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tzar View Drop Down
Baron
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Posts: 472 		  Quote tzar Quote  Post ReplyReply Direct Link To This Post Topic: question about newton
    Posted: 06-May-2010 at 03:28
Hello to everyone! I've been missing for a long time. Yesterday we had about 3 hours argument about this:
Imagine two boats on the river/lake and three persons. The first boat is tied with rope to its prow, the other end of the rope is at the hands of one of the persons. about the second boat  - there is a person on the boat, who  holds the rope and a person on the beach who holds the other end of the rope. If everything is equal- speed, force, friction, weight - which boat would reach the beach first - first or second? The situation is like on this pic. I'm not good painter :) 
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Posts: 4620 		  Quote opuslola Quote  Post ReplyReply Direct Link To This Post Posted: 18-May-2010 at 09:02
Tzar, these words lead me into some confusion, you wrote;

"If everything is equal- speed, force, friction, weight - which boat would reach the beach first - first or second?"

Are you saying that the empty boat weighs the same as the boat with the man in it? And,what do you mean by speed being equal?

But nothing is said about "pulling" the boats?

You see, I find too many variables to make an answer!

Thus if neither of the persons on the beach pulls the boat, then the boat will "only reach the beach" if the wind or current or both cause it!

Now, assuming that with all things, like the weight of the boats, whether manned or not, and friction, etc., are the same we have a little more of a problem. First of all, what if both the person in the boat, and the one on the beach each collects the rope, thus both cause the boat to come towards the beach?

Assuming that the person of the beach does not just hold on the end of the rope and run away from the water (which should increase the speed towards the beach by a large factor over merely collecting the rope at ones feet, while pulling the rope hand over hand, etc.!) Thus I would say the boat with two men, both pulling the rope hand over hand, whould beat the one man on the beach pulling the rope in a like manner! It would be a two human powered boat versus a one human powered boat!

My conclusion, based upon the above is all things being equal, the boat with the man, would beat the boat without the man, etc.!
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Posts: 555 		  Quote DreamWeaver Quote  Post ReplyReply Direct Link To This Post Posted: 18-May-2010 at 09:57
If speed, force, friction, weight etc are all equal then the boats reach the beach at the same time.

The empty boat must be different somehow to the boat with a man in to have the same weight, maybe it has more mass, but if everything else is equal, then the boats weigh the same and are pulled at the same speed with the same force towards the beach then they will arrive at the same time.
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Posts: 472 		  Quote tzar Quote  Post ReplyReply Direct Link To This Post Posted: 19-May-2010 at 12:35
Opuslola well forget about the speed - my mistake. If the speed is equal off course they reach the beach at same time
It is just on theory. At real situation this is impossible, but I think the same as DreamWeaver that the both boat would reach the beach at the same time.
Opuslola BTW my brothers thinks exactly as you - this with man on a board would be first.
 
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Posts: 357 		  Quote Shield-of-Dardania Quote  Post ReplyReply Direct Link To This Post Posted: 08-Jun-2010 at 23:48

My answer:

Assumptions:
(1) The 2 boats (excluding the man in one of the boats) are both equal in weight, say X kg each. The 3 men are also equal in weight, say Y kg each.
 
(2) The boats are not operating on their own power (i.e. engine switched off).
 
(3) Only the force, say F (pulling force of the men, i.e. their 'strength'), weight (weight of men, weight of boats) and friction (between the boats' underside and the surface of the water), say Fr, can be equal.
 
(4) The speed (at which each man is pulling his boat) of the boats cannot be equal, because the total weight each man is pulling is different.
 
The 2nd man (Y kg), pulling the boat only, will be pulling a weight of only X kg, against a friction Fr. While the 1st man (Y kg), pulling the boat with a man on it, will be pulling a weight of (X + Y) kg, against a friction Fr.
 
Therefore, the 2nd man, of the same pulling force ('strength') as the 1st man, will be able to pull his boat (X kg) faster.
 
Just imagine yourself, for exaggeration, pulling: (1) a big ship (engine switched off); and (2) a small wooden boat (engine switched off); over the water. Which one would you pull faster?
 
But if there had been zero friction (a hypothetical situation), then each man would have been pulling with equal speed, because the weight of each boat is acting straight down in the Y-axis, while each man would be pulling horizontally, i.e. in the X-axis. So neither of them would actually be affected by the weight he is 'pulling'.


Edited by Shield-of-Dardania - 08-Jun-2010 at 23:58
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DreamWeaver View Drop Down
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Posts: 555 		  Quote DreamWeaver Quote  Post ReplyReply Direct Link To This Post Posted: 09-Jun-2010 at 04:27
Of course this is all just physics, where we all make wonderful assumptions about friction and tension of the rope etc. Engineers would no doubt have different answers. Hence the ever consistent arguments between engineers and scientists.
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Posts: 4620 		  Quote opuslola Quote  Post ReplyReply Direct Link To This Post Posted: 15-Jul-2010 at 12:06
As I said earlier;

"You see, I find too many variables to make an answer!"

Perhaps I was correct?
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