Science | Science Experiment of the Week 2 | 227 - Boomerangs

Construct your own working paper boomerangs with index cards. The dynamics are a bit different with the paper ones, but they are still lots of fun.

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This Week's Experiment - #227 Boomerangs

These experiments are from Robert Krampf - The Happy Scientist

This week's experiment comes from conversation I had with Dean Bolton and Doug Hullander. They are both talented science educators that live in the Oak Ridge area and I always look forward to chatting with them. We got into a discussion that ranged from gyroscopes to bicycles. The conversation led us into talking about boomerangs, so of course we had to make some paper ones to play with. The dynamics are a bit different with the paper ones, but they are still lots of fun. To make your own, you will need:

an index card or stiff cardboard about 4 inches by 6 inches
scissors
a pen or pencil

You want to cut a boomerang shape from the index card. If you are not familiar with the shape, you want a broad "V" shape. Measure about 1/2 inch up from the lower, left corner of the card. Use the pen to draw a line from this point to the center of the upper edge. Then do the same thing on the other edge, making a line from about 1/2 above the lower, right corner of the card to the same point at the center of the upper edge. Use the scissors to cut along these lines.

Again starting at the lower, left corner, measure about 1/2 inch towards the center and make a mark with the pen. Measure about 1 inch down from the point that is left at the upper, center edge. Draw a line to connect these two points, and again, do the same thing from the lower, right corner. Cut along these lines and you should have a broad, V shaped piece of card, which is about 1 inch thick. This will be your boomerang.

Hold your left hand out flat in front of you, with your palm up. Lay the boomerang on your palm, so that the center point is pointing away from you and the right end hangs at least an inch over the right side of your hand. Tilt your hand upwards, so that your fingers are pointing upwards at a steep angle.

Now we are ready to launch your boomerang. Stick the index finger of your right hand straight out. You are going to slide your index finger quickly along the right side of your left hand, so that it hits the part of the boomerang that is sticking out. This will knock the boomerang hard enough to cause it to fly off of your hand. Because you are hitting it on one end, it will spin as it flies off.

Watch carefully. If you do it correctly, the boomerang should fly upwards and away from you for a few inches and then it should come back towards you. It may not make it all the way back the first time, but with some practice, it will. The more you practice, the better you will get at this, although you will have to make new boomerangs periodically. As soon as they begin to get bent, they don't work as well.

Why does this boomerang come back? Part of the answer has to do with air resistance. Hold another index card so that it is flat out in front of you. Release it and watch as it falls to the ground. Does if fall in a straight line? No, probably not. It has to push air out of the way to fall, and it tends to flutter from one side to the other. Pick up the card and drop it again, but this time hold it by one edge, so that the card hangs straight down. This time when you drop it, the edge slices through the air and the card falls much straighter. It is easier for the card to move through the air edge first.

The other part of the explanation has to do with a device called a gyroscope. If you have ever played with one, you know that once you get the wheel of the gyroscope spinning, it is difficult to tilt it from side to side. The same thing works for bicycle wheels. It is much easier to stay upright on a bicycle while it is moving than when it is sitting still. When the wheels are spinning, the gyroscopic action resists tilting to one side or the other and you stay upright.

The spinning boomerang reacts the same way. It resists tilting while it is spinning. Your hand is angled upwards, the spinning boomerang tries to stay at that angle, and it is easier for the card to move through the air edge first. This causes the boomerang to move upwards as it moves away from your hand. As it moves upwards, gravity is pulling downwards on it, so it slows and eventually begins to fall back down. Once again, the gyroscopic action of the spinning boomerang keeps it tilted, and again, it is easier for the card to move through the air edge first. This causes the boomerang to slide back down the same sloped pathway that it moved up, bringing it back to you. Think of it as almost like rolling a ball up a slope and then having it roll back down to you.

If it does not work the first time, keep trying. If the boomerang tends to droop at the ends, try using stiffer paper. If it does not fly well, try cutting a boomerang that is a bit wider. With practice, you will soon be boomeranging like an expert. Be sure to clean up your scrap paper when you are done, but be careful when you throw it away, just in case some of it comes back.

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