Circular motion icon illustration
Teacher Guide

Circular Motion

Experimenting with forces and variables of circular motion.

What happens to an object moving in a circular motion? What happens when the centripetal force is removed?

This resource was originally published in PhysicsQuest 2020: Force & Motion.

This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.

View the student guide: Circular Motion

What happens to an object moving in a circular motion? What happens when the centripetal force is removed?

  • Balls of different sizes and materials (use the different balls from previous experiments to see if there is a difference)
  • Plates with a rim
  • String
  • Wiffle Ball
  • A camera to record the experiment (optional)

Students will experiment with the forces that act on an object in circular motion and what happens when the circular force is removed.

  • Total time
    45 - 60 Minutes
  • Education level
    Grades 5 - 9
  • Content Area
    Force & Motion
  • Educational topic
    force, motion, centripetal force

In the case of the falling objects from previous activities, we learned that the forces acting on the objects were the gravity pulling the objects down and the drag force pushing the objects in the opposite direction to the fall. In the case of the pendulum, there was a force caused by the pulling of the string and the gravity force pulling the objects down. In the case of circular motion, there is a force that is pulling objects inward, towards the center of the circle. We call it centripetal force.

The existence of a force tells us that there is an acceleration. Even if the object is moving at a constant speed, like the Moon around the Earth, the acceleration comes in the form of the change in the direction of the velocity, without changing the speed. Whatever the object, if it moves in a circle, there is a force acting upon it to cause it to deviate from its straight-line path, accelerate inwards and move along a circular path. In the case of the Moon orbiting around the Earth, the force of gravity from the Earth pulls the Moon towards the Earth as it is also moving forward, causing the circular motion.

The word centripetal only means center seeking, so a centripetal force is any force that pulls the object into a circular path. With a swinging yo-yo, it is the string holding the yo-yo and pulling it towards the pivot point. In the case of orbiting planets or moons, it is the force of gravity pulling the orbiting object towards the object that they are orbiting, that means the force of gravity of the Earth pulls the Moon inwards.

Key terms

These are the key terms that students should know by the END of the two lessons. They do not need to be front loaded. In fact, studies show that presenting key terms to students before the lesson may not be as effective as having students observe and witness the phenomenon the key terms illustrate beforehand and learn the formalized words afterwards. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterwards in the context of what they learned.

However, if these words are helpful for students on an IEP, ELL students, or anyone else that may need more support, please use at your discretion.

  • Mass: A measure of the amount of stuff (or matter) an object has. Not to be confused with weight or volume. Mass only says how much actual stuff there is, not how big an object is or how hard something is pulling on it.
  • Weight: Mass (amount of stuff) times how hard the planet is pulling on it (gravity). This means that your weight on the moon will be 1/6 of that on Earth (gravity on the moon is 0.166 times of that on Earth). However, your mass will still be the same.
  • Force: The push or pull an object feels because of interactions with other objects. If the interaction stops, then there is no force. It is formally defined as mass times acceleration. For example, gravity is a force that represents the pull the Earth has on all objects.
  • Centripetal force: The generic name of any force that pulls objects into a circular path.
  • Drag force: A generic name for any force that resists the movement of the objects. For example, the air resistance that we saw when objects fell in activity one, or the resistance you feel while swimming in a pool when the water slows you down.
  • Velocity: A measure of how fast something is going in some specific direction. Not to be confused with speed, which is only how fast something is moving. “The car was going 65 mph south on I-95” is a measure of velocity. “The rollercoaster was moving at 65 mph when Billy got sick” is a measure of speed.
  • Tangential velocity: The instantaneous velocity in a straight line of an object moving in a circular motion. It is said to be instantaneous because, in a circular motion, the direction is constantly changing, which is why there is an acceleration and a force.
  • Acceleration: How fast the velocity is changing. When something accelerates, it changes how fast it is going or the direction in which it is moving. For a positive change in acceleration means that the object is moving faster, a car going from 30 mph to 40 mph. A negative change means the object is moving slower, the car is going from 40 mph to 30 mph. Finally, a change in the direction of the object’s velocity without changing speed, such as if a car is moving North and turns East still moving, then the car accelerated because the direction of the car’s velocity changed. Remember that velocity is a vector with direction and magnitude, therefore changes in any (or both) of those factors will produce an acceleration.
Experiment 1
Objective

Students will experiment to understand what happens to an object in circular motion.

It is important to understand that student goals may be different and unique from the lesson goals. We recommend leaving room for students to set their own goals for each activity.

Before the experiment
  • Use the Snowball Protocol to ask and discuss: Describe a time when you saw an object move in a circular motion. What made it move that way?

    1. Students begin in pairs, responding to a discussion question only with a single partner.
    2. After each person has had a chance to share their ideas, the pair joins another pair, creating a group of four.
    3. Pairs share their ideas with the pair they just joined.
    4. Groups of four join together to form groups of eight, and so on, until the whole class is joined up in one large discussion.
  • Have students record their thinking in their student guides and finish before the experiment section.

Setting up
  • Take a plate with a rim and put one of the balls on the plate. Give the ball a little push and observe the trajectory of the movement.

  • Think about what forces are causing the movement — you can prompt students to think about the contact of the ball with the plate and how it differs from the force of the air in contact with the piece of falling paper from activity one.

  • Think about what the direction of movement is, the direction of the velocity, and how that connects with the direction of the force. Have students share ideas.

  • Use a second plate and make a cut out on the rim of the plate. Repeat the steps from before: give the ball a little push and observe its movement.

During the experiment
Collecting data
  • In the student’s guide we have asked the students to design their own experiment to test what variables affect a pendulum’s swing. The idea is to encourage them to be creative, to understand how to design experiments, and to think like scientists and engineers. They are given a set of materials that they can use to do their experiments. This is to prompt them, but they should be allowed to use other materials in their design. As the teacher, you can ask prompting questions to get them to think about the different aspects of the experiments. We have included instructions for how to set up the experiment with the just materials included in the kit.

Analyzing data
  • In the student guide, they will analyze their results and compare it to their hypotheses.

  • Continue to listen in on each groups' discussion, answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later.

Teacher tip

Suggested STEP UP Everyday Actions to incorporate into activity:

  • When pairing students, try to have male/female partners and invite female students to share their ideas first.
  • As you put students into groups, consider having female or minority students take the leadership role.
  • Take note of female participation. If they seem to be taking direction and following along, elevate their voice by asking them a question about their experiment.
  • Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.
  • As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.

Consider using the RIP protocol (Research, Instruct, Plan) for lab group visits and conferring.

Consider culturally responsive tools and strategies and/or open ended reflection questions to help push student thinking, evidence tracking, and connections to their lives.

Conclusion
  • What happened to the ball when it got to the cut out. Describe the forces acting on the ball.

Experiment 2
Objective

Students will experiment with the forces that act on an object in circular motion and what happens when the circular force is removed.

Before the experiment
  • Use the Turn & Talk Protocol to ask and discuss: If you swing a yo-yo or other object in a circular motion and then let go, which direction would the object fly?

    1. Pair students up
    2. Give them a minute to think quietly
    3. Give students 2 minutes to discuss their thinking
    4. Have students record their answers or share out to the whole group
  • Have students record their thinking by drawing or writing in their Student’s Guide.

Setting up
  • Take the string and attach it to the whiffle ball. Make sure the string is tight so that the whiffle ball will not fly off.

  • Spin the string with the wiffleball attached. Try spinning it in the horizontal and vertical directions. Ask the students to predict where the wiffleball will fly off if they let the string go and if the direction will change depending on the position of the hand/wiffleball when they let the string go.

  • Do several tests of letting the string go when you are spinning the string vertically and horizontally. While still maintaining circular motion, you’ll want to spin the wiffleball as slowly as possible so that your group can actually determine the proper release point. Let the string go at different positions, for example when spinning up or down, or when it is farthest from you when spinning horizontally.

During the experiment
Collecting data
  • Make sure students are put into intentional groups. See above.

  • Students will complete the experiment using the Student Guide where we have outlined the experiment for students and along the way, they record results and answer questions.

  • Have students predict what patterns they think they will see when the laser shines through the different pieces of cloth.

  • (Optional) Have students look at the different swatches of cloth through the microscope and draw the patterns that they see into their notebooks. If you don’t have access to microscopes, try magnifying glasses, or just have students hold the swatches up to a light.

Analyzing data
  • In the student guide, they will begin calculating the wavelength of a laser.

  • Continue to listen in on each group’s discussion, answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later.

Conclusion
  • Use the Concentric Circles Protocol to ask and discuss: Can you draw some conclusions from the two experiments about what direction the force and the velocity are for an object going in a circle?

    1. Students form two circles, one inside circle and one outside circle.
    2. Each student on the inside is paired with a student on the outside; they face each other.
    3. The teacher poses a question to the whole group and pairs discuss their responses with each other.
    4. Then the teacher signals students to rotate: Students on the outside circle move one space to the right so they are standing in front of a new person (or sitting, as they are in the video).
    5. Now the teacher poses a new question, and the process is repeated.
  • Jupyter Notebook for Python coding PhysicsQuest activity
  • Katherine Johnson related Activity: Math for Circular Motion
  • Choice 1: Sign up for Physicists To-Go to have a scientist talk to your students.
  • Choice 2: Create a video for younger students, how would you explain these concepts to them?
  • Choice 3: Have students write their own definition for key terms using evidence from experiments to support their definitions.
  • Choice 4: Conduct a discussion about how experimentation and evidence lead scientists to change their models.

Credits

Coordination, Research, Text, and Editorial Review Claudia Frachiolla, Jamie Liu, Leah Poffenberger, James Roche, Laurie Tangren, Rose Villatoro, David Voss

Graphic Design and Production Meghan White

Illustrations Isabel Bishop

Updated in 2023 by Sierra Crandell, M.Ed. partially funded by Eucalyptus Foundation

Extension by Jenna Tempkin with Society of Physics Students (SPS)

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