Newton's 2nd law: Inquiry approach

Author: Cecilia Tung

Overview: In this lab activity, students act as fellow scientists and colleagues of Isaac Newton. He has asked them to independently test his ideas on the nature of motion, in particular his 2nd Law. The emphasis here is on the process of science rather than the actual results. Students need to focus on how they would design a procedure to test Newton’s hypothesis and then communicate that idea to others.

Lesson concepts:

• Science deals with the natural world and natural explanations.
• The process of science involves observation, exploration, discovery, testing, communication, and application.
• Scientists decide on their own methods of investigation.
• Experimentation involves controlled manipulation of variables in order to establish causal relationships.
• Scientists organize data in tables and display data visually in graphs.
• Scientists aim for their observations and tests to be replicable.
• Multiple trials of an experiment are required to see patterns in the data. Scientists decide how many trials to run.
• Error is present in every scientific experiment.
• Scientific experimentation does not prove a relationship between variables.

Materials: As students will be developing their own procedures, you will want to provide a variety of materials from which they can select. These include standard high school physics lab materials: dynamics carts (small carts with wheels), stop watches, meter sticks, pulleys, string, and masses. Depending on the level and background of the class, you may want to provide a CBL (calculator based laboratory), force sensor, and accelerometer or photogates in order to collect more precise data.

Time: One to two class periods

Grouping: Threes or fours

Teacher background: Review the information in How science works in order to identify key concepts that you wish to reinforce regarding testing of scientific ideas, the logic of a scientific argument, interpreting data, making assumptions, replication, and peer review.

Teaching tips: This is a lab activity that is usually given to students with a predetermined procedure that they follow. In this version of the lesson, students must come up with their own procedures to verify Newton’s 2nd Law. As the emphasis here is on the process of science rather than the actual results, some students may not see this as "real" science. It is in fact a huge part of the scientific process carried on by real researchers. It is important for students to realize how much time can and should go into designing a good procedure.

Within this activity, Newton’s 2nd Law is referred to as a hypothesis as the students are supposed to be acting as contemporaries and testing his ideas.

Instructions have purposely been kept to a minimum to that students come up with their own ideas rather than follow a specified method. You may need to add further guidance for some students.

Procedure:

1. Begin by describing the following scenario to your students: Imagine that you are a contemporary of Isaac Newton. He has been working feverishly on understanding the nature of motion and believes that he has distilled all of his observations and experiments into a few succinct "laws" that seem to govern how objects move. You are one of his most trusted friends and fellow scientists. He has developed a very systematic method of running his experiments, but he wants you to develop your own method independently of his to really test whether his ideas are correct.
One aspect of Isaac’s explanation can be summarized by the following equation:

Fnet = (mass) x (acceleration)

If an object accelerates, it is because a net force acts on it. A net force is the result of combining all the forces acting on an object. Taking into account size and direction, the net force on an object has a size measured in newtons and a specific direction. The resulting acceleration of the object will be proportional to the net force and in the same direction as the net force and inversely proportional to the mass of the object. Here is a specific example. A box full of apples is pulled by a force of 25 N to the right as shown below. There are three other forces acting on the box…gravity, friction, and the support force of the floor. If Newton is correct, the acceleration of the box times the mass of the box will be equal to the sum of the forces on the box taking into account direction. So the support force and gravity cancel each other out and the net force is simply the difference between the applied force and friction. Since the applied force is bigger, the net force will be in that direction.
2.

3. Provide the following directions to your students: With a partner or group, determine a method to test Newton’s hypothesis that a net force produces acceleration in the same direction, proportional to the net force, and inversely proportional to the mass of the object. Write a complete procedure that uses available equipment and measuring devices to test all or part of this hypothesis. The procedure should include some background information about the relationship between the variables and what you are trying to test. Include a data table and any specific assumptions that need to be made in order to complete the experiment (For example, are you assuming that friction is constant throughout the motion?). Provide an explanation of any sort of analysis that is necessary. Be as specific as possible.
• What relationship are you trying to test?
• There are three variables in the equation. Which two will you focus on first to test?
• How will you keep the other variable constant?
• What kind of equipment do you need to measure the other variables?
• What kind of data do you expect to get?
• How will you display and analyze that data?
• Do you need to make any simplifying assumptions? Are they justified?
• What other equations do you need to use to complete the test?
• Will you be testing all aspects of Newton’s hypothesis?
• Will you be "proving" that his ideas are correct?
• What role does "error" play in the design and execution of the experiment?
4. Have students share their experimental method with students in another group. Advise them to look objectively at the second design and note the similarities and differences to their own method.
5. Provide time for student reflection. Prompt: What was the path that you followed in making initial observations and in designing, carrying out, and interpreting your experiment? At which points did you experience your greatest successes? At which points did you experience your biggest challenges? In the end, what did you learn most about the process of science? As a class, discuss the different methods used by each group. In particular, discuss whether all students used the same approach, what adjustments had to be made to their original ideas, and whether or not students felt that they had "proved" Newton to be correct.

Extension: Students can reflect on the process of science by charting their pathways on the Science Flowchart.

Have students write a summary of the steps that they took in their process and number them. Record the numbers for these steps on a copy of the Science Flowchart. Draw a small circle around each number. Then "connect the dots." Students can then compare their pathways.

An Understanding Science lesson
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