## Wednesday, December 13, 2017

### Impulse Saves

As a hockey goalie, it is no secret that I am trying to get hit with pucks as frequently as possible. Recently, in the spirit of physics, I was wondering how my protective equipment protects me. Pucks are shot on average at 75 mph, which means they hit me with a momentum of  5.7 kg*m/s. The

p = m*v = (.170 kg)*(33.528 m/s) = 5.7 kg*m/s

So why is it that goalies aren't always completely covered in bruises? My CCM chest protector is made with D3O foam, a high-tech, shock-absorbing material. It is an engineered material that acts as a non-Newtonian fluid. What this means is that its viscosity depends on the force applied to the material. In the case of D3O, it is more "squishy" when you press on it gently and hard when hit with a puck (or hammer), this is called "shear thickening."

A non-Newtonian fluid is useful in protective equipment because they absorb the impact of the force by spreading it through the material and by increasing the surface area over which the force is applied. This does three things: 1) increases the time of the impulse, which reduces the force and 2) increases the surface area, which decreases the pressure, reducing blunt trauma caused by the impact (bruises), 3) allowing for the dissipation of energy through friction between D3O molecules. As we have seen in in-class problems, even a small increase in time can drastically reduce the force.

1) F = m*(vf-vi) / t             2) P = F / Area

While the physics of Non-Newtonian fluids can become very complicated, it is these introductory level relationships that govern their use as protective equipment in sports.

Additionally, there is another way that goalies increase the time of the impulse. As a goalie, you are taught not to be "scared" of the puck. Being scared of being hit causes you to tense up, therefore decreasing the impulse time (in addition to psychological factors). When you are not afraid of being hit, you can focus on reacting and "cradling" the puck, further increasing the impulse time and decreasing the force experienced (see video below @ 2:07).

Sources:
http://www.explainthatstuff.com/energy-absorbing-materials.html
https://www.d3o.com/
http://www.physics.org/featuredetail.asp?id=35