In this lab, students will investigate the effect of gravity on the speed of a sled and rider sliding downward on an ice-/snow-covered track.

Lesson Objectives:

After completing this lesson, students will be able to:

1. Explain the effect of gravity on objects like a luge moving down an incline on Earth or other cosmic bodies.
2. Relate different gravitational forces to the speed of movement of the luge on an incline.
3. Predict the gravitational force acting on the luge when given speed data from any luge run.
4. Explain why the average speed during the first half of the luge run in always less than that during the last half of the run.

Background:

Introduction: Anticipation of the luge competition at the Winter Olympics brings visions of high speed, sharply banked curves, a trough like track, and tests of skill in maneuvering the tiny sled. The Winter Olympics scheduled for Utah's mountains in 2002 promises to present an event as exciting as the first Olympic luge competition held in Innsbruch, Austria in l964. The approximate 1335 meter-long track at Utah Winter Sports Park starts at an altitude of 2233 meters and descends to a base altitude of 2142 meters. The luge and rider, after an initial pull off aided by start handles, begin the descent of the track. The timing for the race starts at the instant the sled and rider reach the end of the horizontal start area and commence to move downward. Then several forces acting on the sled and rider begin to play major roles on the movement down the track.

Science: Gravity is the force which pulls the luge and rider faster and faster downward. Also acting on the luge is friction of the ice surface on the sled and the drag caused by the air friction on the rider and sled. Other factors which play roles are the slope of the track, the conditions of the ice surface, types and numbers of curves and the initial start velocity as the luge begins the descent. In this lesson, all frictional forces have been reduced to zero, curves have been eliminated from the course, and so the acceleration of the luge and rider are affected only by the slope of the track. With no frictional forces on the track surface, the acceleration is equal to g x sinθ where θ is the slope angle and g is the acceleration due to gravity on the particular cosmic body. Remember, as θ approaches 90 degrees, the acceleration approaches 9.8 m/s2 which is the acceleration due to gravity for a free-falling body on Earth. In the case of the luge course depicted in this lesson, θ is approximately 4.5 degrees and the corresponding acceleration is about 0.76 m/s2 on Earth. Likewise, after the 55 seconds for the luge and rider to reach the finish line, the final speed is approximately 44 m/s ( ie. 0.76m/s2 x 55 s) on Earth.

Preliminary Knowledge: Prior to working on these lessons, students will be expected to know the relationship between speed, distance and time and should be capable of using the formula, average speed = distance/time. The term "speed" will be used in the On-Line Student labs, however, teachers may want to review the difference between "speed" and "velocity".

• Computer with Internet connection
• Lab notebook
• Calculator
• Student Lab Packet - This is a printable version of the lab materials (instructions, tables, and questions)

1. Teachers may introduce this lab by reviewing what students know about the luge and luge competition. Contrast the luge and the bobsled. Remember, contestants lay flat on their back on the luge sled but sit up in a bobsled. Students may visit the web site http://www.luge.com for general information on luge competition world wide. Also, http://www.SLC2002.org/sports/html/winter_park1.html will give specifics for Utah's luge and bobsled track to be used in the 2002 Winter Olympics.

2. Continue introduction by asking students what force or forces cause the luge and rider to move down the track and gain speed. If gravity is mentioned, explore student ideas about this force, i.e. Is gravity always present? Is there any way to escape its effects? Can it be changed in intensity? What happens to the luge run if it were held on the moon?

3. Be certain that students can tell what variables need to be measured so speed or average speed can be calculated for the run. Review the formula, average speed = distance/time. Practice several hypothetical problems on speed, distance and time. Challenge students to speculate on how they might actually measure distance and time for the luge moving down a mountain side.

4. Ask students to give examples and explain what other forces may be acting on the luge and rider and how these forces affect the speed. It is expected that students will mention air resistance and friction of the sled's runners on the ice. Other factors they may mention could be conditions of the ice, the types of curves, and the slope of the track. Each teacher may explore these and additional factors in different ways depending on experience of the teacher and interest of students.

Pre-Assessment:

An optional group of questions to test your students' present knowledge of the effect of gravity on moving objects.

1. If a skier slides down a steep slope, what will happen to the speed of the skier as he/she proceeds further down the slope?
2. What force or forces act on the skier while on the slope?
3. If the skier could try the same slope on a cosmic body the size of our moon, what effect would less gravity have on the speed of the skier? What if the cosmic body where larger than our Earth, what effect on speed would you predict?
4. Gravity is a force produced by our Earth and all other cosmic bodies. If you could turn the force of gravity off, what would happen to a skier on the same steep slope?

Directions for Teaching the Lab:

1. Invite students to proceed to the beginning of the student lab and sign in as a member of the Cosmic team.
2. After the initial sign in, students will see the luge run screen appear. It will show the luge course, free of curves, and descending through a distance of approximately 1300 meters. (Note: the actual luge run at Winter Park in Utah descends approximately 91 meters in altitude through a distance of 1335 meters). The course is marked with a half-way point and a finish line.
3. To the right of the luge course is a rectangular area containing scalers which measure time, speed, and distance for the luge on each run. Below the scalers is a Gravity Dial Meter which allows students to choose the amount of gravity that will act on the luge and rider. At the bottom is a data table which will record the force of gravity, relative to Earth's, the half-way time, and the final time.
4. Students may click on the luge at any point for an instantaneous measurement of speed, distance, and time. If these values are needed for future use, they must be recorded in the student's notebook before clicking to continue the run.
5. To start the run, press Start Run button. Each time the track is traversed, the time at mid-point and time at the end of the run will be provided. This data will be logged onto the data table provided. Students will be encouraged to copy down this data in their own notebooks. (See Student Lab Packet for a copy of the data table). As the student selects a particular gravitational value to investigate, he/she will be encouraged to make a minimum of 3 trial runs (by clicking Repeat Run) at that value. These may be averaged or totaled to determine the best performance.
6. After several runs at different gravitational values, students may record their data and proceed to a series of questions and problems to apply what they have learned. These questions will also be included in the Student Lab Packet.

Summary:

Students have been invited to join the excitement of the 2002 Winter Olympic luge competition by signing in as a member of the Cosmic team and taking a ride. The lesson has allowed students to vary the amount of gravitational pull and examine its effect on the run. It is expected, after completing the activities and analyzing the data, that students will discover:

1. The higher the gravitational pull, the less total time it takes to traverse the track.
2. During the first half of the run, the time is always greater than the time to complete the second half of the run. This should lead to the idea that the speed is increasing during the run.

Extension:

Students will have opportunities to apply their understanding of gravity on movement of objects like a luge by answering the questions and working problems following the experimental section of the lesson.

Post-Assessment:

A following series of questions will assess student learning:

1. What causes the luge and rider to gain speed during the run?
2. If you could turn off gravity, what is the effect on a luge and rider as they exit the starting gate? Can you explain why they take the path they do?
3. How does average speed during the first half of the run compare to that during the last half?
4. What would be the effect of doubling the gravitational pull on a luge run?
5. Can we change the pull of gravity on our own Earth?