During the summer Olympics, people all over the world become enthralled by the feats of strength, stamina, and skill of the athletes competing in the games. Try these simple experiments to understand a little more of the science behind the sports.

Spinning Chair Project*

Have you ever watched an Olympic diver or gymnast twisting through the air? Did you notice how they often bring their legs and arms in close to their bodies and tuck their chins to their chests? It’s easy to observe the grace and beauty of their movements, but beyond the athletic ability and artistic expression, a more subtle element is at work—science.

What You Need:

  • Rotating office chair on wheels
  • Plenty of space
  • 2 dumbbells, cans of soup or other heavy objects of equal weight
  • A partner
  • Adult supervision

What you do:

  1. Make sure you have plenty of space around and sit in the chair.
  2. Arrange your feet so they’re off the ground.
  3. Stretch your arms out to your sides.
  4. Have your partner give your chair a light spin then quickly step away.
  5. Immediately pull your arms into your chest.
  6. Try the experiment again holding a weight (like dumbbells or cans of soup) in each hand and see what happens.

Be careful when you perform this experiment! If you get spinning too fast, the chair could tip over. And be extra cautious when you get up, since you’ll probably be dizzy!

What Happened:

You should have noticed that once you brought your arms in close to your body, you started spinning faster. This phenomenon is called the conservation of angular momentum. According to Newton’s first law of motion, also called the law of inertia, an object in motion will stay in motion until an outside force acts upon it. Conservation of angular momentum is the corresponding principle that applies to rotating or spinning objects.

When moving along a linear plane, momentum is determined by multiplying an object’s velocity (meters-per-second) and its mass (how much space it takes up). Angular momentum is determined by multiplying angular velocity and moment of inertia. Angular velocity is measured in degrees, or radians-per-second. Moment of inertia refers to how much mass the object has and how the mass is distributed around the rotational axis.

That’s why when you pulled your arms in, you were able to spin faster. You reduced your moment of inertia by redistributing your mass about the rotational axis. With your arms out, your mass was farther from the rotational axis and the moment of inertia was greater. With your arms in, your mass was closer to the rotational axis, and the moment of inertia was smaller. Because of conservation of angular momentum, there was still as much energy involved but each rotation required less energy to execute, so your spinning sped up. This is most clearly displayed by ice skaters in the winter Olympic Games, but divers and gymnasts demonstrate it as well. Watch how they tuck their knees into their chests and drop their chins down during flips and notice how the angular velocity changes.

*Adapted from Exploratorium.org

Science Links


Reaction Time Science Project

For Olympic runners and swimmers, a fraction of a second is often the difference between winning a gold medal or a bronze. Indeed, it’s the distance between winning any medal or returning home with nothing but hopes at another chance in four more years. And while its impact is most dramatic in running events, speed isn’t only a matter of crossing the finish line first. In sports, reaction time, the interval between stimulation and reaction, often determines who wins and who loses. Even more importantly, in real-life situations, like when driving a car, it can mean the difference between life and death. Measure your reaction time with the following project.

What You Need:

What you do:

  1. Have your partner sit or stand with their arm on the flat surface so their wrist extends beyond the edge.
  2. Hold the meter stick vertically above your partner’s hand, with the “0” end of the stick just above their thumb and forefinger, but not touching them.
  3. Instruct your partner to catch it as quickly as possible as soon as they see it begin to fall.
  4. Without warning your partner, drop the meter stick.
  5. Record how far it fell before your partner caught it. Consult the reaction time table to determine reaction time. Repeat at least two more times.
  6. Switch places with your partner and repeat.

What Happened:

In this experiment, your reaction time is how long it takes your eyes to tell your brain that the meter stick is falling and how long it takes your brain to tell your fingers to catch it. We can use the distance the meter stick fell before you caught it to figure out your reaction time. The following formula is the basis: d = ½ gt2.

In this formula, “d” equals the distance the object fell, “g” equals gravitational acceleration (9.8 m/s2), and “t” is the time the object was falling. To simplify the process, we’ve provided a reaction time table with the calculations already done.

Try it again with a dollar bill, only start with the bill halfway between the catcher’s thumb and pointer finger. If you’re really brave, you can up the ante and allow whoever catches the dollar bill to keep it. Unless someone anticipates the dollar bill being dropped, the 6-inch bill should fall completely through the catcher’s fingers before the typical human reaction time (about ¼ second) allows them to catch it.

For further study:

  • Talk about what sports depend on having a fast reaction time. How about real-life situations?
  • Try the experiment on a variety of people of different ages. Whose reaction time is faster? Boys or girls? Adults or kids?
  • Repeat the experiment, only this time, have the catcher whistling throughout. Did that make reaction time faster, slower, or the same?
  • Can you improve your reaction time by repeating the experiment several times daily? Practice for a week then test yourself again to see.