Concept:

A punching bag that plays relaxing music when you hit it. The sound will change according to the direction of the punch, so the user will be able to play with the music and make different melodies with the punches. Also, the force of the punches will be taken into consideration, there will be a difference between hitting harder or softer. Probably, a change in strength will translate into a change in the sound volume.
Philosophically speaking, the system will take ‘anger’ as an input and throw ‘relaxation’ as an output.

Input:

We discussed several alternatives for detecting the punch.
Putting FSRs in the punching bag doesn’t seem good, because that will limit the hittable area. Also FSRs tend to be fragile, we are afraid they are going to break with use in this case, and we don’t know if they are good for detecting different grades of punches.
If we put the FSRs in the gloves, we avoid the limiting surface problem, but, on the other hand, that would require wiring in the gloves.
The best solution, in our opinion, will be putting an accelerometer in the punching bag. This way, we detect the generated acceleration from the punch. Accelerometers, also, give input of the acceleration in two (or even three) dimensions, so with a single accelerometer we can obtain the direction and intensity of a punch by analyzing the two vectors.
Lastly, a switch so it can be easily turned off.

Output:

Sound.
We want to output MIDI protocol through serial communication. There are programs for faking a MIDI port through USB. Then, a program like GarageBand or Ableton will play the MIDI music.
Some links:
https://sites.google.com/site/bharatbhushankonka/home/diy-midi-over-usb-using-arduino-uno
http://spikenzielabs.com/SpikenzieLabs/Serial_MIDI.html
http://lizarum.com/assignments/physical_computing/2012/sound/arduino_garage_band.html

Materials:

Punching bag: sold at $30 in amazon
Punching gloves: sold at $15 in amazon
Accelerometer
Computer
Switch
Speaker