Exploring bouncing balls

Authors: Judy Scotchmoor and Lisa White

Overview: In this lesson, students will explore the physical properties of a variety of balls and how they bounce (i.e., their bounciness or elasticity). The point of the activity is not necessarily to have students arrive at a precise explanation for the phenomena they are investigating, but to provide students with the following experiences: initial exploration (observation, conversation, questioning, wondering); identification of a question to explore; articulating the question; developing expectations/predictions; formulating explanations; testing their ideas; modifying their initial ideas; and sharing their results with others. Students then reflect on the process they used by charting their pathway on the Science Flowchart.

Lesson concepts:

• The process of science involves exploration, testing, communication, and application. Making observations is a critical aspect of the process of science. Often scientists make many observations of a phenomenon before they formulate any hypotheses about it.
• The process of science is non-linear and iterative.

Materials:

• Five different kinds of balls, all of which bounce pretty well: tennis ball, ping pong ball, super balls of different sizes, solid rubber ball, etc. Note: As the mass of the ball is an important factor in this exploration, either have a balance available for student use, or find the mass of each ball prior to the experiment and mark each ball with its mass. Prepare a bag of five different balls for each group.
• A meter stick for each group
• Scratch paper
• A Question to Explore form for each group
• A copy of the Science Flowchart for each student

Time: One class period

Grouping: Small groups and whole discussion group

Teacher background: This lesson is really about initial exploration. However, if your are interested in the physics behind how balls bounce, the following principles apply: The ball begins at rest from height h with potential energy mgh, where m is its mass and g is the acceleration due to gravity. On first bounce it comes up to a height d, corresponding to potential energy mgd. The coefficient of restitution, d/h, is the fraction of mechanical energy remaining after first bounce. Each time the ball bounces, energy is dissipated in the form of heat and acoustical energy and the height is reduced by roughly the same factor: the coefficient of restitution. d1/h = d2/d1 = d3/d2 = …. (Excerpted from Bouncing Balls by Porter Johnson, Physics Department, Illinois Institute of Technology, http://www.iit.edu/~smile/ph9417.html)

Teaching tips: Students should be introduced to the Science Flowchart prior to this activity. For suggestions of how to do so, visit the Understanding Science website:

Procedure:

1. Stand, where all students can see, holding one of the balls about chest high. Ask students what they think will happen when you drop the ball. After they have shared their thoughts, drop the ball. Ask the students if what they expected to happen actually happened.
2. Tell the students that you are going to repeat the dropping of the ball, but this time they should concentrate on what they observe in greater detail. Drop the ball. Ask students for their observations.
3. Hold the ball up as far as you can reach. Tell the students that you are going to drop the ball from this height. What do they think will happen? Drop the ball. Ask the students if what they expected to happen actually happened.
4. Explain to students that they are going to explore how balls bounce. Show them the materials that they will be using (bag of balls and a meter stick). Tell them that first they will be given five minutes to simply explore the bounciness of the five balls. Pass out the materials and let them explore.
5. After about five minutes, ask the students to return the balls to the bag and ask them what they discovered. Capture a summary of their findings on the board. Ask students to identify the variables involved in their exploration (height of the drop, hollow vs. solid, mass, floor surface, elasticity, etc.).
6. Tell the students that they will now have an additional 5 minutes to generate as many questions as they can about how balls bounce. For example, does the height at which I drop the ball effect the height of its first bounce? Tell them that the goal is to write down as many questions as they can in the time provided. The questions should be written on the scratch paper provided.
7. After five minutes, go from group to group and ask them to share one of their questions. Write the question on the board. Continue to go around the room until all questions have been listed. Examine the list and, if necessary, ask the students if any of the questions should be eliminated for now either because they are not possible to investigate given the materials that they have, or they do not directly address the focus of the investigation how balls bounce.
8. Give two minutes for each group to select a question to investigate. Once all of the groups have decided on their question, pass out the Question to Explore form to each group. Have them write their question in Box #1 (e.g., Why do some balls bounce higher than others on the first bounce?). In Box #2 they should write what they think will happen (expectation/prediction; e.g., a hollow rubber ball will bounce higher than solid rubber ball.). In Box #3, they should write why they think that will happen (their explanation or hypothesis; e.g., the air inside a hollow ball acts like a spring and helps push the ball off the floor better than a solid). Formulating an explanation/hypothesis can be difficult. You can help them with this by asking them to fill in the following sentence: I think X will happen because Y. Y represents the hypothesis. Point out that there is a second column called Modifications. Encourage the students to modify their question, expectation, or explanation as they continue their investigation. They may find that their first question needs to be narrowed.
9. Give each group 30 minutes to test their ideas by conducting an investigation. NOTE: You may opt to have one student in each group serve as an observer to the group investigation beginning at step 4. That student can take brief notes of what the team is doing as they are working and then trace the pathway of the team on the flowchart based upon the notes taken.
10. After 30 minutes, have students return all of the balls to the plastic bag and prepare to share their findings with the rest of the class. As they do so, have them state their question, expectation, and explanation, encouraging them to explain any modifications that they made. Then they should briefly explain how they tested their ideas and what they found out. From the group findings, summarize what they have learned about elasticity and how balls bounce.
11. The critical component of this lesson is to provide students the opportunity to reflect on what they did! If you had an observer for each group, that person should share with the group what they did and show them how to chart the group's path on the Science Flowchart. OR the group members can work together to remember what they did and chart their path on the Science Flowchart.
12. Share the pathways developed from the student reflections by posting them in the room and discussing them as a class. Steer the discussion so that the students are aware that the scientific journeys varied from group to group, that they were all nonlinear, that they involved multiple phases of testing and revision of ideas, and that they didn't necessarily produce a firm conclusion. Students probably have many more questions about bouncing balls after this lab than they did before the lab!

An Understanding Science lesson
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