The Slap shot is by far the most powerful type of shot that can be performed in the game of hockey. Not only is it extremely powerful, it is all possible due to physics. By hitting your stick on the ice just before the puck, it allows for full body weight transfer and stick flex to be used in the shot, which is called momentum. The more momentum there is, the harder the shot will be, this generally explains why larger people tend to have a harder and faster shot. Below is Zdeno Chara who has currently won the hardest shot competition the last three years in the NHL, breaking his previous record of 108.8 mph. Chara is the largest player in the NHL at 6' 9" and 255 lbs, according to the Boston Bruins website. With Chara being as large as he is, he is able to create a tremendous amount of momentum by transferring his mass and creating high rotational motion put into his shot. 

Momentum is known as a closed system and it is always conserved. When taking a slap shot, there are many components that cause friction; the stick and ice, the puck and ice and the stick and puck. Many people may not think it is a factor, however there also is resistance from air. Although the air resistance is minimal, it still plays a factor. Due to the conservation of momentum, when a player such as Chara takes a slap shot, the faster he swings his stick and transfers his mass it will result in a harder shot, providing that friction is minimal. The smoother the ice will limit friction and provide a faster shot. 
-Rotational Motion(Momentum)=Inertia * Angular Acceleration
-Linear Motion(Momuntum)=Mass * Velocity

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