Depends on how you want to go about it, and for probably the first time in my life, something I learned from the show Mythbusters is actually relevant.
By placing an accelerometer in the bag, you could measure the rate of acceleration of the bag, which I'm sure can be applied in various equations to come up with desired information (velocity, etc.).
Another option is placing a 'load sensor' in the bag. Taken from the following link, load sensors are capable of:
"Tekscan's FlexiForce single-element load sensors are at work in a variety of applications, performing a multitude of functions. The sensors are utilized to:
* Detect and measure a relative change in force or applied load
* Detect and measure the rate of change in force
* Identify force thresholds and trigger appropriate action
* Detect contact and/or touch"
All of this equipment and principles are used (and where I got the idea from) in engineering such as crash testing: http://auto.howstuffworks.com/crash-test1.htm