Slapshot Physics: Hockey
The slapshot is the fastest, hardest shot in ice hockey--and an excellent illustration of elastic collisions, energy transfer and momentum exchange. NSF-funded scientists Thomas Humphrey of The Exploratorium in San Francisco, and Kathy Flores, an Ohio State University materials scientist, explain, along with U.S. Olympic hockey players Julie Chu and Zach Parise.
LESTER HOLT, anchor: One of the most popular team sports at the Winter Olympics is hockey. More than just a physical game, for scientists like The Exploratorium’s Thomas Humphrey, a physicist funded by the National Science Foundation, hockey is a showcase for physics on ice – especially when it comes to hockey’s hardest shot: the slap shot.
HOLT: In a game known for hard hits, it may be the hardest one of all. The violent collision between stick and puck that produces one of the fastest projectiles in the Olympics: the slap shot.
ZACH PARISE (U.S. Hockey Team): For me, probably about 85, 90 miles an hour, not very fast.
HOLT: More than a feat of physical strength, the Olympic slap shot is a showcase for the physics of elastic collisions – the nearly perfect transfer of kinetic energy from a hockey stick to a frozen six-ounce rubber puck.
Thomas Humphrey is senior scientist at The Exploratorium in San Francisco.
Dr. THOMAS HUMPHREY (The Exploratorium): Two Hundredths of a second is the amount of time that the stick is in contact with the puck. But the average force on the puck is about 100 lbs.
HOLT: To see slap shot physics in action, we went to one of the best and brightest players in the world. Julie Chu is a Harvard grad and two-time Olympic medal winner who will be going for her third medal as a Team USA forward in Vancouver.
For this experiment, Chu allowed us to film her slap shot with a "phantom cam," a special high-speed digital camera that can shoot at up to 15-hundred frames per second.
JULIE CHU (U.S. Hockey Team): The high-speed camera is incredible; like we know that we’re flexing the stick a bit, but you might not get a sense that it bends that much under your power or the weight that you’re throwing into the puck.
CHU: Oh, that’s awesome.
HOLT: The first step to Chu's slap shot is the wind-up. Watch how she rotates her upper body until the stick is high overhead, then how she transfers her weight from back to front skate, swinging at the puck with maximum velocity.
Dr. HUMPHREY: You get the stick moving first. It's got a lot of momentum from all your muscles and your action there, a lot of momentum, and that momentum is transferred to the puck.
HOLT: The second step is the key to the slap shot’s speed. During the swing, Chu's stick actually strikes the ice before it hits the puck. Instead of slowing the shot, it amplifies it - by adding flex to the stick, and loading it with potential energy.
Dr. KATHARINE FLORES (Ohio State University): When you let the-- the stick snap back-- it's going to put all of that energy into the puck. And that energy's going to translate into the velocity of the puck.
PARISE: The flex and the whip and the stick almost shoot’s the puck for you. And if with the good form, and the proper form it gives the puck a lot more velocity.
HOLT: Adding to the snap, most Olympians today use sticks made from aluminum, carbon graphite and other materials that are stronger, lighter, and much more flexible than wood.
Dr. FLORES: When we saw it on the test frame, we were actually applying about 300 lbs of load to the stick, and deflecting it through about three inches of displacement.
HOLT: The final step to the slap shot is the follow-through, which ends a bit like a wrist shot. Watch how Chu uses her wrists and the curve of the blade to impart spin to the puck. Like a gyroscope, the spin helps keep the puck stable in flight.
Dr. HUMPHREY: At the very end of a slapshot you also do a little bit of a wrist shot actually to get the puck rolling off the blade. So you have a little bit of control on it, not as much as you have with the wrist shot.
HOLT: In slow motion, slap shot physics is revealed. In real time, it unfolds almost effortlessly.
Announcer: Darwitz back to Chu, walking on, firing, she score!
CHU: When you hit the puck just perfect on the slap shot, it really takes off with some zing, you can just feel it.
HOLT: Especially when the slapshot finds its way to the back of the net.