2.4 — Jumping Rope – Part III

App Features

Using the force lens students will:

• Use previously recorded performances.
• Open the graph drawer.
• Add push/pull stickers.

Expected Activity Time

• Total Activity Time: 45 minutes
• Introduction: 15 minutes
• Investigation: 20 minutes
• Discussion: 10 minutes

Materials and Prep

• iPad with the Playground Physics app
• Videos created by students in the app
• Projector with iPad adaptor
• Force: Jumping Rope Part III Worksheet

Activity

Introduction: 15 minutes

• Tell students that when scientists study the world they look for trends or patterns in natural phenomena. These patterns are things that happen consistently because they are controlled by the physical properties of Earth. Tell students that today they are going to be physicists and look for the patterns in forces.
• Remind students that in the last lesson they learned about Newton’s Third Law and force pairs. Tell students that they are going to build on those skills by thinking about force pairs in relation to speed using the graphs that the Playground Physics app creates.
• Project the “Thinking about Graphs” portion of the worksheet and have students follow along on their own copies. This worksheet is intended to help students learn to read the graphs in the app. Work through the worksheet as a class.
• “Thinking about Graphs” asks questions that require students to know the difference between balanced and unbalanced forces. Balanced and unbalanced forces explain why force can cause changes in motion when force is also present when someone sits still on a chair. Balanced forces are in equilibrium, which means that for every push, there is a pull of the same size. Unbalanced forces, however, cause changes in motion. An unbalanced force does not have an opposite force that is equal in magnitude and so, while the bigger force will slow down on account of the smaller force, it is still great enough to overcome the smaller force. An object moved by an unbalanced force will slow down and stop eventually due to gravity or interaction with a different, greater force (e.g. a wall).

Investigation: 20 minutes

• Tell students that they are going to look for patterns in the same video they used in the last lesson. Pass out iPads to groups of students. To access the videos their group recorded initially, students should work in the same group and should use the same iPad as in previous lessons.
• Have students pull up the video of someone in their group jumping rope that they used for the last lesson. The path of motion for the person jumping should still be marked on the video so they won’t need to re-annotate the video. Make sure that students have annotated the path of motion for multiple jumps. Because the lesson looks for patterns in forces, students will not see this unless they have tracked the path of motion for multiple jumps in the video.
• Have students open the graph drawer so they can see the graph “along path” of the motion in the video and draw the graph for speed on the space in the worksheet.
• Have students circle the places on the graph where the person was moving the fastest and place a box around the places where the person was moving the slowest. The graph of motion should be cyclical with the speed increasing and decreasing each time the person jumps.
• Have students revisit the place push/pull stickers in the video that they placed in the last lesson. Students may want to redo the stickers now that they have spent more time thinking about forces. How many stickers are appropriate to place on the video will depend on how many times the person in the video jumps.
• Have students describe what is happening when the person is moving the fastest, what is happening when the person is moving the slowest, and the overall patterns that exist in the force pairs in association with the speed of the person who is jumping.
• Have students use their observations to complete the “Patterns in Forces” portion of the worksheet together in their groups.

Discussion: 10 minutes

• Have the class come back together and have each group share the patterns they noted in the investigation.
• If there are discrepancies in the patterns, discuss as a class what could cause the differences groups saw in their patterns. Ask the class if everyone should have the same patterns. Have students share their reasons for why or why not.
• Remember to check the “Parking Lot” of questions at the end of the class period. Remove any questions that have been answered and add any new questions that may have come up.

Answer Key: Thinking about Graphs

1.  
2. Amna’s push on Delenn’s chair is greater than the force of friction from the floor acting on Delenn’s chair, so Delenn moves forward.
3. Delenn slows down because the friction from the ground opposes the direction of Amna’s push force.

Answer Key: Patterns in Forces

1 – 6. Answers will vary

Below are some of the overall patterns in the forces on a person jumping rope that students may notice.

Forces on the person jumping:

• When the person jumps, their feet push down on the ground and the ground pushes back on them. This is the force students are most likely to label with a push sticker in the Playground Physics app.
• The person jumping moves up until gravity pulls them back down towards the ground.

Other forces that are present in the act of jumping rope:

• Friction between the jump rope and the ground.
• Since we are jumping rope on Earth, gravity will be pulling down on everything in the scenario.
• The hands of the people holding the jump rope are exerting a force on the handles of the jump rope as they turn the rope.
• If the person jumping rope doesn’t jump high enough, their leg will exert a force on the rope and vice versa, bringing the rope to a stop.
• Friction between the rope and the air as it turns around. The rope applies force to the air molecules.

Worksheet Previews