Key concepts
Aerodynamics
Drag
Flight
Forces

Introduction
Have you ever wondered how a parachute works—or which design features are most important in slowing someone's descent? Parachutes come in many different shapes and sizes, but they work based on the same general principles. In this activity, you will test differently sized parachutes to see how changes in the size of the parachute affect flight. What do you think will work better: a bigger parachute or a smaller one?

Background
In the sport of skydiving, a person jumps out of an airplane from a very high altitude, falls through the air, and releases a parachute to help the skydiver slow his or her way down and land safely on the ground. How does the parachute break the free fall so well?

As the skydiver is falling, the force of gravity is pulling the person and his or her parachute toward the earth. The force of gravity can make an object fall very fast! The parachute slows the skydiver down because it causes air resistance, or drag force. The air pushes the parachute back up and creates a force opposite to the force of gravity. As the skydiver falls, these "push and pull" forces are nearly in balance.*

Materials
•     Heavy-weight garbage bag
•     Scissors
•     Ruler
•     String
•     Four pennies
•     Tape
•     A safe, high surface, about two meters from the ground. A good place may be a secure balcony, deck or playground platform.
•     Stopwatch (accurate to at least 0.1 seconds)

Preparation
•     Cut open the garbage bag to make a flat sheet of plastic.
•     Cut two squares out of the garbage bag. Make one be 20 centimeters by 20 cm (about eight inches by eight inches) and one be 50 cm by 50 cm (20 inches by 20 inches).
•     Tie a knot in each of the four corners of each square.
•     Cut the string into eight pieces that are 40 cm (about 16 inches) long each.
•     Tie one end of each piece of string around each of the knots, positioning the string right above the knot.
•     For each square, hold the center of the square in one hand and pull all of the strings with the other hand to collect them. Tie the free end of the strings together with an overhand knot.
•     Securely tape two pennies to the end of the strings on each square. What do you think the purpose of the pennies is? Hint: you can try letting the squares float to the ground without the pennies first and see what happens.
•     Your parachutes are now ready to test!

Procedure
•     Bring a stopwatch and the parachutes to the safe, high surface you found that is about two meters from the ground.
•     Release the smaller parachute from high above the ground and time how long it takes for it to fall to the ground. Try this parachute two more times, releasing it from the same height each time. About how long did it take to fall on average?
•     Release the larger parachute from the same height and time how long it takes for it to fall to the ground. Try this parachute two more times. About how long did it take the larger parachute to fall?
•     Which parachute took longer to fall to the ground? Why do you think this is?
•     Extra: Make some more parachutes out of differently sized squares from the garbage bag. Test out your new parachutes. Do you see a clear trend between the size of the parachutes and how long it takes them to fall?
•     Extra: In this activity, you tested one variable—the surface area of the parachute—but there are a lot of other variables that affect how well a parachute works. Try this activity again but this time vary the material that the parachute is made out of (you could try nylon, cotton, tissue paper, etcetera), the shape of the parachute, the length of the string, the weight of the string, the load (by increasing or decreasing the number of pennies), or the height at which the parachute is dropped. How does changing one of these other variables affect how well the parachute works?

Observations and results
Did the larger parachute take longer to fall to the ground than the smaller parachute did?

How large a parachute is (in other words, the parachute's surface area) affects its air resistance, or drag force. The larger the parachute, the greater the drag force. In the case of these parachutes, the drag force is opposite to the force of gravity, so the drag force slows the parachutes down as they fall. Consequently, the larger parachute, with its greater drag force, takes longer to reach the ground than the smaller parachute. Although the force of gravity is greater on the larger, slightly heavier parachute than the smaller, lighter one, the relative increase in the drag force on the larger parachute is greater than the increase in the force of gravity.

*Correction (9/19/12): The last sentence in this paragraph, which erroneously described the effect of drag and gravity on the skydiver, was removed after posting.

More to explore
Calculating Drag from Aerocon
Skydiving from the Physics Classroom
Parachute Descent Calculations from Randy Culp
Parachutes: Does Size Matter? from Science Buddies

This activity brought to you in partnership with Science Buddies

3 Comments

1. 1. pwbpwb 08:10 PM 9/6/12

   The last line in the "Background" section is incorrect. It reads, "The drag force from the parachute is slightly less than the force of gravity, so the skydiver floats slowly to the ground."
   
   Forces cause changes in velocity (acceleration), however, and not velocity itself. If a skydiver is slowly floating to the ground without speeding up or slowing down, then the drag force exactly cancels the weight. In reality, of course, inconsistencies in the atmosphere and in the parachute cause slight up and down accelerations, experienced as buffeting, because the drag force is sometimes bigger than the weight and sometimes smaller.
   
   More details:
   Bigger parachutes lead to bigger forces, and bigger speeds lead to bigger forces, too. If you are in freefall when you open your parachute, something like the following happens:
   
   1. In freefall you move fast, so initially there is a drag (upward) force much bigger than your weight (downward) force. The drag overcomes the weight, so there is a net upward force that slows you down, at first relatively quickly.
   
   2. Slower speeds lead to smaller drag forces, so as you slow down, the drag force gets smaller, though still larger than your weight. You come to slow down less and less quickly, but you're still falling.
   
   3. Eventually you approach a point where you fall slowly enough that the drag force is equal to your weight. You stop slowing down and continue to fall at terminal velocity.
   
   4. If, at some point, there is an inconsistency in the air density or the parachute shape or something, the drag force can be less than your weight. In that case you speed up a little, because the net force is down.
   
   5. Speeding up increases the drag force, until it pretty much cancels out your weight again.
   
   6. Such inconsistencies can also cause a bigger drag force, which slows you down a little.
   
   7. Slowing down decreases the drag force, until it pretty much cancels out your weight again.
   
   This repeats until you're ready to land.

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2. 2. waterbergs 10:21 AM 9/11/12

   As per pwbpwb above it is rather tragic that purveyors of physics educational material don't get Newton's First law: "An object will stay at rest or continue at constant velocity unless acted on by a resultant force" Constant velocity = no resultant force = gravity equally and oppositely matched by drag. Not rocket science.

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3. 3. Johnay in reply to waterbergs 10:49 PM 9/22/12

   I have no doubt it's used in rocket science, though. :)

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