Hard, Heavy Shots Versus Fast Slapshots—
Why Some NHL Players have Shots that are 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.
So 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?
Al MacInnis with the
Basically, by putting
a lot of spin on the puck, Mr. MacInnis was adding 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:
where:
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:
where:
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 the angular momentum is. Obviously, the mass of the puck
and its radius do not change.
When people calculate the momentum of a
puck, they usually only calculate it as:
mv
where:
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. So the total momentum of his shot should be
measured as the sum of angular momentum and momentum. 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 the 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.
So if a player wants to score more goals,
follow Al MacInnis' example!
Dr. Bruce M.
Firestone,
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 will 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 pay check, a faster car and a pretty girl
to go with it too...
pps. thanks to Wikipedia.com for
some of the above equations.
http://www.dramatispersonae.org/
http://www.dramatispersonae.org/ShortFormResumeParsed.htm