How does physics keep you zipping along on your in-line skates?

Dave improves his skating technique with a lesson in physics.
Segment length: 7:30

Insights

Push and glide. Push and glide. Faster and faster, until you're cruising along somewhere near 25-32 kilometers (15-20 miles) per hour. The wind whistles around your helmet. The wheels on your in-line skates whisper as you race along. Science and math are a long way from your mind. But they aren't a long way from the sport. Side-surfing, crossovers, backward skating, swizzling, arabesques, and roller hockey all depend on physics.

Physics is the science that tries to explain things like matter and energy. Energy added to matter can produce motion. Motion can be changed by force. And when you're having fun skating, serious forces come into play.

For starters, you push and glide to increase your speed. But you aren't just moving, you're moving in a certain direction-hopefully forward, though backwards works and "down" happens a lot when you're first learning. Motion in a particular direction is velocity. You increase your speed while trying to pass the slowpoke on the bike in front of you, decrease your speed to let the five-wheeled in-line racer pass you, or swerve to avoid a pothole. All of these actions involve changes in velocity and are therefore accelerations.

Still not convinced that something so popular could have anything to do with physics? Then how about center of gravity? You may not realize it but you're finding your center of gravity every time you try to keep your balance while you're slaloming, tweaking, or wall-riding. And if you don't find it, you experience the forces of gravity (and friction) firsthand. Ouch!

When you're stair-riding or checking someone into the boards during a roller hockey game, you're dealing with another principle of physics-inertia. Newton's first law of motion talks about inertia, which is the tendency of a moving object to maintain a constant velocity. It's the principle you benefit from in the "glide" part of "push and glide." After you push, you'll keep on going until friction within the wheels' bearings, between the wheels and the ground, and between you and the air rob you of your forward motion.

The physics of motion-acceleration, velocity, center of gravity, inertia, and friction-are all part of every in-line race, hockey game, or zing around the park. Who knows-understanding the science better may help you become a faster, better, and more powerful skater!

Connections

1. How would lowering your center of gravity (by squatting down) change your velocity? How is doing an arabesque the ultimate exercise in finding your center of gravity?
2. How might the hardness of the wheels on the in-line skate affect your speed?

Key Words

acceleration change in the speed or direction of motion
center of gravity point around which the entire mass of an object can equally balance
force push or pull exerted on or by an object
friction force that acts against forward motion
inertia tendency of an object in motion to remain in motion and an object at rest to remain at rest
mass amount of matter an object has
newton unit used to measure the amount of force you need to accelerate a one-kilogram object to a velocity of one meter per second in one second
velocity the speed and direction of motion of a body

Resources

1. Freeman, I.M. (1990) Physics made simple. New York: Doubleday Books.
2. Giancoli, D.C. (1991) Physics: Principles and applications. Englewood Cliffs, NJ: Prentice Hall.
3. Gutman, B. (1992) Blazing bladers. New York: Tor Books/Tom Doherty Associates.
4. Powell, M. & Svensson, J. (1993) In-line skating. Champaign, IL: Human Kinetics Publishers.

Rappelfeld, J. (1992) The complete blader. New York: St. Martin's Press.

5. Sullivan, G. (1993) In-line skating: A complete guide for beginners. New York: Cobblehill Books.
6. Walpole, B. (1987) Fun with science: Movement. New York: Warwick Press.

Additional sources of information

1. In-line magazine
   1919 14th St., #421
   Boulder, CO 80302
2. International In-line Skating Association
   Lake Calhoun Executive Center
   3033 Excelsior Blvd.
   Minnetonka, MN 55416
3. U.S. Amateur Federation of Roller Skating
   4739 South St.
   PO Box 6579
   Lincoln, NE 68506

Main Activity

Arnold Schwarzenegger is a powerful human being, no doubt about it. But how much force do you think he can generate in newtons? When he holds a 300-pound barbell above his head, he's exerting 1,336 newtons of force. Here's an activity that lets you figure out how much force in newtons you generate on a pair of in-line skates.

Materials

• in-line skates
  . stopwatch
  
• masking tape or chalk
  
• meter stick
  
• calculator
  
• human weight scale

1. Weigh each person in your class.
2. For this activity, you need to know the mass of each person. To do that, divide the weight in pounds by 2.2. This is each person's mass (m) measured in kilograms. Record the mass of each person on a chart. (For example, if Emily weighs in at 88 pounds, her mass is 40 kilograms.)
3. Predict which students in your class will be able to generate the greatest amount of force (F) on a pair of in-line skates.
4. On a gym floor (with masking tape) or in a parking lot (with chalk), mark a starting line and another line (your distance, or d) 20 meters away.
5. Have someone stand at the finish line with a stopwatch. Signal the skater to go, pushing as hard as she or he can for the full 20 meters. Make sure the skater and the stopwatch start at the same time.
6. Stop the stopwatch when the person crosses the 20-meter mark. That number (in seconds) is that skater's time (t).
7. Do this for as many students as you want, keeping careful records. When everyone is done, you're ready to figure the average acceleration (a). Acceleration can be calculated using the following formula: a = (2 x d)/t 2. (If Emily covers 20 meters in 5.0 seconds, her average acceleration is 1.6 meters per second squared. This means that she is adding one meter per second to her speed every second.)
8. Now you're ready to figure the force generated by each skater. Force is measured in newtons. One newton is the amount of force you'd need to get one kilogram (2.2 lb) of mass accelerating at one meter per second squared. You can figure out the force by using this formula: F = ma. (Emily's force would be 64 newtons.)
9. Once you've figured out the force measured in newtons, convert newtons to pounds (newtons x 0.22 = pounds). Remember that these are very rough calculations. We are assuming that acceleration is constant and that all the force exerted by the skater goes to increase speed (rather than to overcome friction and to push downward to avoid slipping).

Questions
1. What do the final numbers mean? What does it mean to have generated a force of so-many newtons or pounds? Compare this to your weight in pounds.
2. Who generated the greatest amount of force? Why? Were your predictions correct?
3. Did each person's mass affect his or her acceleration? How?

Spin a raw egg on its side in a bowl. Stop it, then let go of it quickly. What happens? Why would the egg start to spin again?

Make a number of odd shapes out of cardboard. Poke three holes in each one. Pin the shape to a bulletin board through one of the holes. Hang a weighted string from the pin and draw a line on the cardboard along the edge of the string. Repeat this using a different hole. Wherever the lines cross is the center of gravity. What would happen if you pinned the cardboard shape at the center of gravity?

Tape a quarter to the inside wall of a can. On a board that is slightly inclined, place the can on its side and turn it so that the quarter is slightly past the top position on the "uphill" side. Let go. Why would the can roll up the board? Can you explain this in terms of center of gravity?

Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.