measuring force

I want to create a force sensitive training bag that will give me feed back on how hard a student is hitting it.

would i use a piazo buzzer to measure force and the accelerometer to measure speed?

I would use an accelerometer to measure acceleration (which is directly proportional to force).

Probably, what you really want to measure is impulse rather than instantaneous force. In that case you would use a threshold comparison to detect the start/end of the impact and integrate the acceleration during the impact to get the speed change, which is proportional to the impulse.