73 replies to this topic

[Barrie]

At 0:45, when the camera zooms closer to the net, the net moves before the puck makes contact. Not only is it fake, but its not even faked well.

I just saw that. I was wondering if the video was enlarged the string pulling the net can be seen.

Speaking of commercials, I like the Call of Duty (heheheheh I just said duty) Black Ops Commercial with Kobe.

[KillrBuckeye]

Did someone say physics!?

I like your thinking, but there are a couple significant mistakes in your analysis.

In order for the net to move, a shot (force) hitting the net's center of mass would have to overcome the initial static friction

Yes, this is true.

(which we can assume there is none due to the ice's slipperiness).

Coefficient of static friction for ice is certainly low, but definitely quantifiable http://iopscience.io...43C5E35A3EAC.c2. If it were truly zero, then any amount of horizontal force applied to the net would set it in motion.

Essentially all there is then is to say that a horizontal force must exceed the downward force due to the mass of the net and gravity, in order for the net to move at all.

This is incorrect. Draw a free body diagram of the net. Downward gravitational force by itself will do nothing to resist horizontal motion of the net. It is only the force of friction that will resist the motion. We can assume a static coefficient of friction somewhere around 0.05 based on the article abstract I linked.

Friction Force = mass*acceleration of gravity * coefficient of static friction

F_fric = (56.69kg)*(9.81m/s²)*0.05 = 27.8 Newtons

Thus, the force of the puck needs to exceed 27.8 Newtons to be able to move the net.

I corrected the above calculations accordingly.

The hardest shot recorded is 105.4 mph = 47.12 m/s

The kinetic energy of a shot with this speed is KE = (1/2)*m*v² = (1/2)*(0.17kg)*(47.12m/s)² = 188.72 Newtons*m.

Yes, I'm with you.

In order to equate it to the Force applied, you must divide this number by the distance, which can be said from the top of the circle, is about 35 feet = 10.67 m.

No. Neglecting air resistance, no work is done while the puck is flying through the air. Work is done on the net only while the puck is making contact with it. So most of the kinetic energy is transferred to the net over a distance of a foot or so (however much the net stretches).

So (188.72 N*m)/(0.3 m) = 629 Newtons

. Corrected this calculation as well. The force exerted on the net is roughly 140 lbf, which seems pretty reasonable to me for a shot of that velocity.

I think it's perfectly reasonable that the net could move. However, it should be kept in mind that the assumptions for coefficient of friction and the distance over which the puck energy is transferred to the net are just ballpark estimates, and even slight changes to the values can drastically change the results. The other huge simplification of the problem is the transfer of puck energy to the net. In reality, it will not be linear. The force exerted will start very small as the puck first makes contact with the net. As the net stretches, tension in the strings will start to pull harder on the metal frame. The force exerted by the puck is growing, but at the same time its kinetic energy is decreasing since it is being slowed down. It is likely at a given point in time, the instantaneous force might be even higher than 140 lbf, but for most of the time the puck is in contact, the force would be significantly lower.

Edited by KillrBuckeye, 12 November 2010 - 04:25 PM.

[Smite]

Agreeing with KillrBuckeye......

I am not going to get all scientific. I was able to move the net as a peewee (and I did not have the hardest shot on the team) if I got a good shot off and hit the middle of the post high enough in practice it would move maybe an inch at most.

Mind you the pegs were not in and I am sure we did not use "NHL regulation" nets and it was around 1987. But you need to factor the ice in the equation. Something else is the puck is make of rubber and the netting is elastic.

Can it be done..... yes

Is it still a fake... I think so or some fancy editing was done.

[Legendary D In 03]

Nevermind

Edited by Legendary D In 03, 12 November 2010 - 06:32 PM.

[Doggy]

At 0:45, when the camera zooms closer to the net, the net moves before the puck makes contact. Not only is it fake, but its not even faked well.


Reebok sucks, and Crosby is still a ******.

I think the puck was supposed to have hit the post and then the net, which is why it looks that way. Not suggesting it's real, but there was no mistake made there. It clearly hits the bar, the net moves, then hits the twine.

[Rivalred]

[lookalive07]

I like your thinking, but there are a couple significant mistakes in your analysis.

Yes, this is true.

Coefficient of static friction for ice is certainly low, but definitely quantifiable http://iopscience.io...43C5E35A3EAC.c2. If it were truly zero, then any amount of horizontal force applied to the net would set it in motion.

This is incorrect. Draw a free body diagram of the net. Downward gravitational force by itself will do nothing to resist horizontal motion of the net. It is only the force of friction that will resist the motion. We can assume a static coefficient of friction somewhere around 0.05 based on the article abstract I linked.

I corrected the above calculations accordingly.

Yes, I'm with you.

No. Neglecting air resistance, no work is done while the puck is flying through the air. Work is done on the net only while the puck is making contact with it. So most of the kinetic energy is transferred to the net over a distance of a foot or so (however much the net stretches).

. Corrected this calculation as well. The force exerted on the net is roughly 140 lbf, which seems pretty reasonable to me for a shot of that velocity.

I think it's perfectly reasonable that the net could move. However, it should be kept in mind that the assumptions for coefficient of friction and the distance over which the puck energy is transferred to the net are just ballpark estimates, and even slight changes to the values can drastically change the results. The other huge simplification of the problem is the transfer of puck energy to the net. In reality, it will not be linear. The force exerted will start very small as the puck first makes contact with the net. As the net stretches, tension in the strings will start to pull harder on the metal frame. The force exerted by the puck is growing, but at the same time its kinetic energy is decreasing since it is being slowed down. It is likely at a given point in time, the instantaneous force might be even higher than 140 lbf, but for most of the time the puck is in contact, the force would be significantly lower.

I had a feeling someone would be able to further explain it, and explain it better at that. All in all, the net has many different parts that affect the problem in different ways, such as hitting the back bar of the net as opposed to the actual netting. Several things would slow the puck down upon impact and some of the force would be transferred in different ways. I was unable to find the coefficient of static friction so I was going on the fact that ice is slippery enough to neglect it. I knew in my head that this would probably throw off my analysis quite a bit, but I was willing to take it with a grain of salt. All in all, reasonable maybe, but does it make the video any less fake? No.

Edit: now that I look at the video a few more times, it does appear that the net moves before the puck hits the back of the net a few times, but it almost looks like a couple of those shots hit the crossbar and near side post before going into the net, which makes it look pretty fake, but it actually does move at the correct times. I'm still not buying it though.

Edited by lookalive07, 13 November 2010 - 02:58 PM.

[edicius]

I don't think "lethal" is the right word. "Dangerous" seems more correct. See below.

[Jedi]

I don't think "lethal" is the right word. "Dangerous" seems more correct. See below.

Hmmm. I must question the logic of your chart.

I would rank as halfway between "Very" and "Mildly" trustworthy. Yet I'm sure there are those here on LGW that would disagree...

[Travis]

Hmmm. I must question the logic of your chart.

I would rank as halfway between "Very" and "Mildly" trustworthy. Yet I'm sure there are those here on LGW that would disagree...

I hope that means you have a Wilford Brimley mustache.

[Jedi]

I hope that means you have a Wilford Brimley mustache.

Lol, nope. I'm apparently slightly more trustworthy than Mr. "Di-uh-beet-us".

[Original-Six]

Lol, nope. I'm apparently slightly more trustworthy than Mr. "Di-uh-beet-us".

Whats wrong with him?

[HudlerFanatic]

This thread is offensive to LGW! Why is it still here?!

[eva unit zero]

overcome the initial static friction (which we can assume there is none due to the ice's slipperiness).

The coefficient of friction for steel on ice is .01.