What is the physics behind movie stunts?

Insights

Hardly a movie made today is without some kind of amazing stunt work. We all are held breathless when a person falls out of a 20-story building or when a heart-stopping car chase ends in a spectacular crash. While these may seem to be spontaneous events, every moment of every stunt is carefully planned and controlled by the scene directors, who have an understanding of the basic scientific principles at work.

In falls, bodies obtain their speed because of the net acceleration due to the forces of gravity and air drag. On the earth, the acceleration rate of a free-falling body is 32 feet per second per second of fall (9.8 meters per second squared). This means that for each second the body is falling, its velocity increases 32 feet per second, up to a limiting velocity of approximately 125 miles per hour.

As the velocity of a falling body increases, so does its momentum. Momentum is calculated by taking the mass of a body and multiplying that number by its velocity. When two things crash, it's the rate of change of momentum that determines the force (or "wallop") and does the damage.

To reduce the chances of damage or injury, stunt designers use devices that stretch out the time it takes to stop a body's momentum. These devices "soften the blow," so to speak, both figuratively and literally. The longer the period of time used in changing the momentum, the less force will be released upon impact. Air bags are one of the devices used because they slow down the impact of the falling body by allowing the body to displace a large volume of air. The greater the displacement, the slower the final impact, and the less chance of injury. For very long falls of several hundred feet, stunt people often use decelerators: long elastic ropes that serve as a brake to limit the maximum velocity.

In addition to having the proper equipment, stunt people must know about human anatomy; they must practice a lot; and they must wear the right protective gear. Even with all this knowledge and training, however, stunt people can get injured--the scientific principles they use to their advantage can be just as effective against them.

Connections

1. What are some of the ways that the human body naturally absorbs energy during exercise?
2. Why is it important that stunt people fall on their backs instead of face down?
3. What properties should a material have in order to reduce impact or to prevent a stunt person from being injured by fire?

Vocabulary

acceleration the rate at which the velocity of a moving object changes over time
drag the retarding force acting on a body moving through a fluid, parallel and opposite to the direction of motion
force a push or pull that causes a body to accelerate or change shape
gravity the force that makes objects tend to move toward each other
insulator a material that blocks the flow of heat energy from one region to another
mass the amount of matter a body or object contains; a measure of the inertia of a body or object
velocity the speed of a body moving in a certain direction.

Resources

Baur, T. (1989) Special effects and stunt guide. Beverly Hills, CA: Lone Eagle.

Dittman, R., and G. Schmieg. (1979) Physics in everyday life. New York: McGraw-Hill.

Ehrlich, R. (1990) Turning the world inside out. Princeton, NJ: Princeton University Press.

Grant, C. (1990) Stunts. New York: Tor Books.

Hewitt, P. (1991) Conceptual physics, 2d ed. New York: Addison-Wesley.

Additional sources of information:

Universal Studios Hollywood
100 Universal City Pl.
Universal City, CA 91608
(818) 777-3801

Hollywood's Stuntman's Hall of Fame and Museum
P.O. Box 277
Moab, UT 84532
(801) 259-6100

Main Activity

Test several different materials to see which are the best insulators of heat energy. Find the material that will take the longest amount of time to heat up and the least amount of time to cool down.

Materials

• 3 cups polystyrene-foam packing peanuts
• 2 wool socks
• 3 cups sand
• empty 1-lb. coffee can with lid
• long Celsius lab thermometer
• watch with a second hand OR a stopwatch
• paper and pencil
• scissors
• portable 1000-watt hair dryer

1. Set up a log sheet as suggested here:

Material

Temp up 10deg.

Temp down 10deg.

Wool
Polystyrene foam
Sand

2. Stuff the woolen socks inside the can. Put the lid back on the can.
3. Using scissors, poke a small hole in the lid. Slip the thermometer through the hole until it is about half-way into the can.
4. Allow the can to sit for five minutes. Then read the temperature on the thermometer and record this as your "starting temperature."
5. Begin heating the outside of the can by moving the hair dryer slowly around all sides of the can. Using a watch or stopwatch, start timing the experiment now. See how many seconds it takes for the thermometer to go up 10 degrees. Record this on your chart as "time up."
6. When you hit the 10-degree mark, turn off the hair dryer and time how many seconds it takes for the temperature to go back down to the starting point. Record this on your chart as "time down." (The temperature may continue to rise after removing the heat source. Record this on your chart as the "maximum temperature.")
7. Repeat the experiment again using the sand and foam peanuts in place of the wool socks.

Questions

1. Based on your experiments, which materials would make the most effective liner to protect a stunt person from heat? Why is it necessary for the material to heat up slowly and cool down quickly?

2. Did any of the materials get significantly hotter after you turned off the heat? What would cause this to happen?

3. What other materials could you test?

What types of protective gear developed for stunt people have been incorporated into our daily lives? Which types of protective sports gear use the same principles as air bags and decelerators? How might you modify this equipment so that it is even better at reducing the force of impact?

Experiment with how different types of bungee cords and springs could be used as decelerators. Attach a large weight (e.g., a pail full of sand with a lid on) to one end of a cord or spring and let it freefall. Try to quantify your experiments by attaching a spring scale to the upper end and measuring the maximum force. What properties make for the best results?

Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.