Physics Secrets for Hula Hooping

1. Measure the height from the ground to somewhere between the hula hooper's belly button and the middle of their chest. This will be the height (or diameter) of the hula hoop.
    
2. 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 π (or 3.14). (The circumference of a circle equals π times the circle's diameter.)
    
3. Measure the length of polypipe needed and have an adult carefully cut the tubing at the right spot using a PVC cutter or sharp utility knife.
    
4. Use a poly insert coupling (or wooden dowel) to connect the ends of the tubing so they form a circle. If it is a tight fit, use a hairdryer 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 hairdryer to heat the tubing if it is ever difficult to insert the coupling.

5. Put a short strip of wide plastic tape over the connection where the two ends of tubing meet.

6. Follow this process to make a second hula hoop that is the same size, but this time add 1 cup of sand to the hula hoop (to increase its mass) 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 pour any spilled sand into the tubing) and/or have a helper assist with this. 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 and fastening it with tape.

7. When you are done making both hula hoops, hold each in your hands.
   How do the hula hoops feel compared to each other?
    
8. If you want, you can decorate your hula hoops with colorful tape, paint, or anything else you can think of.
    
9. Now you will do some hula hooping with your homemade hula hoops. Pick up the lighter hula hoop to try first. Have one person 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 hula hooper spin it around? Does it seem awkward or does it spin well?

10. Repeat this process with the heavier hula hoop, again timing how many spins the hula hooper can do in one minute. Make sure the hula hooper does not 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?
     
11. If you want, you can repeat this process a few more times for each hula hoop. Are your results consistent?
     
12. If you want, the hula hooper and helper can switch roles and repeat this process for both hula hoops.
    Did the other person get similar results?

What Happened?

The hula hoop with sand added to it should have clearly felt heavier than the hoop that had no sand in it. Because of its greater mass, the heavier hoop was being pulled down more than the lighter hoop was when they were spun around in the air. The hula hooper probably felt like they needed to work harder to keep the heavier hoop up and spinning, and it might have even been difficult to keep it up around their waist for more than 30 seconds. This is because when the same force, or push, applied to a lighter object is applied to a more massive object, that force will cause the more massive object to change its motion less. As a result, more effort is needed to keep the heavier hoop going. The heavier hoop probably spun much slower than the lighter hoop. For example, one of our hula hoopers clocked in at 60 to 70 turns per minute for the heavier hoop compared to 100 to 120 turns per minute for the lighter hoop, although there can be a lot of variability depending on the hula hooper and the hoops.

Digging Deeper

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 are exerting an upward force (from their hips) and a turning force known as torque. Torque is a twisting, outward force that is basically needed to cause the hoop to spin. (More technically, torque is needed to keep the hoop spinning because it is 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 friction. Friction between the hoop and the hula hooper's clothes and the air will slow the hoop's spinning down. However, friction also helps to keep the hula hoop up on the hula hooper's body while the force of the hula hoop's mass pulls it down (this downward force is due to gravity). The heavier (more massive) the hoola hoop, the greater the downward force and the more work it takes to keep the hula hoop spinning.

For Further Exploration

• In this activity you compared hula hoops that were the same size but had different masses. You could repeat this activity again but this time make hula hoops of different sizes. Tip: The smallest hula hoop you would want to make would have a diameter up to the hula hooper's navel, and the largest would have a diameter to the middle of their chest. How does the size of the hula hoop affect how it spins?
   
• You could make a bigger range of hula hoops, such as by adding different amounts of sand and/or making several different sizes. Is there a relationship between speed and the mass or size of the hula hoop? If so, what is it?
   
• Try hula hooping in different types of clothes. Do different types of clothing affect the speed at which the hula hoop rotates? Can you correlate your results to the force of friction?

Credits: Teisha Rowland, PhD, Science Buddies

Related Links:

If you enjoy learning about the physics of hula hooping and explaining it to others, you may want to consider a career as a physics teacher

For a printable version of this activity visit Science Buddies.

For more physics fun, try these activities too: 

• Theme Park Science with Jell-O® Loop-De-Loops.  
• Circus-Trick Science: How to Balance Anything

How does the distribution of mass affect the rotation of, say, an egg? Science Friday explores this question with astronaut Don Pettit in the video “Spinning Eggs in Space”