Student 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.

Woman demonstrates a ball in 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)
Experiment 1
Intro

We have already looked at the motion of objects falling in one direction as when you drop them or in two directions as part of a pendulum. We have also looked at what happens when objects fall while also moving in the horizontal direction. In this last activity, we are combining all that knowledge to see what happens when objects move in a circular motion: there are constant changes in the horizontal and vertical direction at all times during the movement, making the object follow a circular path. Examples of circular motion are carousels or merry-go-rounds in parks, a car going around a roundabout, the Moon orbiting around the Earth, or the Earth revolving around the Sun.

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

After reading the introduction, what is your essential question or objective for this activity?

Before the experiment

  • What do you think happens when you give a little push to a ball while it is in a circular container?

    • How will the ball move?
    • What direction does it go?

  • After your push, what is the force causing this circular motion?

  • What do you think will happen? Write a hypothesis.

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. Student: shared 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 what direction is the ball moving when you give it a push? Think of the definition of force and indicate what forces you think are acting on the ball. Draw this in your notes.

  • Draw a labeled sketch or write the process of your experiment

  • How will your design help you confirm or deny your hypothesis?

  • What variables are present in your experiment and how are you controlling for them?

  • Collect your data in a table. Record your results.

Analyzing data

  • What were your results?

  • Did your results support your hypothesis and why or why not?

  • Did you get the same results every time? Why or why not?

  • Additional observations

Conclusion

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

Experiment 2
Intro

For activity two, we need you to design an experiment to test what direction a whiffle ball and string will fly if you let go of the string when you are spinning it. Before you start, think about what force or forces are acting on the Wiffle ball as you spin it. What is the specific force directly causing the Wiffle ball to follow a circular path? Hint: though you are applying a force to the string, you are not connected to the Wiffle ball. In what direction is this force? Draw a diagram on the next page to show the force and direction.

Before the experiment

  • If you swing a yo-yo or other object in a circular motion and then let go, which direction would the object fly?

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 whiffle ball 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

  • Draw a labeled sketch or write the process of your experiment.

  • How will your design help you confirm or deny your hypothesis?

  • What variables are present in your experiment and how are you controlling for them?

  • Collect your data in a table. Record your results.

Analyzing data

  • What were your results?

  • Did your results support your hypothesis and why or why not?

  • Did you get the same results every time? Why or why not?

  • Additional observations.

Conclusion

  • 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?

  • What direction will the whiffle ball fly if you let go while you are spinning it?

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