Drag acts upwards on falling objects.
1 pair of scissors
Drag Shapes Handout, one per group
What a Drag! Worksheet, one per student
1 stopwatch or a watch with a second hand
1 meter stick for all groups to share
1 balance; share among teams as necessary
The type of drag that we are most familiar with is form drag. This is the resistance (or pushing) sensation you feel when you walk into the wind. It is caused by all of the air molecules running into your body. Form drag is dependant on the shape of an object, the cross-sectional area of an object, and the speed of an object. In the case of drag, the cross-sectional area is the area of an object that is facing the direction of its movement. For example, if you hold your hand out of a car's window with your palm down, you do not feel much push from the wind. If you turn your hand right, so that your palm faces the direction the car is moving, the wind will push your hand back much harder. This increase in resistance (push) is due to the increase of the cross sectional area of your hand, not the overall size of your hand.
Engineers often consider drag in designing things like airplanes and cars. They try to design these things as streamlined as possible. Streamlined means that the shape of the object, airplane or car can reduce the drag of the object (reduce the force opposing forward motion). This is why airplanes have rounded nose cones and why they pull up their landing gear after liftoff (to remove the wheels from being in the way and creating unnecessary drag). In this activity, we will demonstrate how shape and size affect the drag on something as simple as a piece of paper.
Before the Lesson
Gather materials.
Make copies of the Drag Shapes Handout (one per group) and What a Drag! Worksheet (one per student).
Divide the class into groups of four students each. Hand out the materials, handouts and worksheets.
Experiment setup: Direct the students in each group cut out the shapes from the handout.
Construct the cones by taping one side of the pattern to the other to hold the paper in the cone shape.
Construct the boxes by folding on the solid lines and taping the tabs in place. Leave the boxes with one open side.
Press a mound of clay (or tape pennies or washers) to the bottom inside of each object. If using clay, use the balance to make sure each shape is the same mass; add or remove clay as necessary.
Within each group, each object should have the same mass. To demonstrate that mass does not affect drag, have group 1 make all their objects 10 grams, group 2 = 20 grams, etc.
Experiment and data collection: Select one person to stand on a chair and drop the objects from 2 meters above the floor.
When the person with the stopwatch says "go," the timing begins and the object is dropped at the same time. When the object hits the floor, the timer stops the watch.
Conduct three trials (take three measurements) of how long it takes for each object to fall. Be meticulous about dropping the objects from the same height each time.
Add up the results for each object and divide by the number of trials to get the average for each object.
Have students record their observations on the worksheet. Which shapes fell faster? What sizes fell faster? What does this tell you about the drag on each of these objects?
Data analysis and conclusions: Create a class data table on the board starting with data from the group with the large cone of the least mass to the group with the large cone of the greatest mass along with the average times it took those cones to fall.
As a class, examine all the data. Compare the mass of the different groups' cones to the average amount of time it took to fall for those cones. Does a relationship exist between mass and time? (Answer: Mass does not affect the amount of time it takes an object to reach the ground. Neither drag nor acceleration due to gravity depends on an object's mass.)
Drag Shapes Handout (pdf)
What a Drag! Worksheet (pdf)
Make sure that students stand on stable chairs or tables so that they do not fall.
It may be useful to drop the objects and just record the time it takes for them to fall past a certain point, like from the top of a desk to the floor. Doing this helps to reduce the error caused by the student starting the stopwatch at a slightly different time than the object is dropped. This way, the timer anticipate when the object will pass the tabletop and start and stop accordingly.
To further reduce error, have groups use the same type of tape on each of their objects.
It helps to practice using the stopwatch before timing the trials.
If students have problems with measuring clay on a scale, or if a scale is not available, use pennies or washers with a pre-determined mass. This ensures that all students are using the same mass.
Use a yardstick to measure the heights in the classroom. Students need to drop their object from the same height as the other groups.
To add height to the drop, have students drop their objects from a slide or other playground apparatus.
Prediction: On a piece of paper or on the board have students predict how long they expect each object to take to hit the ground. Which one do they think will be the fastest? Slowest?
Worksheet/Pairs Check: Have students work individually or in pairs on the What a Drag! Worksheet, recording the time it takes for each object to hit the ground, and answering the questinos. Direct students who work in pairs to check each other's answers.
Question/Answer: Ask students questions and have them raise their hands to respond. Discuss as a class.
Which shapes fell faster? (Answer: More streamlined shapes; in this activity, the cone.)
Which mass fell faster? (Answer: Mass does not affect how fast an object falls.)
What does that tell us about the drag on each of these objects?
Have students repeat the experiment with other shapes. For example, use various balls with similar weights to the other objects; try adding clay to an uncut/unfolded piece of paper.
Challenge students to design and complete an experiment to show what happens if clay is placed in the center of a piece of paper so that it falls flat side down compared to if clay is placed on the side of the paper pulling it down by its edge.
Challenge students to create a shape that goes even slower using one piece of paper and tape.
Have students add streamers to their shapes and see if that increases the time to drop.
For younger students, do not add mass to the shapes. Have the class write their times on the board and discuss as a class. Have the teacher calculate the averages for the class as an example before students do their individual averages on their worksheets. As a class, graph the relationship of large cone weight vs. time to drop.