Activity Two Teacher Page:
The Effect of Gravity on Motion
Invitation To Learn |
Lab Procedure |
In this lab, students will investigate the effect of gravity on the speed
of a sled and rider sliding downward on an ice- and snow-covered track.
After completing this lesson, students will be able to:
- Explain the effect of gravity on objects like a luge moving down an
incline on Earth or other cosmic bodies.
- Relate different gravitational forces to the speed of movement of
the luge on an incline.
- Predict the gravitational force acting on the luge when given speed
data from any luge run.
- Explain why the average speed during the first half of the luge run
is always less than that during the last half of the run.
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.
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 (vertical), 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 42 m/s ( ie. 0.76m/s2 x 55 s) on Earth.
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 (See our Technical Support page for
minimum requirements and assistance.)
- Lab notebook
- Student Lab Packet - This is a printable
version of the lab materials (instructions, tables, and questions).
- Optional for Invitation To Learn: Pictures or resources specific to
winter sports. (See #1 below.)
Invitation To Learn
- 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
- 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?
- Be certain that students can tell what variables need to be measured
so average speed can be calculated for the run. Review the formula,
average speed = distance/time. Practice several hypothetical
problems on average 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.
- 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.
An optional group of questions to test your students' present knowledge of
the effect of gravity on moving objects.
- 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?
- What force or forces act on the skier while on the slope?
- If the skier could try the same slope on an extra-terrestrial body
the size of our moon, what effect would less gravity have on the
average speed of the skier? What if the body where larger than our
Earth, what effect on average speed would you predict?
- Gravity is a force produced by all bodies. If you could turn the
force of gravity off, what would happen to a skier on the same steep
Directions for Teaching the Lab:
- Invite students to proceed to the beginning of the student lab and
sign in as a member of the Aspire team.
- 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.
- 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
Selection Box 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.
- 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.
- 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, s/he 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
- 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.
Students have been invited to join the excitement of the 2002 Winter
Olympic luge competition by signing in as a member of the Aspire 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
- The higher the gravitational pull, the less total time it
takes to traverse the track.
- During the first half of the run, the time is always greater
than the time to complete the second half of the run. Assuming the
value of gravity is not zero. This should lead to the idea that the
speed is increasing during the run.
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.
The following series of questions will assess student learning:
- What causes the luge and rider to gain speed during the run?
- 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?
- How does average speed during the first half of the run compare to
that during the last half?
- What would be the effect of doubling the gravitational pull on a
- Can we change the pull of gravity on our own Earth?