# Swiveling Science: Applying Physics to Hula-Hooping

A weighty wager from Science Buddies

Twisting physics: Spin homemade Hula-Hoops to learn about force and gravity--and have some summertime fun in the process! Image: George Retseck

Key concepts
Physics
Speed
Forces
Weight
Gravity
Geometry

Introduction
Have you spent time this summer spinning a Hula-Hoop around your waist or arm? Could you do it easily or was it difficult? Have you ever wondered how Hula-Hoops work or what makes them able to spin around a person's waist or arm—seeming to defy gravity? The answer can be explained by physics, which can help you determine what makes an effective Hula-Hoop. In this activity you'll get to create your own Hula-Hoops and investigate how their weights affect how they spin. Which do you think will spin better, a heavy hoop or a lighter one? Get ready to do some hula-hooping to find out!

Background
What makes a Hula-Hoop spin around a person's waist? It comes down to a combination of several forces at work. When the person inside of the hoop moves their body to propel the hoop around them, they're exerting an upward force (from their hips) and a turning force known as torque. Torque is a twisting, outward force that is needed to cause the hoop to spin. (More technically, torque is required to keep the hoop spinning because it's needed to keep the centripetal force going.) Another force involved in the Hula-Hooping process is friction. For example, if a ball is rolling along a flat surface, it eventually stops due to the friction created by its contact with the surface. Friction between the hoop and the hula-hooper's clothes and the air will slow the hoop's spin down. Friction, however, also helps to keep the Hula-Hoop up on the hooper's body while the force of the Hula-Hoop's weight pulls it down. (This downward force is due to gravity.) How do you think the hoop's weight affects how it spins?

Materials
• Tape measure
• Calculator
• PVC cutter (preferred), or a sharp cutter knife or some other type of cutter that can safely cut through polypipe
• Polypipe, which is hard black tubing usually used for irrigation (five-eighth, three-quarter or one-inch diameter, about 25 feet in length, although it normally comes in rolls of 100 feet). Available from a garden supply or home improvement store.
• Poly insert coupling or wooden dowel that fits snugly inside of the polypipe tubing. For example, if you are using three-quarter-inch tubing, you will want to use a three-quarter-inch poly insert coupling. This should also be available from a garden supply or home improvement store.
• Hair dryer
• Wide plastic tape such as duct tape
• One cup of sand
• Measuring cup
• Funnel with an opening that can fit into the polypipe or a thick sheet of paper and tape
• Bucket (optional)
• Timer or stopwatch
• Another helper to assist in testing the Hula-Hoops (optional)

Preparation
• Measure the height from the ground to somewhere between the hula-hooper's navel and the middle of their chest. This will be the height (or diameter) of the hoop.
• To figure out the length of polypipe you want to cut to make your Hula-Hoop, take the number you just measured and multiply it by pi (3.14). (The circumference of a circle equals pi times the circle's diameter.)
• Measure the length of polypipe needed and have an adult carefully cut the tubing at the right spot using a PVC cutter or sharp cutter knife.
• Use a poly insert coupling (or wooden dowel) to connect the ends of the tubing so they form a circle. If it's a tight fit, use the hair dryer to warm the tubing ends (one at a time) for about two minutes before inserting the coupling inside of the tubing. Insert the coupling in one end of the tubing until the coupling is about halfway inserted, then insert the other half of the coupling into the tubing's other free end. Push the tubing together until little (or no) coupling is visible, using the hair dryer to heat the tubing if it is difficult to insert the coupling.
• Put a short strip of wide plastic tape over the connection where the two ends of tubing meet.
• Follow this process to make a second Hula-Hoop that is the same size, but this time add one cup of sand to the tube (to increase its weight) before connecting the ends with the poly insert coupling. To add the sand, use a funnel to carefully pour the sand into the tubing. You may want to do this over a bucket and/or have a helper assist with this. (You should pour any spilled sand into the tubing.) If you do not have a funnel, you can make one by rolling a thick sheet of paper into a cone shape with a hole at the end, fastening it with tape. When you're done making both Hula-Hoops, how do they feel compared with one another?

Procedure
• Now you will do some hula-hooping with your homemade Hula-Hoops. If you need directions or tips on how to hula-hoop, you can watch an instructional video online, such as this one .
• Pick up the lighter two Hula-Hoops to try first.
• Have one person (yourself or a helper) hula-hoop with it around their waist while a helper starts timing the hula-hooper as soon as they reach a steady pace. Have the helper time the person for one minute and count how many full turns the hoop makes during that time. If the hooper cannot hoop through the full minute with the Hula-Hoop, try starting over again or try collecting data for only 30 seconds. How many times could the hula-hooper spin the lighter hoop in a minute? How well could the hooper spin it around? Does it seem awkward or does it spin well?
• Repeat this process with the heavier Hula-Hoop, again timing how many spins the hula-hooper can do in one minute. (Make sure the hooper doesn't change clothes while collecting data.) Did the hula-hooper spin the heavier hoop faster or slower than the lighter one? Why do you think this is? Did this hoop feel more or less awkward to spin, or about the same as the other hoop?
Which Hula-Hoop spun faster, the lighter or heavier one? Was the faster Hula-Hoop also the one that was the easiest to spin, once it got going?
• If you want, you can repeat this process a few more times for each Hula-Hoop. Are your results consistent?
• For even more data, the hula-hooper and helper can switch roles and repeat this process for both Hula-Hoops. Did the other person get similar results?
Extra: In this activity you compared Hula-Hoops that were the same size but had different weights. You could repeat this activity again, but this time make Hula-Hoops of different sizes. Tip: The smallest Hula-Hoop should have a diameter up to the hula-hooper's navel, and the largest should have a diameter to the middle of their chest. How does the size of the Hula-Hoop affect how it spins?
Extra: You could make a broader range of Hula-Hoops, such as by adding different amounts of sand and/or making several different sizes. Is there a relationship between speed, the weight and/or size of the Hula-Hoop? If so, what is it?
Extra: Try hula-hooping in different types of clothes. Do different garments affect the speed at which the Hula-Hoop rotates? Can you correlate your results to the force of friction?

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