Speed of a slapshot on the moon?

Factual Questions
1

Greetings.

There’s a beer TV commercial here in Canada (could be aired in the USA too) with two guys on the moon. One of them takes a slapshot and the puck flies into the other astronauts face mask and cracks it. Pretty funny commercial, but the question my friends and I are debating is how fast a puck would go on the moon.

Our ignorant thoughts:

Since gravity is lesser on the moon, there would be less of a force acting on the puck, thus the net force (using vector arrows like in high school) would be greater in the direciton of the puck, giving it more speed.

Also, swinging the stick would be less because there isn’t as much gravity assisting you with bringing the hockey stick down. Say your muscle force is 100N, plus on earth 9.8, giving a total force of 109.8, but on the moon it would be less (I don’t know what the force of gravity is on the moon, so I’ll just use 2), only 102.

Other ideas we had were that because the force pulling hte puck down, and the force pulling the stick down cancel each other out, it doesn’t matter what the force of gravity is on the moon, the net external forces would be the same, so the puck would go the same speed.

We agree that the puck would go further (obviously), but what we’re concerned with is top speed. Assuming you could hit the puck with exactly the same force each time, say using a computer that swings, would the puck go faster, slower or the same speed on the moon?

Thanks,
Bodi

2

Not only is there less gravity, but there is no atmoshpere causing drag to slow the puck.

The problem that would be run into is the fact that spacesuits aren’t really designed to allow a full range of motion. I doubt that an astronaut could get a full swing at the puck. The guy who hit the golf ball on the moon had to make major modifications to the tool he used (it wasn’t a real club, but a club head on a rock picker). He held onto the shaft with one hand and had some sort of cable coming off the end that he used his other hand on.

3

Okay, so assuming it was possible, strictly from a physics point of view (and if you can write the math out that’d be cool too), what would occur?

4

Sorry, but IANAP.

5

The force acting on the puck is the force of the stick when it strikes the puck. Since you refer to a commercial in which the puck flies in to someone’s face, and I assume the person was not lying down, we’re talking about a slapshot that raises the puck off the surface.

This is important, because the friction of the lunar surface does not come into play (if it were a straight surface shot, the friction would be important, but in general, the speed would be greater on the moon’s surface because gravity plays a role in how much friction a horizontal surface presents to a given object). So we are talking about an initial vector that is at some upward angle relative to the surface of the Moon.

Your point about gravity assisting the downward swing is not without merit, but I would guess that the effect of gravity is negligible (You might also argue that the reduced gravity assist on the moon might be counterbalanced by the lack of air resistance on the stick to to lack of atmosphere, but, really I think both effects are minimal enough in the space of time we’re talking about that they both can be ignored). So given a uniform swing such as you describe, I would say the vector applied to the puck as it is being struck is identical in both locations.

This situation is a simple F=ma equation (Force=mass x acceleration). Let’s say on Earth we have

F(e)=ma(e),

and on the Moon we have

F(l) = ma(l).

I use lower case “L” (for luna) as an abbreviation for the Moon to avoid confusion with the m for mass, which by the way is uniform and does not change. We have established in the above paragraph that the force applied to the puck by the stick is identical in both cases, so F(e)=F(l)=F. So our equations become

F=ma(e)
F=ma(l)

Because F and m are constant, then by the power of mathematics, the acceleration given to the puck is identical in both cases.

Now for the actual velocity of the puck. The once the force stops being applied to the puck (i.e., the puck flies away from the stick), the acceleration due to that force stops, and the puck moves at a constant velocity, which I will call v(f). The equation that describes the velocity is:

v(f) = v(i) + at.

v(i) is the initial velocity of the puck before acceleration takes place, which is equal to zero in both cases. t is the amount of time that the puck undergoes acceleration. I can’t think of a reason that t would be different in the two different settings, so I would say the velocity from the force of the stick is identical in both places.

NOW gravity and lack of oxygen really come into play. Because the puck has taken flight, its velocty vector at any given time can be broken into horizontal and vertical components. Both components are subject to the velocity equation above for any accelrations they undergo, and the components would be added using vector mathematics. The horizontal vector remains constant on the moon and undergoes gradual negative acceleration on earth due to the force of its friction with the air (drag). Vertically, the puck’s velocity accelerates downward with the standard acceleration, which is something like 80% less on the moon. So at any given time, the puck experiences less reducing acceleration on the moon than on the earth, so in general its velocity will be greater at any given time.

6

Well, you set up the experiment in two seperate ways:[ul][li]The hockey player is on an icy surface in an arbitrarily large dome on the lunar surface, in which there is a breathable atmosphere, or[/li][li]The player is out in the open on the lunar surface, on an smooth icy patch, though wearing a protective suit that allows full range of motion[/ul]You’ll also have to try to match temperatures, since these can greatly affect the puck’s resilience and the ice’s smoothness.[/li]
In one-sixth gravity, air resistance becomes greatly significant, so youll have to choose to account for it or not. Assuming you do (you should, actually, since in a vaccum, I’m not sure if a thin layer of melted water could form on the icy surface. This thin layer of water makes the ice a lot “slicker”), there’s another major factor to consider: The player’s footing won’t be nearly as secure. Even on Earth, on a near-frictionless ice rink, a player can place his skates so he feels as little “recoil” as possible. This will be much more difficult if he weighs only one-sixth normal. It may not be possible for a player to put his full energy into a slapshot without sending himself tumbling. You could try to correct this by locking the player’s feet down.

Given solid footing (well, as solid as standing on ice ever gets), weight was never a major concern for a slapshot. The puck, after all, is gliding across a near-frictionless surface already. I figure the initial speed of the puck would be about the same (once the player had enough practice time to perfect a low-gravity swing) but it would travel much much further since the friction (already pretty low) would be even lower. The kinetic coefficient of friction for a puck across ice is pretty low, and I’ll assume it would be only one-sixth as strong on the moon, and this might not even be accurate, since there may be water adhesion and whatnot…

Sigh. Okay, my best guess is that the moon-puck’s initial speed would be about the same as Earth-puck, but moon-puck would travel at most (and possibly a lot less than) six times as far. Not very exciting, I know. Trouble is, I can’t find any records for the longest slap shot. I can find a few speed records, but no indication of how far the puck would travel, since I guess anything beyond the 200-foot standard length of a hockey rink is judged irrelevant.

Clear as mud?

7

I’ll just add that the top speed would be the same, just that on the moon the puck would decelerate more slowly.

8

Well, looking at scotandrsn’s post, submitted while I was reseaching and writing mine, it’s clear that when the puck is lifted off the surface, it’ll travel considerably further. A bit of rough calculation shows me that when gravity is halved, “hang time” is doubled, so dropping gravity to one-sixth would increase hang time by a corresponding amount. The puck would travel six times as far before landing, and lower gravity would let it tumble much further than normal gravity. If you take the shot in a vaccum, you’ll get much longer distance, though the hang time will be the same. I just don’t know how well ice would behave in a vaccum. If the temperature was a few degrees above freezing, the sublimation mght make the ice surface irregular, and if the temperate was significantly below freezing (as the lunar surface is most of the time), the puck might shatter or break the stick when the slapshot was atempted.

9

Guys if you don’t know the answer don’t just guess most of the posts in this thread (including the OP) are ignorant of basic physics.

We can ignore air resistance becuase the highest velocity of the puck with be the instant after the collision. Gravity is also irrelevant becuase we are not looking for the maximum distance traveled but highest velocity. The only effect that gravity and air resistance have on the question posed is their affect on the stick.

The relevant physic principle in collisions momentum is conserved mathematically this means m1v1i+m2v2i=m1v1f+m2v2f. Solving for v2f we get v2f=(m1v1i-m1v1f)/m2This equation holds true for all collisions but since we have two unknowns (v1f and v2f) we need another. The other property of objects in collisions that we need is kinetic energy mathematically shown as 1/2mvv (vv=v squared). Depending on the objects in question collisions can be elastic (kinetic energy is conserved) or inelastic (some kinetic energy is lost).

The situation presented in the OP we know that some energy is lost becuase we can hear the stick strike the puck (I know we can’t hear stuff on the moon but this is irrelevant). We are comparing collisions with identical sticks and pucks therefore the mass of the two objects is irrelevant.

From the first two paragraphs we can determine that the only way to increase the final speed of the puck is to increase the sticks velocity (doing so increases KE and momentum) or decrease the Kinetic Energy lost in the collision. Gravity only adds a small amount of momentum to the stick becuase the time the swing takes is not very long so I would argue that gravity does not affect the velocity of the stick. Since there is no air on the moon the stick does not encounter any resistance during the swing. I believe this is more significant than gravity being less however I do not have numbers to calculate to verify this. I believe the most important thing would be that the stick and the puck would be signifcantly colder on the moon decreasing the elasticity of the collision. Therefore I would argue that the puck would actually travel faster on earth.

10

I agree with Treis

11

I not sure what your gripe is, but most of the posts here were more meaningful to me than yours. Your post seems to emphasize the “coldness” when the OP made it clear that he was interested in the effect of gravity (i.e., it was a thought experiment).

And the “including the OP” comment is just rude. So the only questions allowed here are by people who know the answer already?

I’ll stop now before I get in trouble.

12

Though Treis may seem icy, his observation about the lowered elasticity in cold is on target.

That being acknowldeged, there is the question of his assumption that it is always colder on the moon than on earth. If so, the velocity of the puck would be greater on earth.

Once source I found says the temperature on the moon varies from approximately -233 F (-147 C) to 212 F (100 C), with a mean temperature of a balmy -9 F (-23 C)*. Sounds like perfect hockey weather to me. Another source says “The temperature on the moon can range from 127 degrees Celsius to -173 degrees Celsius.”** Knowing this allows us to stipulate that the temperature is identical in both places and therefore is irrelevant.

If we skip the imaginary layer of water on top of the ice making it “slicker” (once the ice is fully frozen, an appreciable water layer actually only appears as a result of pressure heating the ice up to the melting point, as under a skate blade) and stipulate the player being able to move equally well in both locations, or that it’s a machine doing the striking, we’re back to the biggest real difference being the lunar lack of air resistance to both stick and puck (or to just the stick, if you don’t care about the puck after it’s struck). The same force applied to the same mass with no friction from air resistance will result in a higher velocity puck on the moon. Also, might one consider that the mass of the puck on the moon is only about 1/7 of that on earth, so it should be able to be propelled faster.

*http://www.asi.org/adb/02/05/01/surface-temperature.html
**http://visitor.broaddaylight.com/spacekids/FAQ_44_5598.shtm

13

I apologize for my comment regarding the OP but I stand by my other comments, if you don’t know the answer you shouldn’t post. The ignorance of physics in this thread is pretty bad. People are talking about forces which aren’t relevant in this discussion. The relevant topic is the impulse or change in momentum of the objects in question.

hack cough what?

Mass is a property of the object it does not change.

Did you read my post particularly these two parts?

In light of ** hyjyljyj’s ** information about the vast temperature changes on the moon I would amend my answer to if you are at the extreme high temperature the puck will travel faster if you are at the extreme low the puck will travel slower.

14

OK technically it’s the weight of the puck that is 1/7 on the moon and not the mass which remains constant at 170 g.

thx

15

Thanks for all your answers ppl. Even if some of them were only pseudo ones :stuck_out_tongue:

I guess the bottom line here is I’d have to hitch a ride with NASA to find out the answer.

rsa was correct when he mentioned that it was only a thought experiment. There was mention of being able to get a foothold on the ice, and the water tension on the ice, etc. None of these mattered to us, were we just concerned with the top speed “all things being equal and possible”.

I have a small education in physics, mostly from my own interest. It didn’t bother me that I didn’t know all of the “basic physics” that was required to really solve this problem, in my own mind I was satisifed to make it into a very simple vector problem. Less downward force means more forward force. The velocity of the stick at impact was my only variable. But since most ppl seem to say that gravity would be insignificant here, I am content to leave it out and just use the lack of a dense atmosphere as the evidence that the puck will reach a higher top speed.

Thx, see ya when somebody corrects this, lol…
Bodi:p

16

I don’t think there’s anything wrong with posting guesses. That’s what you are doing too; criticizing each other’s guesses is useful for discussion.

Do you have any numbers on how much the elasticity would change, and how that affects the puck speed? It’s not obvious that the effect greater than those of gravity and air resistance.

17

There is a difference between posting blatantly wrong information and type of guessing I did. It isn’t possible to exactly determine how the elasticity would change without knowing which type of puck/stick we are talking about. I used my real world experience (hitting a baseball and golf ball in cold weather) to determine that being significantly colder has a noticable impact on the elacsticity of collisions. I also used my basic physics knowledge to determine that gravity and wind resistance have a negligible impact on the speed of the hockey stick at impact.

This is where I believe you are making your mistake, the puck does not reach a higher speed it starts with a higher speed. I am guessing the time the puck is accelerated to its top speed by the stick is measured in microseconds. In this short timeframe gravity has almost no effect on the puck.