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Motion Mania: Applying Physics to Hula-Hooping

Difficulty
Time Required Short (2-5 days)
Prerequisites You should know how to (or be willing to learn to) hula hoop around your waist or arm. Note: Hula-hooping is fairly easy if you have the right hula hoop for your body. This science project provides guidelines to make such a hula hoop.
Material Availability Readily available.
Cost Very Low (under $20)
Safety Adult help is required to cut the polypipe tubing.

Abstract

Practice makes you better at most things, and knowledge makes practice so much easier! Can you swirl a circular toy called a hula hoop around your waist or arm? Is it hard? What knowledge can you apply to find ways that make hula-hooping easier? Physics! Yes, physics will help you determine what makes one hula hoop a winner and another a flop.

In this science project, you will create your own hula hoops, spin them, and draw conclusions. The road will then be open to your becoming a hula hoop expert.

Objective

To make hula hoops of different circumference and mass and study how these factors affect the speed at which the hula hoop spins around.

Credits

Sabine De Brabandere, Ph.D., Science Buddies

Cite This Page

MLA Style

Science Buddies Staff. "Motion Mania: Applying Physics to Hula-Hooping" Science Buddies. Science Buddies, 16 Nov. 2013. Web. 25 July 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p088.shtml?from=Blog>

APA Style

Science Buddies Staff. (2013, November 16). Motion Mania: Applying Physics to Hula-Hooping. Retrieved July 25, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p088.shtml?from=Blog

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Last edit date: 2013-11-16

Introduction

Have you ever spent time spinning a hula hoop on your waist or arm? Have you ever wondered how hula hoops work? This science project will not only teach you how to make a hula hoop, but it will also guide you in finding the hula hoop that is right for you and describe the physics that make a hula hoop spin. Add a little spinning creativity and practice, and you could become a hula hoop expert!

So, what is there to know about hula hoops? How can physics help you become a winning hooper?

While we can "hoop it up" on various parts of our body, we focus here on traditional waist hooping. Once you understand this, you can extend the reasoning to arm hooping. But before we analyze what makes the hula hoop spin, let us think about how we are going to measure the spinning motion of the hoop. Any ideas? Would the speed, or how fast the hula hoop spins, be a good factor to measure? If so, how would you do it? (In this project, the speed will be measured as the number of times the hula hoop makes a full turn per minute.)

Now, let us study what makes the hoop spin. In any hooping, you are the source of the hoop's movement. When you move your body to propel your hoop around you, you are actually exerting an upward force (from your hips) to keep the hoop from falling and a turning force known as torque to keep it spinning. Torque is a twisting, outward force that is basically needed to cause something to spin (or rotate), and, once spinning, torque is needed to keep it spinning. Here, it is interesting to note that the amount of force needed to keep up a good spinning motion depends upon the weight and mass of the hoop and the size of the hoop as well as how it fits around your waist.

Weight and mass? Are these not the same? No, even though they are closely related, they are different. Mass is a measure of how much "stuff" an object holds. It is expressed in kilograms (kg) or grams (g) and is a measure of how easily the object reacts to a force. Say we have a 2000 kg car and a 100 g (or 0.1 kg) toy car. If you give both the same push (a scientist would say "you apply the same force to both objects") to get each to move forward, which one will react quickly and move faster? Also, once they are both in motion, which will be easier to stop when applying the same force to both? The one with a lot or a little mass? You see, a lot of mass needs a lot of force to make its motion change (for instance, slow down or speed up). How would this apply to your hoop? Would a heavy hoop react strongly or just a little to a small push? Would it have a greater or lesser change in speed?

If mass is a measure of "the amount of stuff," what is weight? Weight is a force, measured by how strong the planet you are on (e.g., the Earth or the Moon) "pulls" the object toward itself. A hula hoop has the same mass here on Earth as on the Moon because it consists of the same amount of material. The weight of the hula hoop, on the other hand, is greater on Earth than on the Moon. Why? Because the force of gravity is stronger on Earth than on the Moon, so the Earth pulls harder on the hula hoop than the Moon does.

Of course, we can not go to the Moon or any other planet to practice with our hula hoops, but weight still plays a role. How? Well, do you think a hula hoop with more mass will be pulled more strongly toward Earth than a hula hoop with less mass? Think of the car: Does a loaded toy car (loaded means more mass) feel heavier than an empty one? The feeling of "heavy" is the Earth pulling stronger on it. So more mass (more stuff) does mean more weight (a stronger pull of the planet) if we compare objects on the same planet. Because a more massive hula hoop is heavier, it will tend to fall down more rapidly. What can a hula-hooper do to prevent it from falling? The hooper needs to give it a stronger push up to overcome its weight (or to balance the force pulling the hoop down, as a scientist would say).

So far, we have the weight pulling the hoop down and our body giving it a push up to prevent it from falling, and we have torque giving the hula hoop a spin... so will it spin forever? Is there anything slowing down the turning motion? Think for a minute: If a ball is rolling, does it roll forever? No, of course not, but what stops or slows the ball down? Friction stops the ball. The ball slows down (or decelerates) because of friction between the ball and the air surrounding it and between the ball and the surface it rolls on. The same is true for the hula hoop. Because of friction, you need to periodically give the hoop a little boost, or torque, to overcome the friction that stops the hoop from turning and keep it spinning. At the same time, friction also works against gravity to help keep the hula hoop up on the hula-hooper's body.

We still need to explore one important aspect, the size of the hoop and its effect on speed. Would a big hoop (larger circumference) turn around your body slower or faster than a small hoop? If the question seems difficult, maybe exaggerating will help provide the answer. Would a hoop that is much bigger than your waist take more time to make a full turn around your waist than a hoop that barely fits around you? Of course, it is easy to visualize that it will take more time for the larger hoop to make a full turn than the smaller one. This means that the larger hoop has less speed than the smaller hoop.

Now that we have explained it all, let us review what we have learned:

  • Weight pulls the hoop down. The hula-hooper prevents this downward pull by periodically pushing up and by pushing out, causing a turning force called torque.
  • Friction works to slow the hoop down, and the hula-hooper needs to prevent this by applying torque. Friction also works to keep the hoop up.
  • The greater the mass of the hoop, the greater the weight and the more the hula-hooper needs to push for the hoop to react and stay up. The same force or push to a lighter object when applied to a more massive object will cause the more massive object to change its motion (speeding up or slowing down) less.
  • The larger the hoop, the slower it will spin.

Can you put it all together and make a prediction? What would be easier: working with a hoop with more mass or less mass? Working with a bigger or a smaller hoop? In fact, there is no right or wrong answer to this question! What works for one person does not always work for another. This science project challenges you to find what works for you (or your test person). Disappointed? While what works well for one person may not be best for another, science does enable you to make other types of predictions! In this physics science project, you will investigate how the size and weight of a hula hoop affects the speed at which it spins around. Can you predict which one would turn fastest (or which has the highest speed)? The small lightweight hula hoop? The heavy, bigger version? The heavy, small hula hoop? Or the lightweight, big hula hoop? Here, there is a right answer! Try to find it from the explanation above. If you still hesitate, no worries! You will find the answer by doing the science project. Once you have finished, try to explain your findings by reading the Introduction again. If you still have a hard time explaining the results, do not hesitate to ask your science teacher or contact Science Buddies' Ask an Expert forum for help.

Now, go and explore! Time to spin into action!

Terms and Concepts

  • Physics
  • Speed
  • Force
  • Torque
  • Mass
  • Weight
  • Gravity
  • Friction
  • Decelerate
  • Diameter
  • Circumference

Questions

  • What forces act on a hula hoop in motion? Which work against hula-hooping and which work in favor?
  • Will a heavier hoop (i.e., one with more mass) rotate slower or faster around your body than a lighter hoop?
  • Will a bigger hoop take less or more time to rotate around your body?

Bibliography

Materials and Equipment

  • Polypipe, which is hard, black tubing usually used for irrigation (⅝ inch, ¾ inch, or 1 inch diameter; 100-foot coil). Available from a garden supply store or home improvement store.
  • Metric tape measure
  • Calculator
  • Sharp cutter knife or PVC cutter
  • Poly insert couplings that fit the size of your tubing (4, same size as your polypipe; for example, ¾-inch poly insert couplings work for the ¾-inch tubing). Wooden dowels with a diameter that just fits inside the tubing with a length of about 1½-inch work as well. Both can be found at a garden supply or home improvement store.
  • Hairdryer
  • Wide plastic tape (min. 2.5 cm wide)
  • Optional: Colorful electrical tape
  • Funnel with an opening that can fit into the polypipe or 1 sheet of thick paper
  • Cup measure
  • Dinner knife with flat edge
  • Sand (2 cups)
  • Kitchen scale, such as the Fast Weigh MS-500-BLK Digital Pocket Scale, 500 by 0.1 G, available from Amazon.com.
  • Bucket
  • Timer or stopwatch
  • Helper or person who wants to hula hoop for you
  • Adult helper

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Experimental Procedure

Make Your Big, Lightweight Hula Hoop

Start by making the larger size, lightweight hula hoop.

  1. Cut a piece of tubing of the appropriate length.
    1. Measure the height from the ground to the middle of your chest in centimeters (cm) — this will be the diameter (or height of the circle) of the biggest hula hoop.
    2. Calculate the circumference (or distance around the circle) of the hula hoop using following formula:
      Circumference = π x diameter
      Circumference = 3.14 x diameter

      The circumference is the length of tubing you will need to cut to create this hula hoop.
    3. In your lab notebook, make a data table similar to Table 1 below and record the diameter and circumference in it.
    4. Measure the length (circumference) needed from your roll of polypipe tubing and mark where the tubing should be cut. Using a piece of electrical tape can help mark the spot.
    5. Ask an adult to cut the tubing at the indicated spot using a sharp knife or a PVC cutter.
  Diameter
(cm)
Circumference
(cm)
Mass of Added Sand
(g)
Hula 1 (Big, lightweight hula)  0 g
Hula 2 (Big, heavy hula)   
Hula 3 (Small, lightweight hula)  0 g
Hula 4 (Small, heavy hula)   
Table 1. Record the data about the four different hula hoops in this data table.
  1. Connect the ends of the tubing so they form a circle, to be fastened using a poly insert coupling or wooden dowel.
    1. If you are using poly insert couplings or a dowel that is a tight fit, you will probably need to first use a hairdryer to warm the tubing ends (one at a time), for about two minutes. Heat will expand the tubing, enabling the connector to slide in easily.
    2. Once they can fit inside the tubing, put the poly insert coupling or dowel in one end of the tubing (the one you warmed) till the coupling is about halfway inserted.
    3. Insert the other half of the connector into the remaining free end of tubing. See Figure 1 below to help visualize this step.
    4. Push the tubing together until little (or no) coupling is visible. If you are using poly insert couplings, you may have a small ridge of plastic still visible in the middle.
    5. Tip: If it is ever difficult to insert the coupling, try warming the tubing more in the area where the coupling appears to be stuck. Warm the tubing for roughly two minutes at a time. You may want to have an adult help you push the coupling inside of the tubing.
Picture shows how to connect the tubing ends to form the hula hoop.
Figure 1. This picture shows how to connect the tubing ends to form the hula hoop using a wooden dowel.
  1. Secure the connection. Put some wide plastic tape over the connection where the two ends of tubing meet, as shown in Figure 2 below.
    Picture showing how the sealing is secured with wide, brown tape.
    Figure 2. This picture shows how the sealing is secured with wide, brown tape.
    1. Decorate your Hula Hoop with colorful electrical tape any way you like (optional). Examples of decorated hula hoops are shown in Figure 3 below.
Homemade hula hoops decorated with colorful tape and ready to use to investigate the physics of hula hooping.
Figure 3. Finished hula hoops can be decorated (optional) with different colors of electrical tape.

Make Your Big, Heavy Hula Hoop

Now that you know how to make the bigger size hula hoop, make a heavier version of this hoop.

  1. Cut off a piece of tubing the same length as previous section. (See step 1 of the first section of this Experimental Procedure.)
  2. Create a funnel that can be used to fill the tubing with sand - skip this step if you have a funnel that fits in the hole of your tubing.
    1. Cut of a piece of tape (about 5 cm. long) and keep it handy.
    2. Roll the sheet of thick paper into a cone (like an ice-cream cone), leaving a small hole at the end. The hole should be big enough to leave room for sand to flow trough but small enough to fit inside your tubing. Figure 4 below shows what the funnel should look like.
    3. Fasten the funnel with tape.
Make a funnel from thick paper and tape (blue tape was used to make it visible, but any tape works).
Figure 4. Make a funnel from thick paper and tape (blue tape was used to make it visible, but any tape works).
  1. Measure and determine the mass of one cup of sand.
    1. Use the scale to weigh the measuring cup alone (in grams [g]) and note the result.
    2. Scoop up 1 cup (c.) of sand and scrape off any excess sand by pushing the flat side of a dinner knife against the rim of the cup.
    3. Use the scale to weigh the sand and measuring cup together, and note the result. Is this the mass or the weight of the cup with sand?
  2. Fill the tubing with sand.
    1. Have a helper hold one end of the tubing up.
    2. Hold the tubing over a bucket.
    3. Using the funnel, pour the sand into the tubing.
    4. Pick up any spills in the bucket and pour those in the tubing as well.
  3. Finish this hula hoop by following steps 2–3 of the first section.
  4. Calculate the actual added weight to the hula.
    1. Does the reading from the scale in step 3c represent the mass added to the hula hoop? Does the cup holding the sand play a role in the reading? Yes it does! So, can you find a way to obtain the mass of the sand added to the hula with the data you obtained? If you subtracted the mass of the cup (the reading you took in step 3a) from the mass of the cup filled with sand (the reading you took in step 3c), you got it right!
    2. Note the mass added to the hula hoop in the data table in your lab notebook in the column "Mass of added sand."

Make Your Small, Lightweight Hula Hoop

Make a smaller hula hoop with approximately the same weight as the big, lightweight hula hoop.

  1. Calculate the length of tubing needed for the smaller hula hoop
    1. If the diameter of the smaller hula hoop is measured from the ground to your navel, can you find the length of tubing needed? (See step 1 in the first section of this Experimental Procedure, "Make Your Big, Lightweight Hula Hoop," for hints.)
  2. Create your small hula hoop using the new calculated length (circumference) of tubing.
  3. Put some thought in how the masses of the two lightweight hula hoops (one big, the other small) compare to each other.
    1. Would this small hula hoop be equal in mass to the bigger hula hoop? If not — what accounts for the difference?
    2. Can you calculate how much extra tubing goes in the bigger hula hoop with respect to the smaller hula hoop? Remember: You have noted the circumference of the big and the small hula hoops in your data table.
    3. Would the extra tubing in the big hula hoop provide the extra mass? If so — is this extra amount of mass large with respect to the total mass of the hula hoop?
    4. Is it safe to say the two lightweight hula hoops are approximately the same in mass? (Note: You can weigh a small piece (e.g., 10 cm) of tubing and calculate from there how much the hula hoops differ in mass.)

Make Your Small, Heavy Hula Hoop

Make a smaller hula hoop with approximately the same weight as the big, heavy hula hoop.

  1. Create another small hula hoop as described in the previous section ("Make Your Small, Lightweight Hula Hoop"), this time filling it with one cup of sand, following steps 3 and 4 in the section above entitled "Make Your Big, Heavy Hula Hoop."
  2. If you enjoy a little challenge, put some thought into how the masses of these hula hoops compare. Without knowing the exact numbers, can you reason which relative mass difference would be bigger — the one for the heavy hula hoops or the one for the lightweight hula hoops?

    Use the following formula to calculate the relative mass differences:

    Relative Mass Difference  =   Mass Big Hoop - Mass Small Hoop
    Mass Big Hoop

    To see which one is bigger, look at how the numerators compare (cross out the symbols that do not fit):

    Mass Big Heavy Hoop - Mass Small Heavy Hoop
    {

    }
    Mass Big Light Hoop - Mass Small Light Hoop

    Now, how do the denominators compare?

    Mass Big Heavy Hoop
    {

    }
    Mass Big Light Hoop

    Knowing this, can you make a conclusion about how the fractions compare?

    Mass Big Heavy Hoop - Mass Small Heavy Hoop
    Mass Big Heavy Hoop
    {

    }
    Mass Big Light Hoop - Mass Small Light Hoop
    Mass Big Light Hoop
  3. As you now know, the relative difference is smaller for the heavy hoops than for the lightweight hoops. So can you safely say the two heavy hula hoops are approximately the same in mass?

Test the Performance of the Different Hula Hoops

Time to put the hula hoops to the test! How? One person will hula-hoop and the other will count how many full turns the hula hoop makes around the body during one minute of hula-hooping. You will test all four different types of hula hoops in this way. How do scientists refer to the number of turns per minute? Can you recall it from the introduction? (That is right: It is referred to as "the speed" of the hula.) You will measure the speed of all four hula hoops.

If you need directions on how to hula-hoop, check out the following clip:

Video  of Introduction in how to hula-hoop.
This video is an introduction on how to hula hoop.

Before we go into the tests of the hula hoops, we need to introduce some terms. The person who hula-hoops will be referred to as the hooper. The other person will help by counting and watching the timer. This person is referred to as the helper. The helper will count the number of times the hoop goes around the arm or waist. This "number" is called the data . The helper will collect data for all four hula hoops. This set of four numbers will be referred to as a set of data . At the end, we will compare or analyze the obtained data and try to draw conclusions.

Can you predict how the speeds of the different hula hoops (or how fast each hula hoop goes) will compare to one another? Which hula hoop do you expect to be fastest? Which one will be next fastest, and so on?

Ready? Here you go:

  1. Decide who will be the hooper and who will be the helper. You can switch roles after a set of data has been collected (see step 6).
  2. Decide whether to take measurements for hooping around the waist or hooping around the arm.
    1. Give it a quick try. Can the hooper spin the four hula hoops in the chosen way (around the waist or around the arm)?
  3. Count the number of turns the hooper can turn this hoop around during one minute.
    1. The helper sets the timer to 1 minute.
    2. The hooper starts hula hooping.
    3. The helper starts the timer as soon as the hooper reaches a steady pace.
    4. The helper counts how many full turns the hoop makes around the body or arm of the hooper.
    5. The helper stops counting as soon as the timer rings. Time for the hooper to take a break.
    6. If the hoop falls before the timer rings, repeat with the same hula hoop from step a. If the hooper is still not able to hoop through the full minute with this hula hoop, don't worry — a failed test can provide valuable information as well. Note the observation in your notebook. If hooping one minute with most hula hoops is too difficult for this hooper, collect a set of data for 30 seconds in stead of one minute and/or switch to hooping on another body part.
    7. In your lab notebook, make a data table like Table 2 below and write down the number of turns counted for this hula hoop in your new data table.
      1. If the hooper did not reach the full minute but instead spun the hoop for 30 seconds, multiply the number of turns they did in 30 seconds by two to get how many turns would have been done in one minute.
  Results Set 1
(Number of Turns in One Minute)
Results Set 2
(Number of Turns in One Minute)
Results Set 3
(Number of Turns in One Minute)
Results Set 4
(Number of Turns in One Minute)
Average
(Number of Turns in One Minute)
Hula 1 (Big, lightweight hula)     
Hula 2 (Big, heavy hula)     
Hula 3 (Small, lightweight hula)     
Hula 4 (Small, heavy hula)     
Table 2. Note the results of the tests in a data table like this.
  1. Repeat step 3 for a different type of hula hoop.
    1. Note: The hooper should not change clothes while collecting one set of data. Why? Because, the friction between the hoop and the hooper's clothng may affect the data. Can we compare data collected under different circumstances?
    2. Repeat this step until all four hula hoops have been tested by this hooper.
  2. Repeat steps 2–4 to collect a new set of data. You can switch roles at this point, or call in a different person to be the hooper.
    1. Do you see why you should not switch roles while taking one set of data, but why it is OK to switch when collecting different sets of data? Would you be able to compare the numbers collected within one set if different hoopers were executing the test?
  3. Repeat step 5 two more times for a total of four sets of data. If you enjoy this testing, go ahead and collect a couple more sets of data. Be sure to keep the same attention and eye for precision.

Analyze the Data

Now that you have all the data, see what it tells you.

  1. Consolidate the data by calculating the average number of turns measured over the different sets of data collected.
    1. Add the numbers collected across one row (for example, data sets one through four for the big, lightweight hula hoop) and divide by the number of filled columns in that row. For example, if you recorded 60, 80, 65, and 75 as the number of turns for a given hula hoop in one minute, the average would be (60+80+65+75)/4 = 280/4 = 70 for that hula hoop.
    2. Write the resulting average in the last column of your data table.
  2. Analyze the data.
    1. Can you see a clear difference in the number of turns recorded for the different hula hoops? Is one clearly faster or slower than the others?
    2. Is there a clear pattern (fastest / slowest) in each set of data taken? (For example, does each data set show Hula 1 to be the fastest and Hula 3 to be the slowest?)
  3. Draw conclusions or explain your observations if possible.
    1. Is there a link between the characteristics of the hula hoop (e.g. mass, diameter) and the speed at which it turns?
    2. If you see a pattern (fastest / slowest), does it match the expectations set before you started the testing?
    3. Can you explain why you obtained these results? Hint: If you are having trouble explaining this, try re-reading the Introduction in the Background tab.

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Variations

  • Make a bigger range of hula hoops — for example, four different sizes of hoops or four different weights — and analyze the relationship between the speed and the chosen variable (size or speed) graphically.
  • Analyze if different types of clothing affect the speed at which the hula hoop rotates around the waist or arm.

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