 to Video  # Inline Skating

• Do in-line skates really go faster than roller skates?
• 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?
How might the hardness of the wheels on thein-line skate affect your speed?

## Activity

Find out how many newtons you can generate with your in-line skates. 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
• 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? ## Resources

• Giancoli, D.C. (1991) Physics:
Principles and applications. Englewood Cliffs, NJ: Prentice Hall.
• Gutman, B.
(1992) Blazing bladers. New York: Tor Books/Tom Doherty Associates.
• 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.
• Sullivan, G. (1993) In-line skating: A complete guide for beginners.
New York: Cobblehill Books.
• Walpole, B. (1987) Fun with science: Movement. New
York: Warwick Press. 