A Science Question About Gravity

Electric Skeptic

Well-known member
We all know that (barring wind resistance) objects fall at the same speed, regardless of their weight.

So why is putting extra weight in things like bobsleds illegal? Adding weight to them won't make gravity pull them any faster.
 

Nouveau

Well-known member
Wouldn't the same be true of a vertical drop? And we know that doesn't help.
A bobsled is accelerated by human effort, so more weight means more mass to accelerate to a higher momentum that would then maintain speed longer against friction slowing it down. But that's just my 2c. I'm not a physicist.
 

Komodo

Active member
A bobsled is accelerated by human effort, so more weight means more mass to accelerate to a higher momentum that would then maintain speed longer against friction slowing it down. But that's just my 2c. I'm not a physicist.
Suppose we had a flat stretch of ice, two sleds with different weights, and two machines set to give each an identical push towards the finish line. (I think this is a fair analogy to the bobsled acceleration, since the push the sledders give to the heavier sled would be equal to the push they'd give to the lighter sled: in each case, they will be giving it all the force they can.) If more mass -> greater momentum -> longer period at high speed vs. friction, then we would expect the heavier sled to reach the finish line first, wouldn't we? But that can't be right, because then a sled weighed so heavily that the machine could barely get it started would also finish first.

(I'm also not a physicist, in case that wasn't obvious.)
 

Nouveau

Well-known member
Suppose we had a flat stretch of ice, two sleds with different weights, and two machines set to give each an identical push towards the finish line. (I think this is a fair analogy to the bobsled acceleration, since the push the sledders give to the heavier sled would be equal to the push they'd give to the lighter sled: in each case, they will be giving it all the force they can.) If more mass -> greater momentum -> longer period at high speed vs. friction, then we would expect the heavier sled to reach the finish line first, wouldn't we? But that can't be right, because then a sled weighed so heavily that the machine could barely get it started would also finish first.

(I'm also not a physicist, in case that wasn't obvious.)
I would expect some trade off in terms of the extra effort needed to get the extra mass moving. As well as not being a physicist, I'm also not a bobsled competitor, so I have no idea whether extra weight is desirable, or at what point the extra weight makes it too hard to accelerate the sled. But the OP is right about gravity, so added momentum is the only thing that would make sense to me.
 

Temujin

Well-known member
Suppose we had a flat stretch of ice, two sleds with different weights, and two machines set to give each an identical push towards the finish line. (I think this is a fair analogy to the bobsled acceleration, since the push the sledders give to the heavier sled would be equal to the push they'd give to the lighter sled: in each case, they will be giving it all the force they can.) If more mass -> greater momentum -> longer period at high speed vs. friction, then we would expect the heavier sled to reach the finish line first, wouldn't we? But that can't be right, because then a sled weighed so heavily that the machine could barely get it started would also finish first.

(I'm also not a physicist, in case that wasn't obvious.)
Imagine a sled as light as a feather. The force given it is constrained by the speed at which the crew can run. Conversely a sled weighing a ton would be constrained by the strength of the crew getting it moving. The optimum weight is where both strength and speed are maxed out. I'm not sure what the regulations are, but a fixed weight means that all variations in speed are down to the physical capabilities of the crew. (ignoring all the other design constraints). The question is surely, does the medal go to the fastest strongest crew, or the cleverest designer?
 

Komodo

Active member
Imagine a sled as light as a feather. The force given it is constrained by the speed at which the crew can run. Conversely a sled weighing a ton would be constrained by the strength of the crew getting it moving. The optimum weight is where both strength and speed are maxed out.
But aren't they maxed out in both scenarios? The crew will give all the leg-force it can for both the feather-light sled and the one-ton sled. (In both cases, at least as it appears from the little bobsledding I've seen, the arms do little but hang on.)
I'm not sure what the regulations are, but a fixed weight means that all variations in speed are down to the physical capabilities of the crew. (ignoring all the other design constraints). The question is surely, does the medal go to the fastest strongest crew, or the cleverest designer?
I don't know the regulations either, but there are sports in which design advantages can be critical. (Race car driving, for example. The best driver in the world won't win with an inferior car.)
 

Komodo

Active member
I would expect some trade off in terms of the extra effort needed to get the extra mass moving. As well as not being a physicist, I'm also not a bobsled competitor, so I have no idea whether extra weight is desirable, or at what point the extra weight makes it too hard to accelerate the sled. But the OP is right about gravity, so added momentum is the only thing that would make sense to me.
It just occurred to me that if extra weight in the sled were an advantage, shouldn't extra weight in the crew be as well? Wouldn't two heavyweight bobsledders then have a big advantage over two lightweights?
 

Temujin

Well-known member
It just occurred to me that if extra weight in the sled were an advantage, shouldn't extra weight in the crew be as well? Wouldn't two heavyweight bobsledders then have a big advantage over two lightweights?
They do. That's why they are built like brick sh1thouses.
 

Komodo

Active member
I would expect some trade off in terms of the extra effort needed to get the extra mass moving. As well as not being a physicist, I'm also not a bobsled competitor, so I have no idea whether extra weight is desirable, or at what point the extra weight makes it too hard to accelerate the sled. But the OP is right about gravity, so added momentum is the only thing that would make sense to me.
Maybe it would help if we broke the acceleration here down into horizontal and vertical components.

In the horizontal component, with a frictionless environment, extra mass would be a disadvantage because equal force divided by larger mass yields lesser acceleration. In a low-friction environment, like ice and snow, my guess is it's still a disadvantage; wouldn't a hockey player be able to slap a lighter puck faster than a heavier puck?

For the vertical component, as ES notes, the masses accelerate equally in a frictionless environment. In a very low friction environment like air, IIRC, it's surface area rather than mass which determines resistance and thus speed. (A steel ball weighing one kilogram will fall faster than a steel sheet weighing one kilogram.) So two identically shaped bobsleds should accelerate equally in the vertical component, even if one is heavier than the other.

So I still don't see how extra mass becomes an advantage. (And I'm still not a physicist, so it's likely I'm missing something.)
 
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Nouveau

Well-known member
It just occurred to me that if extra weight in the sled were an advantage, shouldn't extra weight in the crew be as well? Wouldn't two heavyweight bobsledders then have a big advantage over two lightweights?
I expect the extra muscle mass would be more valuable for getting the sled going at the start, but the limitation is that they all have to fit in a very cramped space quite quickly once moving.
 

Nouveau

Well-known member
Maybe it would help if we thought about the horizontal and vertical components of acceleration here.

In the horizontal component, with a frictionless environment, extra mass would be a disadvantage because equal force divided by larger mass yields lesser acceleration. In a low-friction environment, like ice and snow, my guess is it's still a disadvantage; wouldn't a hockey player be able to slap a lighter puck faster than a heavier puck?

For the vertical component, as ES notes, the masses accelerate equally in a frictionless environment. In a very low friction environment like air, IIRC, it's surface area rather than mass which determines resistance and thus speed. (A steel ball weighing one kilogram will fall faster than a steel sheet weighing one kilogram.) So two identically shaped bobsleds should accelerate equally in the vertical component, even if one is heavier than the other.

So I still don't see how extra mass becomes an advantage. (And I'm still not a physicist.)
For the horizontal component, friction counts in the tight turns so that is where extra mass/momentum might help.

Maybe we need confirmation that cheating by adding extra weight is actually a thing here?
 

Temujin

Well-known member
I expect the extra muscle mass would be more valuable for getting the sled going at the start, but the limitation is that they all have to fit in a very cramped space quite quickly once moving.
Probably why they tend to be built like rugby players rather than sumo wrestlers.
 

Electric Skeptic

Well-known member
For the horizontal component, friction counts in the tight turns so that is where extra mass/momentum might help.

Maybe we need confirmation that cheating by adding extra weight is actually a thing here?
Okay, at this point I have to admit that I assumed that cheating by adding extra weight is actually a thing here. My belief was based solely on the movie Cool Runnings, where the John Candy character had been suspended for doing just that previous to the movie. In retrospect, using a John Candy movie as the basis for my belief might not have been the wisest move (much as I like John Candy). But who new that a movie might include something that's not actually true?

So I did a bit of research (limited to the net because I'm lazy). There's a page here that discusses cheating in bobsled, but there's no mention of using extra weight as a method to do so. The wikipedia page here states that there's a maximum weight for the bobsled which includes the racers themselves (which addresses the question above of the racers being physically big - it doesn't really matter). If the sled is underweight (because of lighter racers, e.g.) they add weights to the sled to take it up to the maximum. It also states that "[t]he bobsleighs themselves are designed to be as light as possible to allow dynamic positioning of mass through the turns of the bobsleigh course", which doesn't make much sense to me. If the whole package is allowed to weight X, what does it matter of 20% of that is the sled and 80% the racers, or vice-versa?

There is a reddit asking the same question as I did here. Some interesting stuff there, but it seems that the answer is still uncertain.

But I couldn't find anything about actually cheating by adding weight to the sled - and from what I read about the pre- and post-race inspections (similar to the post-race weighing of handicapped race horses) - it wouldn't be easy. There seem to be easier ways to cheat (which are actually used).

So it all may be moot anyway.
 

Temujin

Well-known member
Okay, at this point I have to admit that I assumed that cheating by adding extra weight is actually a thing here. My belief was based solely on the movie Cool Runnings, where the John Candy character had been suspended for doing just that previous to the movie. In retrospect, using a John Candy movie as the basis for my belief might not have been the wisest move (much as I like John Candy). But who new that a movie might include something that's not actually true?

So I did a bit of research (limited to the net because I'm lazy). There's a page here that discusses cheating in bobsled, but there's no mention of using extra weight as a method to do so. The wikipedia page here states that there's a maximum weight for the bobsled which includes the racers themselves (which addresses the question above of the racers being physically big - it doesn't really matter). If the sled is underweight (because of lighter racers, e.g.) they add weights to the sled to take it up to the maximum. It also states that "[t]he bobsleighs themselves are designed to be as light as possible to allow dynamic positioning of mass through the turns of the bobsleigh course", which doesn't make much sense to me. If the whole package is allowed to weight X, what does it matter of 20% of that is the sled and 80% the racers, or vice-versa?

There is a reddit asking the same question as I did here. Some interesting stuff there, but it seems that the answer is still uncertain.

But I couldn't find anything about actually cheating by adding weight to the sled - and from what I read about the pre- and post-race inspections (similar to the post-race weighing of handicapped race horses) - it wouldn't be easy. There seem to be easier ways to cheat (which are actually used).

So it all may be moot anyway.
"Dynamic positioning of mass" means that the crew lean in on curves. In yacht racing, crewmen are picked for their size because they can then affect the stability by moving about. Ask AN for further details. 😉
 

Komodo

Active member
Okay, at this point I have to admit that I assumed that cheating by adding extra weight is actually a thing here. My belief was based solely on the movie Cool Runnings, where the John Candy character had been suspended for doing just that previous to the movie. In retrospect, using a John Candy movie as the basis for my belief might not have been the wisest move (much as I like John Candy). But who new that a movie might include something that's not actually true?

So I did a bit of research (limited to the net because I'm lazy). There's a page here that discusses cheating in bobsled, but there's no mention of using extra weight as a method to do so. The wikipedia page here states that there's a maximum weight for the bobsled which includes the racers themselves (which addresses the question above of the racers being physically big - it doesn't really matter). If the sled is underweight (because of lighter racers, e.g.) they add weights to the sled to take it up to the maximum. It also states that "[t]he bobsleighs themselves are designed to be as light as possible to allow dynamic positioning of mass through the turns of the bobsleigh course", which doesn't make much sense to me. If the whole package is allowed to weight X, what does it matter of 20% of that is the sled and 80% the racers, or vice-versa?

There is a reddit asking the same question as I did here. Some interesting stuff there, but it seems that the answer is still uncertain.

But I couldn't find anything about actually cheating by adding weight to the sled - and from what I read about the pre- and post-race inspections (similar to the post-race weighing of handicapped race horses) - it wouldn't be easy. There seem to be easier ways to cheat (which are actually used).

So it all may be moot anyway.
On the general topic of sports and physics, I remember once hearing Erich Segal (author of Love Story, but also a classics professor) commenting on the Olympics and saying that the ancient Greek long jumpers would carry a couple of lead weights and then throw them behind them when they were in mid-air, giving them a kind of "rocket" type boost. And I thought, wait a minute... wouldn't the extra weight handicap their liftoff at least as much as the discard would help them? If that sort of thing could work, couldn't you use the same principle to create a perpetual motion machine? So was Segal botching the physics, or was I missing something?
 
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