the kite becomes even greater. This phenomenon, that the faster air moves over an object the less the air pushes on the object (the air pressure is lower), is called Bernouille's principle.

Since the kite has a larger area below the attachment point of the string than above, the air pushes more there. This causes the kite to be angled upwards. Then the air pushes the kite backwards/upwards. But since you are holding the kite with a string, the kite doesn't move so much backwards, but instead mostly straight up. However, in your hand you can feel the force that arises when the air actually pushes the kite backwards/upwards.

The tail of the kite is not only meant to look good, but has an important function. It stabilizes the kite. When the kite wiggles in one direction, the tail wiggles in the other direction, and counteracts the kite's movement. However, it must be long enough to have any effect, but not too long either, because then it no longer swings rhythmically with the kite. A rule of thumb is that the tail should be seven times as long as the kite.

The weight of the tail also plays an important role. The heavier the tail, the more the kite is kept upright, and the better it gets at catching the wind. However, stronger winds are then required for it to rise high, beacuse it's heavier. Therefore, you should not use sticks that are too heavy or a string that is too heavy.

Test and improve

You have now built your first prototype. There are probably improvements to be made. In order to keep working on your design, try answering any of the following questions.

What happens if you run faster with the kite?
What happens to the kite when the wind changes direction?
What happens if you make a bigger or smaller kite (but with the same proportions)?
What happens if you change the proportions of the kite, for example, make it wider?
What happens if you tie something to the tail?
What happens if you use a lighter string or lighter sticks?

Variations

There are many different ways to build kites. The website My Best Kite contains good step-by-step instructions of different models.

An awesome project is to attach a video camera to the kite and film some views from the air.

A more environmentally friendly altenative is to use paper instead of a plastic bag. This paper kite doesn't fly quite as well as one made of plastic, but still good enough.