This lesson begins the study of circular motion, its causes and applications. This sets the foundation for circular motion where students figure out that the force causing circular motion is directed toward the center of the circle while the velocity of the object moving in the circle is tangent to the circle.
Students explore the forces acting on an object that goes in a circle by performing two investigations. One investigation involves two plastic plates and a marble. One of the plates has a section cut out of it.
The other investigation involves a rubber stopper with a string tied to it. I have enough sets for my class of 24 students, who are working in groups of 2, so that everyone is engaged with either one investigation or the other. I also have goggles for every student as there will be rubber stoppers flying around the room.
This activity involves Science Practice 3: Planning and Carrying Out Investigations. No matter what the object is, if it undergoes uniform circular motion, the force and acceleration are in the same direction. By the end of the activity, students will apply Math Practice 8: where students look for and express regularity in repeated reasoning.
I start off the class displaying the Centripetal Acceleration Power Point. I show the learning objectives which are:
Then I have students participate in a Buzz Groups activity. This is a brainstorming activity where they have 3 minutes to talk to their neighbors and think of as many situations as possible that involve objects going in a circle. One group member writes down the groups answers on a small white board that is at each lab table. This is a great way to stimulate student interest and generate ideas. I spent 2 minutes calling on various groups to share what they came up with.
3 minutes is too much time to spend on the brainstorming aspect of the Buzz Group activity. After about 1 minute students exhausted examples of object that move in a circle and they lost focus on the lesson. I tried out 1 minute with another class; that was the right amount of time for the brainstorming part of the activity.
I tell students: "Today, you will explore the TWO essential conceptual ideas involved with circular motion. These central ideas are applicable to ALL of the situations we just discussed and beyond."
I introduce the investigations students are to do: the first with the plates and marble and the second with the rubber stopper on a string. I instruct the students to break into groups of two; everyone gets a pair of goggles and the Circular Motion Instructions. Each group has 7 minutes to do one of the investigations. Then, the groups switch supplies and spend 7 minutes doing the second investigation. I have the power point slide titled "Circular Motion Activities" displayed to reinforce my verbal instructions.
Working in groups of two, the students are in Student Learning Teams which help foster relationships among students, increase confidence in participating, and encourage the students to discuss what they witness during the investigations.
As students get their supplies, I leave the "Circular Motion Activity" slide up on the board from the Centripetal Acceleration Power Point. Half the class works on the first activity of the plates and marbles. They gently push the marble along the rim of the plate and hopefully notice that the force making the marble go in a circle is the normal force of the plate rim on the marble. They repeat the experiment with the plate that has a section cut out of it. When the marble reaches that section it no longer travels in a circle but goes off in a straight line. The Plate and Marble Movie shows two students engage in this activity.
The second activity involves a rubber stopper tied to the end of a string. The goal is for students to spin the stopper in a vertical circle and to figure out where to let go of the string so that it hits the ceiling directly above them. This demonstrates that the velocity of an object experiencing uniform circular motion is tangent to the circle. The Rubber Stopper Movie shows a student successfully completing this activity. While students are doing these activities, I circulate the room and provide help and guidance where needed.
After all of the groups have completed the two activities, which only takes about 15 minutes, I instruct them to return the supplies and to go to their seats. I call on various groups to review the answers they placed on their sheets, asking if other groups agree with their solutions. The goal here is to get everyone's answers in agreement. Sometimes, it is nessessary to demonstrate the activity for the whole class to see and discuss it further.
Then I instruct students to get out their notebooks and I give a formal lecture from the rest of the PowerPoint (Centripetal Acceleration Power Point) on what they witnessed during the activities. They experience the motion and forces first hand and now we put it into the language of physics using vector diagrams and formulas.
Once the presentation is over, I give the students the Circular Motion Homework to be completed for class review tomorrow. We finish the period with a humorous clip that shows some foolish teenagers using a gas powered moped to spin a playground ride. It is a great example of what happens to an object going in a circle if the centripetal force is removed. The clip is embedded in the PowerPoint (and is also available on YouTube.)
The variety of student answers to the questions are amazing. Students put in things such as gravity or friction being the cause of circular motion for the mable on the plate. My response to an incorrect answer is, "if I removed that force, would the marble not go in a circle?" I lead students to the correct answer of the normal force through a process of elimination.
For many of the common mistakes on the sheets, I am able to catch during the activity itself. However, looking at Student Work - Circular Motion Activity, we can see that a few still remain. For instance, question two asks students what direction the force is in. In this case, I am looking for "inward" or "toward the center". On future sheets, I will give choices so that we can avoid answers like up or left.
Also worth carefully correcting with the students is the velocity vector. Many students like to draw a curved arrow that shows the path of the object. It is important to communicate that the vectors must be straight lines and that they show the velocity at a particular moment. A strategy I like to use is called, "my favorite no" where I show an incorrect answer such as the curved arrows on the second Student Work - Circular Motion Activity example. This answer makes a lot of sense to many students as it does show the path of the ball. But if we discuss what happens to the ball when the centripetal force is removed, it makes more sense to them that the velocity vector must be straight and tangent to the circle at that point.