Conservation of Momentum using Rollerblades and a medicine ball

Peter Somers
Triton High School
Dodge Center, MN 55927

This is believed to be an original activity.
Author Profile

Summary

In this guided inquiry demonstration, a person on rollerblades and a medicine ball are used to teach the concept of conservation of momentum. Various sorts of "collisions" (throws and catches) are used to show how momentum is conserved within a system. This macro-scale demonstration works very well to introduce a unit on impulse and momentum.


Used this activity? Share your experiences and modifications

Learning Goals

This activity is designed for students to get a visual demonstration to associate with the Law of Conservation of Momentum. Students make their predictions about what is to happen in several scenarios and then see if their predictions match what transpires during the activity. Various different scenarios can be analyzed when the medicine ball and rollerblader collide, such as changing the mass of the rollerblader using a different person, mass of the medicine ball, speed of the throw and friction of the floor surface.

Upon completion of this unit, students will be familiar with the concepts of the law of conservation of momentum when an impulse is applied.

Key vocabulary

force
time
mass
velocity
momentum
impulse
friction

Context for Use

This activity is designed to take place during one 45 minute class period. It can be applied in a 9th grade physical science class or physics course. The best location for this is in a long, tiled hallway because the smooth nature of the tile allows for the rollerblader to coast better and makes the changes in velocity more noticeable when the medicine ball is thrown. This is best done at the beginning of a unit on Newton's Laws and momentum because it is fairly basic and qualitative. It works very well to describe elastic and inelastic collisions in a physics class.

Description and Teaching Materials

The hook to this lesson is to be in rollerblades when students walk into the classroom. They are instantly intrigued by the sight of the teacher on wheels. Start the period out with an introduction to the words impulse and momentum. Create working definitions of momentum and then explain what you are going to do. You will be examining multiple "collisions" using a thrown medicine ball and an individual on low friction rollerblades. The activity handout goes through four different scenarios. Give the students 5 minutes to write their predictions about what will take place. Then have them share with their neighbor to see if they agree or if there are differences that exist in their predictions. Then take the medicine ball to the location that you are going to use to teach the lesson. I like a wide open, tiled surface because of the low friction. The best spot in my opinion is in front of the office if possible because it shows how much fun science classes are. Have students line up in a single file line so that they will all get to see the change in motion. Choose a student to assist with the throwing of the medicine ball. You want a student who can throw hard enough to give a decent change in momentum, but who is not malicious and wants to seek revenge for a bad test.

For each scenario on the handout, make sure that you come to a consensus as a class about what happened. Depending on the mass of the medicine ball and the rollerblader, the changes in velocity may or may not be easy to see in certain circumstances, so you might have to repeat the trials a couple of times to make sure students see what they are supposed to see. I have had the best results with the heaviest medicine ball I could find-ask the track or weight-training coach if your school has them. Give students enough time to record their observations on their sheet before moving on. Once you have finished the scenarios on the sheet, have the students create different scenarios that they would like to test. Often times the students will be able to create many good questions that they want to answer to prevent themselves from going back to class. Student Handout for Conservation of momentum with a rollerblader and medicine ball (Microsoft Word 48kB Aug6 08)

Teaching Notes and Tips

One common problem that I have with scenario 4 on the sheet is the medicine ball does not have enough mass to slow the rollerblader down so that it is a perceptible change in velocity. There is a change, and you can feel it if you are the rollerblader, but it is hard to see. A solution for this is to substitute a heavy dumbbell for the medicine ball in this part of the activity. You just need to be a little careful because a dropped dumbbell will crack a tile.

Assessment

Questions similar to the scenarios presented in this activity could very easily be used on any sort of exam format.

Standards

9-12.II.D.1,2 Newton's Laws, forces, momentum, friction

References and Resources