Physics students pls help

[pkt]

I'm just curious about the impact a hockey puck exerts on the surface where it hits. If you can help please give my the force in Newtons.

These are the dimensions of a hockey puck:
weight 8 oz. - or whatever that is in grammes.
thickness: 1 inch;
diameter: 3 inches.

So assuming the puck was shot at various speeds. I was told a slap shot's speed is around 90 mph.

Let's go with that to start with.

Thank you,
PK

[KShan5[PrFC]]

You should Pm epeemike81, he is a physics major at Umass.

[prototoast]

You ask for the force in newtons, yet you give information in ounces, inches and miles per hour!

I can give you a bunch of numbers, but I won't be that nice until you at least convert your data into kilograms, meters and meters per second!

[neevel]

8 oz is 1/2 lb-- about 0.23 kg.
(90 mile/hour*1.6km/mile*1000m/km)/3600 sec/hour = 40 m/s.

At that speed, kinetic energy is 0.5*(0.23 kg)(40 m/s)^2 = 184 joules.

Is that velocity the velocity of the puck leaving the stick, or the velocity at impact?

To know the force exerted, we need to know the distance over which the puck is decelerated from its impact velocity to zero. Let's make a SWAG of about 3 cm = 0.03 m. The force is (184 joules)/(0.03 m) = 6133 Newtons.

Now, here's the ringer-- I assume you're wanting to compare this with the FIE Newton ratings for penetration resistance on uniforms & mask bibs, right? Well, the problem is that Newtons are actually the wrong units for that. You see, the relevant value for penetration resistance is going to be pressure: Newtons/m^2.
The implied meaning of that FIE Newtons ratings is actually "Newtons applied on the penetrator specified in the rulebook". IIRC, that penetrator is 3 mm X 3 mm section square rod with a 130 degree pyramidal point. So, the pressure from 1600 N applied to that penetrator is going to be much greater than the pressure caused by the 6133 Newtons applied across the impacting surface of the puck.

-Dave

[Artisan]

Originally posted by pkt
I'm just curious about the impact a hockey puck exerts on the surface where it hits. If you can help please give my the force in Newtons.
Thank you,
PK

See the bike helmet thread:

You need to know the properties of the rubber and that of the surface its hitting to calculate the force with which it hits (F=ma). Without that, how can you predict how much surface area is in contact? Now if you want to now how much kinetic energy the puck has before it hits.... thats just mass times velocity.

[Artisan]

Originally posted by neevel
Let's make a SWAG of about 3 cm = 0.03 m. The force is (184 joules)/(0.03 m) = 6133 Newtons.

I'll bet that a puck doesn't smush that much. I'd SWAG it at 1 cm max.... oh yeah... and pucks are kept cold too - so you gotta throw in a temperature coeffiecient for the rubber.

I was always suprised that a baseball can be thrown faster than a puck can be shot, especially with the leverage provided by a stick. I was told once that it is because of the shorter time in contact with the puck for the energy to be transferred.

[Dave]

You have to define the surface that it is hitting as well. So far we have decided that it is an impvable incompresible object (This sounds good).

Then the puck
If it is made of rubber its compression will obey Hookes law. You can then calculate the compresion of the puck by intergrating
through the compression taking into account the shape of the puck and therefore the change in its area. The elasticity of the rubber should be fairly easy to discover (remember to quote your sources).

So for a given energy, mass, size and elasticity of a puck this should be calculable exactly with out too much dificulty.



But you really should be doing your own home work not us.

[pkt]

I profess I was an arts major then a Business Admin undergrad. So, you can assume whatever you want to assume. But methinks Dave got it correct to think that I want to compare the effect of a frozen puck striking whatever surface, be it a goalie's mask's cage, a goalie's pad, a person's face - ouch!, the boards, whatever.

The only reason I gave all those info about the puck in Imperial is because that's what the NHL uses. Thanks Dave for converting them and doing most of the work.

As they're so fond of saying in the Biz world, the executive summary provided by Dave is, "So, the pressure from 1600 N applied to that penetrator is going to be much greater than the pressure caused by the 6133 Newtons applied across the impacting surface of the puck."

As they say, the pressure exerted by a woman's high heel is greater than that of an elephant's foot on a per sq mm basis.

Thank you all. You may stop now.

PK

[Soldier]

Originally posted by pkt

Thank you all. You may stop now.

PK

You are dismissed...

[Artisan]

Originally posted by pkt
I profess I was an arts major then a Business Admin undergrad. So, you can assume whatever you want to assume. But methinks Dave got it correct to think that I want to compare the effect of a frozen puck striking whatever surface, be it a goalie's mask's cage, a goalie's pad, a person's face - ouch!, the boards, whatever.

Thank you all. You may stop now.

PK

But I don't wanna stop now!
This is more interesting than the endless: "Who's arm extended first" threads.

"As they're so fond of saying in the Biz world, the executive summary provided by Dave is, "So, the pressure from 1600 N applied to that penetrator is going to be much greater than the pressure caused by the 6133 Newtons applied across the impacting surface of the puck."

Just like a business major / future manager type. Listen for the easy first answer they like and accept it without regard for its veracity.


An like the elephant's foot and the high heeled shoe, surface area DOES matter, and Neevel's 6133 N doesn't account for the contact surface area of the puck. The actual value is probably much higher.

I can say from unequivocal first hand experience that even a 75 mph puck hurts way more, and does more tissue damage than any epee hit I have ever received, so have to dissagree with Neevels conclusion - better check the math.

...now I'm done, thankyou

[neevel]

I was specifically referring to penetration resistance (what the FIE Homologation testing refers to), not total energy dissipated against the target, so we're not in any disagreement. If you re-read my post, you'll see I did consider the surface area. The contact surface area of the puck will almost certainly be much greater than the 130-degree point of that 3 mmX3 mm test probe, so the pressure (Force/surface area) will be greater for the test probe at 1600 N than for the puck at 6133 N. But the puck will have more total energy to dissipate against the target, and so will likely hurt a lot more, and have the potential to cause more damage due to blunt trauma than an epee point (which will have more contact area then the FIE test probe, but still less than the puck). If a broken epee blade were to penetrate the jacket and skin then, of course, the injury could be more severe since the smaller amount of energy would be going directly to a far more vital and vulnerable spot (internal organs or blood vessels) than the blunt surface trauma of the puck.

Fencing jackets are designed far more to resist penetration than to prevent transmission of the impact force to the body, whereas hockey gear is going to be designed more to fully absorb the impact energy of a puck and prevent it from being transmitted to the wearer's body than to resist penetration from a small-cross-section object like a blade. For example, a big block of styrofoam strapped to my chest would probably cushion the impact of a hockey puck far better than my 800-N fencing jacket, but I certainly would not want to count on that styrofoam stopping a broken blade driven by a full lunge.

-Dave