Demo Highlight: Newton's Cradle
C. Daetwyler
Welcome to the fall 2024 semester at UMD Physics. The Lecture Demonstration Facility & staff are excited for the fall semester!

If you have any questions about finding the right demonstrations or other resources for your class, access to the order form, or anything else we can help with, be sure to call or email.

Please remember to order your demonstrations before the cutoff deadline for the order form system: For morning classes, before 1PM the previous working day; for afternoon classes, before 4AM the day of the class. Where possible, we appreciate having the orders at least one full working day ahead, to ensure plenty of time to make sure everything is ready for you. Some demonstrations may require more notice if they use particularly complex apparatus or materials that require special handling.

As always, we’ll meet with you before your class to go over the demos. We'll have some new procedures in place, and we look forward to chatting with you all and helping you find the right demonstrations for your class.

Welcome back! Today we’re taking a look at a popular demonstration related to the concept of relativity.

When we observe and measure motion, we are inevitably making the measurement against some frame of reference. An inertial reference frame is the technical term for a frame of reference in which an object is observed to have no outside forces acting on it, so that it is moving freely in space. Sometimes we have to go to great lengths to determine what such a frame of reference might be – and in the case of Demonstration P1-02, it is literally a metal frame!

Read more about this exciting demonstration and how it can be used in class in our latest blog post.

We’re often asked how many demonstrations we have in the collection. That’s a more complex question than it might at first seem.

At last count, we have just over 1,500 demonstrations published to the website – that is, that’s how many demonstration pages exist in the collection. But some pages describe a single setup than can be used in several different ways. Take a look at K2-61: Thomson’s Coil, for example. This single page actually describes four different, related demonstrations that can be performed with this device. They don’t require very different equipment to be delivered, just slight changes in preparation, though, and they’re usually all relevant at approximately the same point in a syllabus, so it’s simpler to list them all in one place. Conversely, there are many demonstrations that use the Optical Board – browse through section L and you will see many of them! Since ray optics is divided into several sections in the demonstrations catalog, each of the configurations of the Optical Board is listed separately, to make it easier to find the one you need; and if you’re only doing one demonstration with it, we can configure it for you in advance to save you time in class.

On the other hand, consider M1-12 and H2-22. These are both listings for Interference Transparencies, a popular way to illustrate the interaction of wavefronts. Here, we made the unusual decision to list the same demonstration twice in two different sections, since otherwise someone planning a course on sound might not think to look for relevant demonstrations in the optics section, and vice-versa. These occasional cross-references make it easier to find the demonstrations you need for your class.

And even aside from the demonstration listings as they stand, we’re often called on to combine equipment in unique ways to demonstrate something new! If it’s a combination that’s likely to be repeated or that proves useful to others, it will be added to the website, but we’re generally open to creatively reinterpreting demonstrations to fit a new class context.

Every year we add more demonstrations to the collection; and occasionally a demonstration is retired, if it no longer meets an instructional need or has been superseded by others. So defining just how many demonstrations we have might not be the right question to ask. Ask, rather, what can we demonstrate for you today?

In support of most classes moving to an online model this year, the Lecture-Demonstration staff are doing our part to help connect you to resources you need for teaching remotely. As one part of this project, we have begun compiling a Directory of Simulations from around the internet, organized by general area of physics. Find it under the Tools and Resources menu above, or click the image below.

There are a tremendous number of simulations out there, that folks have been creating for years. We’re testing them out, choosing ones that we can confirm currently work (always a question as internet technology marches on) and that seem useful for our department’s classes. As of this posting, we have just over fifty simulations collected. Our initial focus has been on physics that is hard to demonstrate in the classroom, or experiments that are difficult to present as static pictures or live video.

This project is ongoing! As we continue to explore we will be adding more subjects and more demonstrations per subject. We also invite recommendations! If you have a favourite simulation, let us know (email lecdemhelp at physics.umd.edu) so we can check it out and add it to the directory.

We’ll have more new projects posted soon; watch the site for news!

In our ongoing work to support remote teaching, we are pleased to announce a new resource. Over the summer of 2020, a Teaching Innovation Grant helped to create our new Demonstration Videos. These can be used for remote, hybrid, and in-person classes to present demonstrations in conjunction with class engagement questions.

The videos have their own YouTube channel, linked both here and on the Tools & Resources Menu above; check them out today!

Science is all about data, and our current pandemic is no different.

Be sure to check the UMD COVID-19 Dashboard for the latest campus data and links to reopening plans and proper safety procedures.

A popular demonstration for illustrating elastic collisions and the conservation of energy and momentum in the classroom is C7-11: Collisions of Balls of Equal Masses. Also popularly called Newton’s Cradle, as it helps us illustrate Newton’s laws of motion even if Newton himself may never have had one, you can find these in many places as entertaining desk toys; but they show us some important physics.

You can see the demonstration in action in our new video featuring Dave Buehrle.

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The simplest and most straightforward explanation for the behaviour of this device is just that – that it is an application of basic conservation laws. The collisions between these hard steel spheres are very nearly elastic, so nearly all of the momentum of the incoming spheres is transferred to the outgoing spheres, and nearly all the energy as well so they rise to the same height on the other side. A pendulum swinging back and forth is a classic illustration of the exchange between kinetic energy (from the velocity of the pendulum) and gravitational potential energy (the potential energy the stationary pendulum has as a result of its position when paused at the top of its swing). And this demonstration is, in a sense, just a set of pendula all swinging together, exchanging their energies and momenta, and we can simplify it be treating only the displaced balls as a single pendulum.

You can likewise see this illustrated in this simulation by B. Surendranath of Hyderabad: https://www.surendranath.org/GPA/Dynamics/NewtonsCradle/NewtonsCradle.html

Try it out at home and see what happens when you change the number of balls you move and how far you move them.

However, we can also explore more complex analyses. We could also analyze the system as a series of coupled oscillators, transferring energy between them much like a phonon in a crystal lattice – the “wave” of motion does have an observable speed, after all, so we could look at it as a propagation problem. Or we could treat each ball as a mostly elastic but slightly inelastic mass, and calculate its interactions with each of the other balls. This might give us an even more accurate picture of

This is a good example of just how the process of doing physics works. No mathematical model of a physical system is every perfect, and different models can be “right” for different situations. We choose the way of modeling a system that bests helps us understand the system at the level we need to understand it at, whether it’s an atom or a galaxy or a desk-top toy.

To explore more about this device, consider reading the article “Newton’s Cradle and Scientific Explanation” by David Gavenda and Judith Edgington in The Physics Teacher. https://aapt.scitation.org/doi/pdf/10.1119/1.2344742