### Gravity is Optional Flying Cups and Curve Balls .. the Magnus Effect

In a vacuum, objects fall based on the law of gravity. The expected trajectory is known as a parabola. However, in the Earth's atmosphere, the presence of the air produces a drag (a slowing force) which changes the trajectory. If an object spins, then the air will provide a force at right angles to both the spin axis and the relative air velocity. (In physics, this is described by the cross product of two vectors.) This is what makes curve balls curve.

Generically, this is known as the Magnus Effect. It is used in all sports that use balls - ping pong, tennis, golf, baseball, and many others. It also affects bullets in flight - besides moving a bullet left or right, a crosswind will make the bullet hit either above or below the target .. depending on the wind direction and direction of bullet rotation.

In this section, we will play with straws and cups and learn how to make them fly.

It is always fun when playing with toys teaches you about real physics.

Straws | Styrofoam Cups | Additional Information

### Straws

We learned this trick at a juggling convention. (Jugglers are people who manipulate objects and love to throw things at each other. In general, they don't understand the concept of growing up. You should try it.)

When thrown overhand, the straw will spin so that the top comes toward you and the straw will "fly" away from you. When thrown underhand, it will fly back toward you.

### Styrofoam Cups

One day, we were showing an acquaintance (Steven Madewell, Interpretive Exhibits Coordinator with the Smithsonian's Lemelson Center) some of our toys. When we showed him the flying straws (above), he told us about styrofoam cups.

Basically, tape the bottoms of two styrofoam cups together and they will fly just like the straws in the previous section. (ooh, I have a new toy for the next juggling convention.)

Just like with the straws above, these fliers can be thrown by hand. When spun with the top of the cups coming toward you (thrown overhand), the cups will move away from you. When spun the other way (thrown underhand), they will return so that you can catch them.

To obtain a higher rate of spin, I wrap sewing thread around the cups and pull it as I throw them. Since sewing thread is somewhat difficult to hold, I wrap it around a stick. (Be creative, I simply picked up a stick in the yard. It should be about as long as your hand is wide.) I have found that the length of the thread should be determined by reaching as far as you can with one hand and placing the other hand next to your side. A little extra is used to wrap it around the stick. Once you are happy with the length (that's right, you should test it first), you can use a half hitch (a simple knot) to keep the string from unwinding. I also tie an overhand knot about an inch from the other end of the string. I find this very useful since the procedure is to lay the string over the cups and then to pull until I feel the knot. Then I know that I have reached the end of the string and should start winding. (I know that that does not make much sense, but after a few tries without the knot you will understand why this is important.)

Eventually, my video will go here.

In the video below, the Children's Museum of Houston explains how to Make Your Own Magnus Glider. Note that the kids use rubber bands to launch the fliers.