A Raindrop Quiz

​Alexandra Siy
Science through the lens

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     Which lunch food has a shape that resembles a falling raindrop?

      a.      orange

      b.     potato chip

      c.      hot dog

      d.     hamburger bun

      e.      all of the above 

      f.       none of the above

               If you chose (f), you’re like most people who think raindrops are shaped like tears. 

               If you chose (e), you’re probably just hungry. 

               In either case you’re wrong. 

               That leaves us with lunch. Let’s start from the top.

               Choice (a), orange, is a sphere. Water droplets are spherical because water is cohesive, meaning it sticks to itself. The “skin” that holds the drop together is surface tension and the reason insects can walk on water.

               If you chose (a), you made a logical choice based on the properties of water, but you are wrong. Notice that you were not asked to identify the shape of a raindrop sitting on a leaf.  You were asked to identify the shape of a falling raindrop. (Always read questions carefully!)

               Moving down the list to (b), we encounter the potato chip. Potato chips come in many shapes, ranging from relatively flat to completely crumpled. Have you ever seen a raindrop that looks even a little bit like a potato chip? If you chose (b) you are wrong, but have a good sense of humor.

               Choice (c), hot dog, is an interesting option. Could a spherical drop of water morph into the cylindrical shape of a hot dog? After all, a hot dog is a cylinder with a hemisphere (half sphere) on each end. Could a water droplet in free fall separate itself into two hemispheres with a long drip of water in between? Although this is an imaginative idea, the laws of physics make it impossible.

               Choice (d), hamburger bun, is the only remaining choice, and is the correct answer. Here’s why:

               A raindrop is acted upon by three forces: gravity, buoyancy, and drag. Gravity is the force that pulls the drop toward the earth, while buoyancy of the surrounding air pushes it upward and keeps it from falling. When the force of gravity is greater than the force of buoyancy, the raindrop falls. The air around it creates drag, slowing the drop down to its maximum speed. In the process, the sphere is distorted into a shape that resembles a hamburger bun.

               Got it? Now, you may go to lunch.

Water forms spherical drops because of cohesion and surface tension. ©Alexandra Siy

Air pressure at the bottom of the falling drop is greater than at the top. (Image courtesy of NASA)

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Bugs bite, drink blood, and rob food from gardens and fields. They can even kill plants, animals, and, occasionally, people. Is bugging a crime? In her book, Bug Shots, Alexandra Siy compiles "rap sheets" on several of the major categories of bugs and takes a very close look at some of the types of insects in an engaging text.   For more information, click here. 
      Alex Siy is a member of iNK's Authors on Call and is available for classroom programs through Field Trip Zoom,  a terrific technology that requires only a computer, wifi, and a webcam.  Click here to find out more.

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