Focus question

How does a parachute affect the speed of an object falling through the air?

Essential materials

• large handkerchief, scarf, bandana or other piece of lightweight fabric about 30 cm (12 in) square
• four pieces of string, each about 40 cm (16 in) long
• metal washer or other similar weight

Optional materials

• Meter stick or measuring tape
• Stopwatch

Main ideas and background information

• When an object moves or falls through air, it has to push the air molecules out of the way. This creates a resistance force, or drag, on the moving object. The drag acts in the opposite direction of the object’s motion.
• The amount of drag on a falling object is due mainly to the speed and to the size or cross-sectional area of the object.
• The faster an object moves through the air, the more drag it experiences. This is because the object has to push the air molecules out of the way faster. For example, gravity causes the speed of a falling object to increase. As the speed increases, however, so does the drag on the object. Gravity keeps trying to speed the object up, but the resultant drag keeps trying to slow it down.
• An object with a large cross-sectional area, such as a parachute canopy, experiences more drag as it moves through the aid than does an object with a smaller cross-sectional area. This is because the air molecules have to travel farther to get out of the way of such a large object. For example, the opening of a skydiver’s parachute increases the drag experienced by the skydiver.
• When the drag on a falling object is so great that its speed cannot increase any more, it has reached what is called the terminal velocity. The downward weight of the falling object is exactly balanced by the upward drag on the object. In general, the greater the drag on a falling object, the slower the terminal velocity.
• A falling object with a lot of drag, such as a person with a spread-out parachute, reaches its terminal velocity more quickly than an object without as much drag, such as a person with a packed parachute. The parachute acts as a brake, establishing a terminal velocity that is slow enough for a safe landing.

Procedural tips

• If no fabric is available, try a square piece of tissue paper or other lightweight paper.
• Lightweight string is recommended rather than heavy twine or cording.\
• You can use any of several knots to tie the washer to the parachute. But if you use a lark’s head knot (as shown in the activity), you can easily undo the knot and add a washer of a different size or weight for more experimentation.
• Release the parachute from a height of at least 2 meters (about 6 feet) above the floor. This should give the parachute time to open.
• Consider releasing a similar object without a parachute at the same time to compare the drop times. Make sure that the object is released from the same height as the object that is suspended beneath the parachute. (See also Air Resistance.)

Discussion

1. Did the object with the parachute fall quickly to the ground? Why or why not? (The object with the parachute fell slowly to the ground. The parachute increased the resistance to falling (drag).)
2. Why did the parachute produce drag? (The parachute produced drag because of its area. Air trapped beneath the parachute had to travel sideways to get out of the way. The parachute could not travel downward until the air moved out of the way.)
3. What do you think might happen if you release a parachute with an object from a higher point? A lower point? (When released from a higher point, the falling time will be longer. When released from a lower point, the falling time will be shorter. In some cases, the falling time will be very short because the parachute will not have time to open fully. Students can use the optional meter stick or measuring tape and stopwatch to investigate this in more detail. Be sure to observe extra safety precautions when releasing the parachute from greater heights.)
4. How might this activity relate to everyday life? (For example, students might notice drag racers using parachutes to slow down at the end of their races. The NASA space shuttle uses a similar technique when returning to earth for a runway landing. Any kind of parachute use, such as skydiving and parasailing, connects directly to this activity. Sailboats and windsurfing also share a relationship to this activity.)
5. From a safety perspective, discuss the importance of attention to detail in the packing of a parachute. (Parachuting is a dangerous activity. However, attention to safety procedures reduces the danger. A parachute must be packed with care. Even then, a parachute can malfunction. Most parachutists also carry a reserve parachute to be used in the event of a malfunction of the main canopy. These reserve ‘chutes have saved many lives.)

Assessment

Are students able to describe how a parachute affects the speed of an object falling through the air? (Because of its greater area, a parachute increases the drag of a falling object. This opposes the falling motion, decreases the falling speed, and results in a lower terminal velocity.)

Extensions and further investigations

• Have students repeat the experiment using
  – other parachute material.
  – other sizes of parachute material.
  – longer and shorter strings.
  – heavier and lighter strings.
  – objects of different weights.
  – different dropping heights.
• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then ask them to compare the results with their predictions and develop possible explanations for any discrepancies.
• Have students use a stopwatch to measure how much time it takes for an object to fall with and without a parachute. Change one characteristic of the parachute and try again. Continue until they find a characteristic that affects the falling time significantly. Ask them to discuss their findings and rationale.
• Have the students use a stopwatch and meter stick to determine the terminal velocity for a variety of parachute drops. Make sure they wait until the parachute is fully open before they start the timer.
• Ask students to design a series of experiments using a pair of parachutes to test variations. For example, predict and then compare the results for a large and a small parachute each carrying the same amount of weight. Repeat using two parachutes of the same size, but each carrying a different amount of weight.
• Challenge students to design a “better” parachute. But first, have them decide what “better” means in operational or measurable terms. Does this mean the slowest falling speed? Or might it be the smallest and lightest parachute possible that does not exceed a certain terminal velocity? What about strength and durability.

Career connections

Drag increase in engineering projects such as

• Parachute and parasail design
• Sail and wind surfboard design
• Configuring parachutes to drop medical supplies and food into a disaster area
• Parachute systems for pilots who need to eject from damaged or out-of-control aircraft

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B - Physical Science: B1, B2

Grades 9-12
Standard B - Physical Science: B2

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

KS2, Sc3, 1a
KS2, Sc3, 13

Glossary/vocabulary

drag
resistance
terminal velocity

Resource links

United States Parachute Association
British Parachute Association

Examples of parachute organizations. Each site contains many useful and interesting resources.

ParachuteHistory.com

A site with a historical perspective. This site is probably best used as a teacher resource.

Hemisphere Parachute Design

A very interesting site with details about parachute design.

GCSE: Energy, Forces and Motion—Friction: slowing things down

A site with a number of excellent simulations. The one on terminal velocity is particularly well done.

Tom Henderson's Physics Classroom: Lesson 3: Newton's Second Law of Motion

Good explanation of terminal velocity with animations.

The Physics Classroom: Newton's Laws—Elephant and Feather - Air Resistance

More animations for free fall and terminal velocity.