Simple harmonic motion is not only a foundational topic in physical science, but also a major player in many different fields from music to engineering to architecture, to sports. The iPad can be used to generate a real-time visual presentation of harmonic motion, both simple and complex, with just a few household parts and your classroom wireless network and projector.
[youtube]http://www.youtube.com/watch?v=HoeL2q9pVwc[/youtube]
The iPad display can be mirrored wirelessly through an Apple TV to a LCD projector or monitor. This allows the iPad to be both sealed in a plastic bag, and moving around. Note that the Apple TV works just fine with a wireless router that is not connected to the internet. In other words, the iPad can talk to the Apple TV through a wireless router independent to the internet as a whole. So this system works as long as both the iPad and the Apple TV are logged into to the wireless router regardless of what the router is connected to.
A free iPhone app called iSeismograph (or the larger pay version for the iPad) works fine, but there are many options. The iSeismograph, among other features, offers a graphical display of X, Y and Z axis movements of the iPad.

Public domain work by  Mazemaster

 
In the iPad demonstration described here and shown in the YouTube video, a full-sized iPad is placed in a large, strong, clear plastic bag. A hole has been punched through the bag just below the resealable closure where a small carabiner will be attached. The closure mechanism adds additional strength so the bag does not tear lose and fall–but be smart, don’t skimp on the bag with a $400+ iPad bouncing above a hard floor.
The bag’s carabiner is attached to a spring or two depending on the stiffness of the springs. The iPad can be operated just fine thorough the plastic bag. Make sure the other end of the spring is attached well since a strong bounce could cause the upper connection point to come unhooked dropping the iPad to the ground.

The above graphic was released into the public domain by its author Oleg Alexandrov

 
With the iSeismograph operating and AirPlay mirroring the iPad screen through the Apple TV to a projector, the iPad is put into motion and its simple harmonic motion is visible onscreen.
Additional motions can be added if desired by spinning the iPad or adding a swing to the bounce.  If precise numbers are desired, explore the settings of your App to see what’s available. It might surprise you just how sophisticated these little Apps are!