How do ski jumpers use physics to get the most out
of their flight?
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Getting
Started
How do the shapes of objects affect the flow of a fluid? Mix one quart of white
Ivory dishwashing liquid with five drops of food coloring and place in a
shallow baking pan. Try dragging different-shaped objects through the liquid
and observe the flow patterns they create. The more swirls, the more turbulence
and the less aerodynamic the shape. Based on this experiment, why do you think
manufacturers don't make skis with square fronts?
Have you ever skied over a large bump and become airborne? Why do you think
that happens? Do you watch the ski jumpers during the Winter Olympics? How far
do the jumpers go before landing? Why do they hold their skis and their bodies
a certain way?
Overview
There are few
feats as breathtaking as a perfect ski jump. Hurtling down a snow-covered ramp
at speeds in excess of 100 kilometers (60 miles) per hour, the skier literally
dives off a cliff, soars through the air, and finally descends back to earth
some 100 meters (328 feet) from takeoff.
To a novice, the steps in a ski jump look deceptively simple. In reality, each
involves a complex balance of forces where only slight changes in equipment or
body position can mean the difference between a gold medal and disaster.
Like a roller coaster, all the energy for a jump comes from gravitational potential energy acquired by going to
the top of a hill - in this case, the inrun.
Coming down the inrun, jumpers try to build up as much speed as possible while
maintaining control. To minimize air resistance, they get in a low crouch, point
their arms forward, and bend their heads slightly downward like a diver
entering the water.
Halfway down the inrun, jumpers begin to re-position their bodies in
preparation for leaping off. Near the end, where the inrun begins to curve
upward, they raise their hips slightly while pressing the chest tight against
the knees. This makes their legs act like a coiled spring storing additional
energy for the takeoff. About three meters (10 feet) from the end of the inrun,
jumpers begin their final adjustments before takeoff, bringing their arms
perpendicular to the ground and rising up slightly.
The most important part of the jump occurs at takeoff. Within a tenth of a
second, jumpers must combine two motions at once, leaping both forward and
upward at the same time. The timing of the takeoff leap is what makes or breaks
a jump. If jumpers spring before they reach the exact end of the takeoff table, their skis will point down, causing
extra wind resistance which results in a short jump. If they spring too late,
their skis are pointed too high, resulting in a serious loss of control.
In the air, jumpers become flying projectiles, using their bodies and skis like
a giant airfoil. They lean forward, producing a
positive angle of attack on the wind.
Traditionally, jumpers always kept their skis straight in line with their
bodies to lessen air resistance and reduce drag.
In 1989, a jumper revolutionized jumping by holding his skis in a large V with
the open end pointed forward. This positioning increases the surface area below
the body, providing more lift toward the end of the flight. It extends the time
in the air and the distance of the jump.
Connections
1. How is a ski
jump like a roller coaster ride? How are the same forces used in different
ways?
2. How do jumping skis act like the wings of an airplane? How would changing
their shape affect the flight?
Resources
Botterill, S. (1994, Feb) V for victory. Life, p. 94.
Finkel, M. (1994, Jan) Leap of faith. Skiing, p. 26.
Friedman, J. (1994, Feb) This joint is jumping. Skiing, p. 22.
Roessing, W. (1995, Jan) Ramp champ. Boys' Life, p. 18.
Rosenberg, D. (1995, Jan 16) High-tech skiing. Newsweek, p. 63.
Ulmer, K. (1996, Feb) To air is human. Skiing, p. 70.
Wolff, A. (1994, Feb 7) Flight of the Finns. Sports Illustrated, p. 82.
Wolff, A. (1994, Feb 28) Jens Weissflog, ski jumper. Sports Illustrated, p. 55.
Giving jumpers a lift:
http://www.oslo.sintef.no/gemini/1993-dec/11.html
Jump: The homepage dedicated to ski jumping:
http://www.cdnsport.ca/jump/
Student
Activity:
Ski Jumping
Launch Control
Vary the angle
of a ramp and chart your flight path.
Main Activity
Ski jumping converts gravitational potential energy to kinetic energy. The
objective is to launch a human projectile as far as possible. By manipulating a
track, you can discover how changing the launch angle will change the direction
and duration of flight.
Materials
* 1 meter (3.3') of Styrofoam pipe insulation, cut lengthwise
* marble or small steel ball
* 8 to 10 thick books or bricks or a chair
* masking tape
* tape measure
* table
* paper and pencil
1. Start building your "inrun" by piling several books on a table so
that they measure about 30 cm (12") high. Place one end of the pipe
insulation right on the edge of the table, and put the other end under one of
the books at the top of the stack. Build up several books under the middle of
the ramp so that it doesn't sag or bend. Secure the insulation to the table and
books with masking tape, making sure you don't tape across the track.
2. Place your marble at the top of the ramp. Without pushing, let it roll.
Observe the flight path and the place where it first lands on the floor. Repeat
this step four more times so that you can get a consistent reading. Remember to
start from the same place each time. Measure and record this distance under the
heading "flat track" and draw the shape of the marble's flight path.
3. Remove the books holding the middle of the ramp and adjust it so that it
curves down to the table and runs flat along the table for about 20 cm
(8") before it reaches the end. Make sure that the end of the ramp still
lines up exactly with the edge of the table and once again secure it with
masking tape.
4. Using the same marble as before, test the ramp again. Remember to start from
the same place. Record the distance under the heading "curved track"
and again draw the flight path of the marble.
5. Repeat step 4 but this time add a book to the end of the ramp so that
instead of lying flat on the table, the ramp curves down and back up a bit.
Record your measurements under the heading "U-shaped/one book" and
draw this flight path.
Questions
1. What happened to the distance traveled by the marble each time you changed
the launch angle of the ramp? Why did this happen?
2. How did the flight path of the marble change each time you changed the shape
of the track?
3. What happened to the total amount of potential energy each time you changed
the ramp?
The lift produced by ski jumpers is affected by the angle at which oncoming air
hits their skis. Position a flat cardboard edge in front of the flow of air
from a hair dryer. Tilt the cardboard progressively higher. Which angle
produces the most lift? Can the angle of attack ever be too steep?
To decrease friction, skiers use waxes on their skis. Using two blocks of wood,
test different substances on their surfaces to see which offers the least
resistance when you slide the two blocks past each other. Try oil, wax, and
ice.
Ski jumpers are always changing the position of their bodies during the jump to
minimize air resistance. To see how this works for yourself, go for a bike ride
and feel the wind blow. How do you have to ride to get the least resistance?
What similarities and differences are there between bike riders and ski
jumpers?
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