CYCLE 3 Developing Ideas ACTIVITY: Gravitational Interactions Purpose We live on a world where, if not supported, everything seems to want to fall to the ground. Even if we throw a ball straight upward, eventually it reverses direction and falls back down. In the rest of this cycle you will investigate the cause and characteristics of this seemingly natural tendency of objects to fall. Why do objects fall? How do they move while they are falling? Initial Ideas Have one of your team hold their pencil about 1 meter above the table, then release it and let it fall. Discuss these questions with your team. While it was falling what do you think the motion of the pencil was like? (Constant speed, increasing speed, decreasing speed, or some combination of these?) Sketch what you think a speed-time graph for the 2005 Physics for Elementary Teachers (PET) Written by Fred Goldberg, Steve Robinson and Valerie Otero 2005 It is About Time, 15 Modified in parts by Armen N. Kocharian, 04/01/2006 3-15
Cycle 3 motion of the falling pencil would look like. Do you think an interaction caused the pencil to fall? If so, what objects do you think were interacting? If not, explain why you think the pencil fell. Participate in a short class discussion about these questions and make a note of any ideas or reasoning different from those of your team. Collecting and Interpreting Evidence Experiment: What is the motion of falling objects like? This experiment may be done as a teacher demonstration. 3-16
Activity 2: Gravitational Interactions You will need: Large ball (such as a soccer ball, or basketball) Access to a Motion Sensor connected to a computer Stand on which to mount the motion sensor vertically Access to the I&M computer simulator STEP 1: Mount the Motion Sensor on the stand. Move the stand to the edge of the table, and orient the sensor so that it is pointing down toward the floor, as shown. (Make sure the sensor is at least 1.5 m above the floor.) Open the motion sensor data collection file for this activity. STEP 2: Now hold the ball about 20 cm below the Motion Sensor and start to collect data. As soon as data collection has started let the ball drop to the floor. (The data collection should stop automatically as soon as the ball gets close to the floor.) Describe the motion of the ball after it was released, for the short time shown on the graph. Was a force acting on the object while it was falling? What evidence supports your answer? 3-17
Cycle 3 What force do you think is responsible for this motion? STEP 3: The experiment with the ball only showed its motion for a short period after it was released. If you allowed an object to fall for a longer time, do you think its motion would change significantly? Explain your reasoning. Sketch the speed-time graph of the falling object and describe its motion below. 3-18
Activity 2: Gravitational Interactions During the time it was falling, was the object involved in an interaction? How do you know? Two students were discussing falling objects. They are not sure about whether a falling object is involved in an interaction with another object, or what that other object involved in the interaction might be. I know things fall because of gravity but I don t think there is an interaction because there is no other object involved it s just gravity. But if there is a force acting on the falling object, it must be interacting with another object somewhere. I m just not sure what that other object is. Daryl Luisa Do you agree with Daryl or Luisa, or neither? Briefly explain your thinking. 3-19
Cycle 3 What causes gravity? By making several observations of falling objects, the English scientist Isaac Newton constructed a theoretical argument to support the idea that gravity is an interaction (or force) that acts at a distance between any two objects that have the property we know as mass. In other words, Newton said that all objects with mass attract all other objects with mass even though they are not touching each other. However, the gravitational interaction between normal everyday objects (such as two people) is extremely weak, and so not noticeable. But, because the Earth itself is so large, the gravitational interaction between the Earth and every other object near it is large enough to be very noticeable. Years later, another English scientist named Cavendish conducted an experiment that actually provided evidence to support Newton's gravity is an interaction idea. Under carefully controlled conditions, Cavendish brought massive lead balls near, but not touching objects on a test balance and showed that they attracted the objects on the balance. A schematic diagram of Cavendish s experimental set up is shown here. Cavendish observed that when the large lead balls (M) were brought near the small balls (m) as shown, the rod holding the small balls rotated slightly. Schematic view of the Cavendish apparatus He observed this rotation by shining a light on the mirror and looking at its reflection on a wall. Cavendish used this particular arrangement because it is very sensitive to weak forces, allowing the hanging objects to react to those forces without the interference of friction. Your instructor will show you a video clip of the Cavendish experiment being performed with bottles of water and boxes of sand. 3-20
Activity 2: Gravitational Interactions Summarizing questions Discuss these questions with your team and note your ideas. Leave space to add any different ideas that may emerge when the whole class discusses their thinking. S1: While an object is falling toward the ground is it an energy source, or an energy receiver (or neither)? Explain your reasoning. S2: How can you account for the observation that objects close to the Earth feel its influence, even when they are not touching it? (Hint: How did you account for this same behavior in magnetic interactions.) S3: In the gravitational interaction between the Earth and a falling object you only see one of the objects involved move (the one that falls). Do you think the Earth moves too? If so, why don t you observe its motion? 3-21
Cycle 3 S4: Complete the energy diagram below for the interaction that caused the ball to fall, when you released it. Interaction Energy Source Field of ball and Earth Mechanical Energy Energy Receiver Ball and Decrease in Increase in motion energy of and impercebtible increase in motion energy of (Note: In future energy diagrams, when the change in motion energy of the Earth is imperceptible, it is permissible to omit this change.) S5: In the past many people have suggested that the rotation of the Earth might be what causes the gravitational force of the Earth. What evidence from your own experience supports or refutes this idea? S6: Some people have also suggested that the Earth s magnetism might be the cause of gravity. Again, what evidence can you think of that might support or refute this idea? 3-22