Why Some NHL Players have Shots that are Heavier and Harder to Stop
Former NHLer Al MacInnis had a secret weapon—in addition to having a very hard shot (around 100 miles per hour), Al was able to put a lot of spin on it too. Goalies around the National League used to say Al’s shot wasn’t just hard, it was heavy. Now what did they mean by that and is there a way players can score more goals using Al McKinnis’ technique?
Heavy v Hard Shot
Basically, by putting a lot of spin on the puck, MacInnis added angular momentum (and torque) to his shot. Al would specially shape his sticks before every game (as all NHL players do) and this would help him get off those well known hard, heavy shots.
Basically, angular momentum, for a body that rotates around an axis (e.g., a puck) is related to the mass of the object, the velocity and the distance of the mass to the axis (its radius).
For an object with a fixed mass (the puck) that is rotating about a fixed symmetry axis (its centre), the angular momentum is expressed as the product of the moment of inertia of the object and its angular velocity vector, viz:
is the moment of inertia of the object,
(pronounced ‘omega’) is the angular velocity (the rate at which the puck rotates).
The moment of inertia of the body (i.e., the puck) is then defined as:
m is its mass,
and r is its perpendicular distance from the axis of rotation (the radius of the puck).
So the faster a puck spins (the higher omega is), the higher its angular momentum is. Obviously, the mass of the puck and its radius do not change.
When people calculate momentum of a puck, they usually only calculate it as:
m is its mass,
and v is its velocity.
That is, they ignore angular momentum.
By shaping his sticks, Al could put a heck of a lot of spin on the puck (former Sens player, Alexei Yashin did this too). So the total momentum of their shots should be measured as the sum of angular momentum and momentum from translation. When goalies say someone has a ‘heavy shot’, they probably mean some players just seem to have a shot that is harder to stop even though it may not register as a faster shot on a speed gun than some other player’s shot. The goalies are right.
There are two further points to make:
a) a shot that is spinning faster will not only have larger total momentum than one moving at the same speed (in terms of translation velocity) but with less spin on it but the ‘heavy’ shot will elude a goalie’s glove more frequently or literally spin off his body or pads more easily and into the net because it is spinning faster;
b) also, think about a modern bullet that spins compared to a musket ball that doesn’t. A spinning bullet has much, much greater impact than a musket ball and a lot more penetrating power too.
So if a player wants to score more goals, follow Al MacInnis’ example!
Let’s see if we can try a sample calculation. Suppose a puck weighs 5.5 to 6 ounces (156 to 170 grams), is 1″ thick (2.52 cm) and 3″ in diameter (76.2 mm).
Now let’s says a NHL player slaps the pucks at 90 mph. That is 132 fps.
So momentum in translation is .359 lbs x 132 fps or 47.388 ft-lbs/sec.
Angular momentum is a bit more complicated. First assume an Al MacInnis clone is able to impart spin on the puck equal to an amazing Rafa Nadal tennis shot: 4,500 rpm or 75 revolutions per second. This has to be converted to radians per second (since radians have no units). To do this we multiply by [2 x Pi]. So now we have spin of 471.24 radians per second.
Angular momentum is thus: .359 lbs x (.25 ft/2)**2 x 471.24 or 2.645 ft-lbs-ft/sec. You will notice that the units for angular momentum and momentum for translation are not the same and are thus not additive. I suspect this is either because there is a flaw in this calculation or I have not corrected for the difference in English units between lbs (force) and lbs (mass).
I will leave this for a clever student to fix for moi. But it appears that angular momentum is a significant factor in the overall momentum picture.
ps. you should also know that angular momentum (and momentum as well) is a conserved quantity: a system’s angular momentum stays constant unless an external torque acts on it. Torque is the rate at which angular momentum is transferred in or out of the system. Torque is a quantity which all gear heads understand (and most (young) NHLers like fast cars and faster girls) and you can easily imagine that a puck carrying more torque will, in fact, help a NHL player score more goals and probably get a bigger paycheck, a faster car and a pretty girl to go with it too…
pps. thanks to Wikipedia.org for some of the above equations.
I have thought about this a lot and have always been told I have a heavy shot … when I used to play.
There may be other elements than angular velocity … Spin and Wobble … spin would likely account for most of the “heaviness” but forceful wobble would create another axis of force.
Perhaps related to spin and wobble … area of impact … if the puck hits flat verses side verses edge. (Please see below.)
Ian Graham [ian @ thecodefactory.ca]