Isaac Newton is well-known for the apple that hit his head and the discovery of gravity. His three Laws of Motion, however, are among the most famous laws of physics. In this lesson, we are especially interested in Newton’s Third Law of Motion—all forces between two objects are equal in magnitude and opposite in direction. We will be studying collisions between two identical carts that are bouncing back-and-forth, much like a Newton’s cradle with just two steel balls. Repelling magnets attached to the front bumpers of each of the carts allow for noncontact collisions.
A PocketLab Voyager on each cart measures x-acceleration during the collisions. Phyphox software records the acceleration data. It creates a real-time scatter plot of the acceleration of one cart versus the other cart. Observable statistical trends on the graph provide strong evidence for Isaac Newton's Third Law of Motion. Figure 1 below shows a bird’s eye view of the apparatus setup for this experiment. The red arrow indicates the positive x-direction for each of the two Voyagers. Balsa wood strips are taped to the table, providing a track to keep the carts on the straight and narrow.
Phyphox (physical phone experiments) is a free app developed at the 2nd Institute of Physics of the RWTH Aachen University in Germany. The author of this lesson has been working with a pre-release Android version of this app that supports BLE (Bluetooth Low Energy) technology to transfer data from multiple Voyagers to the Phyphox app. Since then, a public Android version of the Phyphox app has been released.
The experiment of this lesson is in a file named TwoCarStudy1.phyphox and accompanies this lesson. This file can then be opened in Phyphox and will appear in the PocketLab Voyager category of the main screen, similar to that in Figure 2. For anyone who may be interested in doing a conservation of momentum experiment with a similar setup, please follow this link.
In order to clarify the experimental procedure, the video that follows shows the author performing this experiment. An Android tablet runs the PhyPhox software. A link to a pdf file showing the author's analysis of the data is included with the materials for this lesson.