maths based riddles... See if you can answer them.
Discussion
walm said:
The difference here is that you are DRAMATICALLY slowing the zig-zagging sand yacht by making him avoid the path of least resistance DOWNHILL.
As soon as he veers away from straight down he will start to slow.
That doesn't happen for the boat. It can point any direction and the current will still push it 10 knots.
ok, I get this - my comparison wasn't a fair one.As soon as he veers away from straight down he will start to slow.
That doesn't happen for the boat. It can point any direction and the current will still push it 10 knots.
walm said:
That doesn't happen for the boat. It can point any direction and the current will still push it 10 knots.
NO IT WON'T....If you somehow resist the forward motion of the boat, it will slow down - as it is easier for the water to slip past the hull than to overcome the force pushing it backwards. The only exception to this would be if the hull of the boat occupied a significant proportion of the cross sectional area of the river - so that water would find it very difficult to get past it, rather than continue carrying it along at the same speed.
A small boat on a big river will NOT travel at the same speed as the water - ever.... So the apparent wind in this case will never be 10 knots and the more sail you put up, the greater the resistance against the water and therefore the slower it will go (and the slower the apparent wind will be).
The website everyone is quoting is WRONG... It is possible for something to come out of a University website to be wrong!!!
The only way boat A can win the race is either by perpetual motion - and that has yet to be discovered, as far as I am aware - or the skipper of boat B to be a complete numpty and turn the boat the wrong way to tack into the wind!!!
kVA said:
Surely you understand that the heavier a boat is, the slower it goes - however it is being pushed?
Not meaning to be rude but REALLY?Do you REALLY believe that??????????
Read it again.
And again.
Are you saying that a 10,000hp motorboat with two fat blokes would be SLOWER than a 1hp powered motorboat with one fat bloke???
The problem you are having is that you think the water does the SAME WORK whether it is a light boat or a heavy boat.
It doesn't - it OBVIOUSLY does far more work on the fatties.
Of course if we were talking about THE SAME 1hp engine powering two boats then the fatties lose but that is NOT what is happening here.
kVA said:
The resistance of the hull against the moving water it will still slow down if you make it too heavy - the water will slip past the hull, rather than pushing it along at the same speed. Agreed?
No - absolutely not.Imagine this for a moment.
Water - moving at 10 knots.
A boat is fired by a cannon at EXACTLY 10 knots into the water.
(There is a 10 knot tailwind so still air.)
The boat lands in the water.
What happens?
Absolutely NOTHING.
There are no forces acting on anything except gravity which is perpendicular so won't change the horizontal speed.
The boat will sit in the water BOTH doing 10 knots.
Of course, when you drop in a fattie it depends on how fast he is going when he drops in.
If he jumps in while going 10knots then no problem everything stays the same.
If he really gives it some then the CONSERVATION OF MOMENTUM, means that the boat will actually SPEED UP for a little while until that momentum is dissipated through friction.
Now if he is going ZERO then YES again conservation of momentum means the boat will slow.
It would be like a fatty jumping into your boat (on a pond, not moving) BACKWARDS at 10 knots.
HOWEVER, eventually the friction will slow that momentum and the boat will come to rest again.
Here the WATER HAS DONE MORE WORK.
walm said:
kVA said:
A small boat on a big river will NOT travel at the same speed as the water - ever....
This is your problem, I think.Why on earth not?
Of course it will.
That's the whole premise of the question!
Please give me a force that is slowing that small boat down.
ANY FORCE!!!
therealpigdog said:
walm said:
kVA said:
A small boat on a big river will NOT travel at the same speed as the water - ever....
This is your problem, I think.Why on earth not?
Of course it will.
That's the whole premise of the question!
Please give me a force that is slowing that small boat down.
ANY FORCE!!!
(This is Boat B we are talking about remember.)
Anyone else?
kVA said:
therealpigdog said:
I understand that boat B is becalmed, and can travel at a velocity of 10 knots max - slightly less because the water would flow around the boat, and nothing moves at the same speed as the current...
This is the one essential element that everyone else seems NOT to understand...
This is the one essential element that you clearly DO NOT understand.
Boat in a pond, stationary, no wind.
And boat B.
They are THE SAME.
PLEASE PLEASE explain to us why you think differently?
Is it because you have seen the current pushing a boat along in real life and for whatever reason it wasn't going at the same speed as the water?
Because I will agree - THAT IS POSSIBLE.
For any number of reasons - air resistance, cross currents, it's just been launched etc...
None of these apply in the example we are talking about.
The boat and the water go the same speed.
walm said:
:no: :no: :no:
This is the one essential element that you clearly DO NOT understand.
Boat in a pond, stationary, no wind.
And boat B.
They are THE SAME.
PLEASE PLEASE explain to us why you think differently?
Is it because you have seen the current pushing a boat along in real life and for whatever reason it wasn't going at the same speed as the water?
Because I will agree - THAT IS POSSIBLE.
For any number of reasons - air resistance, cross currents, it's just been launched etc...
None of these apply in the example we are talking about.
The boat and the water go the same speed.
I think you are now confused This is the one essential element that you clearly DO NOT understand.
Boat in a pond, stationary, no wind.
And boat B.
They are THE SAME.
PLEASE PLEASE explain to us why you think differently?
Is it because you have seen the current pushing a boat along in real life and for whatever reason it wasn't going at the same speed as the water?
Because I will agree - THAT IS POSSIBLE.
For any number of reasons - air resistance, cross currents, it's just been launched etc...
None of these apply in the example we are talking about.
The boat and the water go the same speed.
We are talking about boat A and it WILL be slowed by air resistance of anything above the waterline - especially when you put up the sails to tack into the apparent wind!!!
kVA said:
therealpigdog said:
I understand that boat B is becalmed, and can travel at a velocity of 10 knots max - slightly less because the water would flow around the boat, and nothing moves at the same speed as the current...
This is the one essential element that eveeryone else seems NOT to understand...
walm said:
Which bit of "Boat B" is confusing you?
OK, I know it's umpteen pages back, but boat B is the one with the 10 knot tail wind AND a 10 knot current and so will move at 10 knots - whether the sails are up or down.Boat A has only the current driving it forward and if it is in the same stretch of river as boat B, we can safely assume there is no issue of the water not being able to pass its hull, rather than move it at the same speed: Gravitational inertia will therefore play some part in slowing it below 10 knots, as will any air resistance acting on its superstructure. So it does not actually have a 10 knot apparent wind in the first place and, as soon as the sails are raised, it will slow down further... Even if it is possible to sail at a higher speed than the apparent wind, the forward component of that motion cannot possibly be more than 10 knots, else Newtons 3rd Law will have been broken.
kVA said:
Talking about boat A and it WILL be slowed by air resistance of anything above the waterline - especially when you put up the sails to tack into the apparent wind!!!
Of course it will. I agree 100%.Just as much as any boat in a 10 knot breeze will be pushed backwards over the water if it just sits there without sailing.
This doesn't stop it sailing upwind.
If boat A just sat there and boat B just sat there, then indeed boat B (becalmed) would win owing to the minor amount of air resistance (as you call it - everyone else just calls it "wind") pushing boat A's hull and spars backwards.
But boat A doesn't just sit there.
It sails upwind.
kVA there are a lot of confusing concepts flying around, so I go back to the most simple that I can think of.
How would a sailor tell the difference between:
1. 10 knot breeze and still water.
2. Water current at 10 knots and still air.
Please describe JUST ONE thing that would be different in those two situations on the boat. Any force that you can point to.
kVA said:
Boat A has only the current driving it forward and if it is in the same stretch of river as boat B, we can safely assume there is no issue of the water not being able to pass its hull, rather than move it at the same speed: Gravitational inertia will therefore play some part in slowing it below 10 knots, as will any air resistance acting on its superstructure.
FINALLY!!!!!!!!! We are getting there!!!What exactly is this magical "gravitational inertia" you speak of?????
Gravity acts DOWNWARDS.
It can't possibly change its horizontal speed or momentum.
If you mean "inertia" then you are confusing yourself by assuming that the boat has at some point been at rest and needs to be sped up - this isn't necessarily the case and even if it was it would be just the same as the inertia of Boat B.
On the air resistance - I agree, as I have said many times.
So can we PLEASE agree that the ONLY difference between A and B is the air resistance acting on Boat A?????
walm said:
kVA said:
therealpigdog said:
I understand that boat B is becalmed, and can travel at a velocity of 10 knots max - slightly less because the water would flow around the boat, and nothing moves at the same speed as the current...
This is the one essential element that everyone else seems NOT to understand...
walm said:
kVA there are a lot of confusing concepts flying around, so I go back to the most simple that I can think of.
How would a sailor tell the difference between:
1. 10 knot breeze and still water.
2. Water current at 10 knots and still air.
Please describe JUST ONE thing that would be different in those two situations on the boat. Any force that you can point to.
Simple: How would a sailor tell the difference between:
1. 10 knot breeze and still water.
2. Water current at 10 knots and still air.
Please describe JUST ONE thing that would be different in those two situations on the boat. Any force that you can point to.
1.) No matter how many sails the sailor raises in a 10 knot wind, the wind will still be blowing at 10 knots
2.) As soon as the sailor raises a sail, the boat will no longer be travelling at the same speed as the current, as air-resistance will slow it down: The more sails he or she raises, the slower it will go - and so there no longer is a 10 knot wind
Remember it is not only the force of the water that determines the speed of the boat - it will be slowed by aerodynamic drag acting against the current, it will have gravitational inertia with a backward component directly proportional to the speed of the current and the hull is not 100% efficient at stopping some of the water slipping past it, rather than pushing it forwards.
Edited by kVA on Friday 23 September 14:41
kVA said:
Simple:
1.) No matter how many sails the sailor raises in a 10 knot wind, the wind will still be blowing at 10 knots
Absolutely not - RELATIVE TO THE BOAT.1.) No matter how many sails the sailor raises in a 10 knot wind, the wind will still be blowing at 10 knots
From the frame of reference of the water what happens is:
Boat moving downwind slightly from the wind pushing against the hull and spars.
Sails go up and the boat moves even faster downwind as there is more for the wind to push.
Of course stationary on the water the wind will always be 10knots!!
While you are on the boat the wind you feel is obviously 10 knots LESS the speed you are going downwind.
THIS IS EXACTLY WHAT HAPPENS ON THE OTHER BOAT.
walm said:
FINALLY!!!!!!!!! We are getting there!!!
What exactly is this magical "gravitational inertia" you speak of?????
Gravity acts DOWNWARDS.
It can't possibly change its horizontal speed or momentum.
Oh dear, you are starting to look slightly silly now: Do you really believe that? So if you double the weight of your car, that will have no effect on it's speed then, for a given engine power / torque?What exactly is this magical "gravitational inertia" you speak of?????
Gravity acts DOWNWARDS.
It can't possibly change its horizontal speed or momentum.
walm said:
If you mean "inertia" then you are confusing yourself by assuming that the boat has at some point been at rest and needs to be sped up - this isn't necessarily the case and even if it was it would be just the same as the inertia of Boat B.
Gravitational force will have a horizontal component on a boat that is moving - simple vectors will show you thatwalm said:
On the air resistance - I agree, as I have said many times.
So can we PLEASE agree that the ONLY difference between A and B is the air resistance acting on Boat A?????
So can we PLEASE agree that the ONLY difference between A and B is the air resistance acting on Boat A?????
Yes, we can agree on that - as long as both boats are pointing in the same direction!
However, as soon as boat A turns away from the current direction, it will actually have less hydrodynamic resistance than the square end of the stern of boat B, so more of the water will be able to slip past the hull, rather than driving it forward when it encounters air resistance from the front.
kVA said:
Remember it is not only the force of the water that determines the speed of the boat - it will be slowed by aerodynamic drag acting against the current
Yes - this you keep calling "air resistance" and is the same for both 1 and 2 above.It is the 10 knot breeze pushing on the spars.
kVA said:
It will have gravitational inertia with a backward component directly proportional to the speed of the current and the hull is not 100% efficient at stopping some of the water slipping past it, rather than pushing it forwards.
THIS IS THE CRUX.You are simply making stuff up! (No offence.)
Check Newton 1: The velocity of a body remains constant unless the body is acted upon by an external force.
We are in agreement that ex-wind/air resistance or whatever a boat moving at 10 knots will STAY moving at 10 knots yes???? Unless an external force acts on it - yes????
In that case - gravity (the force you keep referring to) can't possibly slow or speed it up (horizontally) because gravity is perpendicular to its horizontal motion!!!!!
I think that you keep thinking that the water needs to keep pushing the boat along.
It doesn't. If the boat is moving 10 knots it will stay moving 10 knots (unless another force is acting upon it).
As above gravity doesn't change anything.
When you add in the complexity of the air resistance (wind) BOTH BOATS EXPERIENCE EXACTLY THE SAME FORCE.
Namely a 10 knot breeze.
Seriously - this "gravitational inertia" thing is your problem.
Check here: http://en.wikipedia.org/wiki/Inertia
The important line is the following:
"On the surface of the Earth inertia is often masked by the effects of friction and gravity, both of which tend to decrease the speed of moving objects (commonly to the point of rest). This misled classical theorists such as Aristotle, who believed that objects would move only as long as force was applied to them."
You are Aristotle AICMFP.
walm said:
Absolutely not - RELATIVE TO THE BOAT.
From the frame of reference of the water what happens is:
Boat moving downwind slightly from the wind pushing against the hull and spars.
Sails go up and the boat moves even faster downwind as there is more for the wind to push.
Of course stationary on the water the wind will always be 10knots!!
While you are on the boat the wind you feel is obviously 10 knots LESS the speed you are going downwind.
THIS IS EXACTLY WHAT HAPPENS ON THE OTHER BOAT.
Mate, the speed of the wind relative to Captain Birdseye stood on the boat is completely and utterly irrelevant in this riddle!!! Neither boat will win the race by having the sailor's hair more dishevelled than the other!!!From the frame of reference of the water what happens is:
Boat moving downwind slightly from the wind pushing against the hull and spars.
Sails go up and the boat moves even faster downwind as there is more for the wind to push.
Of course stationary on the water the wind will always be 10knots!!
While you are on the boat the wind you feel is obviously 10 knots LESS the speed you are going downwind.
THIS IS EXACTLY WHAT HAPPENS ON THE OTHER BOAT.
What matters is the speed relative to land, as that is how the race winner is being determined!
Incidentally, I note that the original objectors to my theory have gone awful quiet? have you just realised I'm right at last?
k off to the Boats, planes and trains forum ? You have killed this thread. kVA said:
walm said:
FINALLY!!!!!!!!! We are getting there!!!
What exactly is this magical "gravitational inertia" you speak of?????
Gravity acts DOWNWARDS.
It can't possibly change its horizontal speed or momentum.
Oh dear, you are starting to look slightly silly now: Do you really believe that? So if you double the weight of your car, that will have no effect on it's speed then, for a given engine power / torque?What exactly is this magical "gravitational inertia" you speak of?????
Gravity acts DOWNWARDS.
It can't possibly change its horizontal speed or momentum.
walm said:
If you mean "inertia" then you are confusing yourself by assuming that the boat has at some point been at rest and needs to be sped up - this isn't necessarily the case and even if it was it would be just the same as the inertia of Boat B.
Gravitational force will have a horizontal component on a boat that is moving - simple vectors will show you thatBring on the simple vectors!
walm said:
On the air resistance - I agree, as I have said many times.
So can we PLEASE agree that the ONLY difference between A and B is the air resistance acting on Boat A?????
So can we PLEASE agree that the ONLY difference between A and B is the air resistance acting on Boat A?????
Yes, we can agree on that - as long as both boats are pointing in the same direction!
However, as soon as boat A turns away from the current direction, it will actually have less hydrodynamic resistance than the square end of the stern of boat B, so more of the water will be able to slip past the hull, rather than driving it forward when it encounters air resistance from the front.
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