SEPARATE AND SIMULATE

 
sim
 
sim0
You can simulate parts of your object if they are not merged together.

sim1

With your object active, press "K". Click "Separate And Simulate" button. If you only want to separate the loose parts you can click "OK". If you also would like to simulate the parts, select "Rigid Body Simulation".
sim menu
Adjust the settings to you liking and click "OK".

sim2

This will separate your mesh object into separate objects, apply the material (for density calculations) you have selected, calculate the mass of each part based on their volumes, center their origins and apply "Rigid Body" mechanics. If you have optionally selected to have a ground object, one will be added below your objects automatically.
sim3
Go to frame 0 and press "Play" and watch your simulation. :)