Breaking down a door is not the exclusive task of burglars and SWAT teams. Some day you might be called upon to break down a door. In case you don’t have special tools at your disposal, you will have to rely on your own body to get the job done.
Note: Breaking a door down should be done only in emergency situations, since trained locksmiths can open the door for you without damaging the door or frame (or your body for that manner). Their service fee is much cheaper than replacing a kicked-in door and splintered door frame.
First, try the doorknob or lever. Push and pull firmly (but not violently) to make sure the door is actually locked and not just jammed shut or warped.
Next, imagine how SWAT teams break down doors, and learn from the physics behind their actions. One or two officers grasp a battering ram and swing it backwards, then slam it against the door near (but not on top of) the lock mechanism. They do not toss the battering ram at the door, or bash the door in random locations. They use steady, consistent blows and follow through with each swing. This is the action you will be imitating with your own body.
Check the hinges, and make sure the door swings into the room. If the door swings towards you, it will be next to impossible to break the door down, since any force you apply will be dissipated throughout the entire door frame. Even if you do succeed in kicking such a door in, you will surely ruin the hinges and the door frame in the process, leading to a costly repair. Don’t bother.
Make sure you’re wearing sturdy shoes with a strong heel. Stand about two to three feet away from the door, balance on your weaker leg, lift your dominant leg, and aim your heel at the area right next to the lock mechanism. Lean forward into the kick and drive your heel straight into that spot, letting the momentum of your body drive the power of your kick instead of relying on your leg muscle to do all the work. (Your knee and leg should be 50% of the kick, and your body weight should be the other 50%.)