Focus question

How do the size and shape of an object affect how fast it travels when dropped in air?

Essential materials

• two identical pieces of paper
• tape
• ruler or meter stick

Main ideas and background information

• When an object moves through air, it has to push the air molecules out of the way. This creates a resistance force, or drag, on the moving object.
• A fast-moving object encounters more drag than a slow-moving object. This is because the fast-moving object has to push harder to get the air molecules out of the way in time for the object to move through them.
• An object with a large cross-sectional area encounters more drag than an object with a small cross-sectional area. This is because the air molecules have to travel farther to get out of the way of a large object.
• A falling object with a lot of drag will travel at a slower speed than a falling object of the same weight (or mass) with little drag. This is because the drag opposes the falling object’s motion.

Procedural tips

• Use A4 paper or similar letter-sized paper. This is a convenient size that can be folded five or six times.
• Accuracy of measurement is important when dealing with variables. The two pieces of tape should be exactly the same length. We found that five centimeters (two inches) long was about right for this activity. Use one to secure the folded paper. Place the other on the second sheet of paper before crumpling.
• For the most consistent results, students should line up the two pieces of paper so that the bottom edges are at the same level before releasing them.
• Listen for the sound of the two pieces of paper hitting the floor. If there is not a noticeable time between the two sounds, try dropping the pieces of paper from a higher level. In our experiment, we had to drop the pieces of paper from a height greater than one meter (three feet) in order to hear them hitting the floor at different times.

Discussion

1. Did the two objects hit the floor at the same time? Why or why not? (The folded paper hit first, and the crumpled paper hit afterwards. The crumpled paper had more drag, so it fell at a slower speed, even though the two paper objects had the same weight.)

2. Why did the crumpled paper have more drag? (The crumpled paper had more drag because it was wider and had more cross-sectional area. This meant that the air had farther to travel sideways to get out of the way. The paper could not travel downward until the air moved out of the way.)

3. What do you think might happen if you release the two paper objects from a higher point? A lower point? (When released from a higher point, the time between the first and second object hitting the floor will be longer. When released from a lower point, the two objects might sound as if they both hit the floor at the same time.)

4. How might this activity be useful in your everyday life? (For example, students might notice bicycle racers wearing streamlined helmets to reduce drag, or parachutists using their parachutes to increase their drag.)

Assessment

Are students able to:

• describe how the size and shape of an object affect how fast it travels when dropped in air? (An object with a lot of drag will fall more slowly than an object with little drag. If an object is wide, it will have more drag than an object with the same weight that is narrow or more compact.)

Extensions and further investigations

• Have students repeat the experiment using
  - other weights of paper.
  - other sizes of paper.
  - materials other than paper.
  - different dropping heights.
• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.
• Develop demonstrations that show how a moving object pushes air out of the way. For example, stand a small sheet of plywood or heavy cardboard on end against the wall. Place small bits of lightweight packing foam on the floor next to the upright sheet. Release the sheet so that it falls over toward the foam bits. Observe the movement of the foam bits as the falling sheet pushes air out of its way.

Career connections

• Drag reduction in engineering projects such as airplane, boat, and automobile design racing clothing and accessories spacecraft traveling through the atmosphere
• Drag increase in engineering projects such as parachute design, sail design

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:

Sc1, Science Enquiry, and the following additional standards:
KS2. Sc4: 2b, c; BoS: 1c, d
KS3. Sc4: 2b, c; BoS: 1c, d
KS4. Revision and introduction to the parachute experiment.

Glossary/vocabulary

drag
resistance

Resource links

GCSE: Energy, Forces & Motion: Friction

Scroll down to the section on terminal velocity.

NASA Glenn Research Center: The Drag Coefficient

Contains background information on the drag coefficient and how the area of an object affects air resistance. Labeled for high school and college use.

Linear Air Resistance on a Projectile

Excellent interactive site with opportunity to explore effects of different initial velocities and drag coefficients. Change the value of the angle to see different effects of the air resistance.

How Does Drag Affect Flight? (Microsoft Education)

This section entitled includes instructions for a slightly different paper activity.

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

This high school physics overview of falling objects and air resistance has an interesting animation.

Simulation Library - Air Resistance

Click on the graphics to run the various simulations that show the effect of air resistance on the falling bodies.