It feels good to punch hard. As long as you maintain good form you will not get injured. Each time you connect with a solid punch the force of your blow reverberates back through your muscles, making them stronger, strengthening your confidence, too. Punching provides you with the experience of being effective. Athletic stores generally sell forty, sixty, or eighty pound heavy bags.
Boxing gyms tend to have one hundred to one hundred fifty pound heavy bags.
The lighter a punching bag is the more it moves around when you strike it. Lighter bags are good for practicing precision and control. The heavier bags move around less, which means you can pound on them without them swinging out of range. The surface of heavy bags is either vinyl, canvas, or leather. You use gloves when you punch, so you do not have to be concerned with whether or not the surface material will be hard on your hands.