Man juggling rocks

littld · 30-06-2006, 16:22

Okay, question for the scientists out there.

I was reading a riddle the other day at this site and it says:

"A man weighing 190 lbs., carrying 3 round rocks, weighing 5 lbs. each, approaches a large ravine. The only way to cross is a rope bridge with a weight limit of 200 lbs. The ravine is too far to throw or roll the rocks across. How can the man cross the ravine in only one trip, with all 3 rocks?"

The answer follows that he juggled them all the way over the bridge.

In bed last night my wife and I argued this at length. You see, I think that even if he is juggling them, they still weigh 15 lbs and that weight can't just disappear. I think he still carries the weight as he has to exert force upwards to keep them in the air. My wife thinks that, as long as he starts juggling before he sets off on the bridge, he will "save" the 5 lb weight of one of the rocks.

Which one of us is right? It is quite important - my way he is a goner but if my wife is right then he's OK.

**Russ** · 30-06-2006, 16:29

Are you on the sub's bench til you find the answer?

Aragorn · 30-06-2006, 16:30

When you are juggling you never have more than two objects in your hand at one (in theory - I'm not that good!). The two stones in hand add to the weight of the man. The third stone does not have any connection with the man or bridge and therefore does not affect his weight. When the one stone is being thrown up, the other is still in mid-air, so temporarily you only have one in hand.

I'm with your wife (metaphorically

)

punky · 30-06-2006, 16:32

She's right.

Newton's 3rd law, which you are refering too, would apply to the air above the bridge, not the actual bridge itself.

30-06-2006, 16:36

I'm reserving judgement, as, although one rock is continually airborne, the reaction to the force required to toss a rock & the momentum of the rock being caught are significant, and would, theoretically, push the total weight over the 200lb limit.

- now if he could keep 2 rocks in the air, while juggling, it wouldn't be so marginal

**Maggy J** · 30-06-2006, 16:44

Good grief! It's a maths lesson...

littld · 30-06-2006, 16:48

Quote:

Originally Posted by homealone

I'm reserving judgement, as, although one rock is continually airborne, the reaction to the force required to toss a rock & the momentum of the rock being caught are significant, and would, theoretically, push the total weight over the 200lb limit.

- now if he could keep 2 rocks in the air, while juggling, it wouldn't be so marginal

Okay now we're getting somewhere. This what I was trying to say, but better articulated.

Is there any way to test it?

Does anyone here juggle? Maybe if you have some bathroom scales at home you could stand on them holding 3 heavy things and start juggling. What happens to the scale? (best do this when no-one else is home or they'll wonder).

Anyone?

danielf · 30-06-2006, 16:54

Quote:

Originally Posted by littld

Okay now we're getting somewhere. This what I was trying to say, but better articulated.

Is there any way to test it?

Does anyone here juggle? Maybe if you have some bathroom scales at home you could stand on them holding 3 heavy things and start juggling. What happens to the scale? (best do this when no-one else is home or they'll wonder).

Anyone?

I think you are right. The force (and movement) required to throw the rock into the air will have some effect, and might tip the balance over 200 pound. Unless, as stated, you juggle in such a way that you only have one rock in your hand at any time. This is perfectly possible, provided you throw the stones up high enough.

littld · 30-06-2006, 16:57

Quote:

Originally Posted by Russ D

Are you on the sub's bench til you find the answer?

Not really - we just got into the argument and I either need to find she's right (in which case I mumble something and try not to make a deal of it) or else I'm right, in which case I can make a big noise about it.

Well, it's one way to run a relationship!

budwieser · 30-06-2006, 16:59

Quote:

Originally Posted by littld

Okay, question for the scientists out there.

I was reading a riddle the other day at this site and it says:

"A man weighing 190 lbs., carrying 3 round rocks, weighing 5 lbs. each, approaches a large ravine. The only way to cross is a rope bridge with a weight limit of 200 lbs. The ravine is too far to throw or roll the rocks across. How can the man cross the ravine in only one trip, with all 3 rocks?"

The answer follows that he juggled them all the way over the bridge.

In bed last night my wife and I argued this at length. You see, I think that even if he is juggling them, they still weigh 15 lbs and that weight can't just disappear. I think he still carries the weight as he has to exert force upwards to keep them in the air. My wife thinks that, as long as he starts juggling before he sets off on the bridge, he will "save" the 5 lb weight of one of the rocks.

Which one of us is right? It is quite important - my way he is a goner but if my wife is right then he's OK.

You were talking about this in bed!?

Couldn`t you find something more `interesting` to occupy you?

littld · 30-06-2006, 17:08

Quote:

Originally Posted by budwieser

You were talking about this in bed!?

Couldn`t you find something more `interesting` to occupy you?

We've been married 12 years tomorrow.

I think we have regular healthy discussions / arguments and I don't find anything wrong with that.

Let's not take this off-topic (a pretend man's life is at stake) but rest assured we can find other interesting things to do when there's nothing to discuss.

budwieser · 30-06-2006, 17:11

Quote:

Originally Posted by littld

We've been married 12 years tomorrow.

I think we have regular healthy discussions / arguments and I don't find anything wrong with that.

Let's not take this off-topic (a pretend man's life is at stake) but rest assured we can find other interesting things to do when there's nothing to discuss.

Sorry mate, just having a laugh. Take it easy.

I wasn`t disrespecting you or your good lady wife.

**Rob** · 30-06-2006, 17:12

Quote:

Originally Posted by Incognitas

Good grief! It's a maths lesson...

Nope it's a physics lesson

30-06-2006, 17:40

I dont think it's possible? there would be added weight to the juggler/bridge due to the force of the stone being thrown up into the air, plus the stone would weigh more (depends on high it was thrown) when caught again adding even more force. this would cause the bridge to snap due to the increasing momentum.

budwieser · 30-06-2006, 17:44

Is this similar to `The Holy Grail` and the discussion about swallows and coconuts.?