Brian hits the slopes to learn how ski jumpers go so high and far.
Segment Length: 7:30

# Ski Jumping

## 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/

# Ski Jumping

### Launch Control

Vary the angle of a ramp and chart your flight path.

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
• tape measure
• table
• paper and pencil

### Procedure

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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