I notice the BB is very quiet these days, so here we go again !
A kayaker sits on a rock at the top of a waterfall in his kayak and seal launches off the rock through the air. He hits the pool below travelling at 4km/hr.
He again runs the waterfall, except this time he comes paddling down the river travelling at 3km/hr in the current and drops over the same waterfall.
He will now hit the pool below travelling at
A: 4
B: 5
C: 6
D: 7
E: 8 Km/hr. Sennen
Proper Phizz Quizz 14
Re:Proper Phizz Quizz 14
A, of course (assuming the usual terms and conditions)
Post edited by: avid, at: 2012/03/18 23:00
Post edited by: avid, at: 2012/03/18 23:00Re:Proper Phizz Quizz 14
Knotman, I recall my terror going over those drops even though its 20 years on! Certainly the paddler will be far more terrified than the seal launcher, if that counts ? I still can\'t pass the Clare Glens on the Limerick road without getting the shivers! Instinct tells me the paddler would love nothing more than to \"check\" and steady before the drop. Instinct tells me that for instance a heavier paddler who catches the lip will hit the pool slower than a lighter paddler who doesn\'t (and is generally better off, but sin scéil eile) ? I know the thing about the gun and the stone but this feels different?
I vote D - 7km/hr (3 + 4).
DW
I vote D - 7km/hr (3 + 4).
DW
Re:Proper Phizz Quizz 14
B.
Horizontal component, 3.
Vertical component, 4.
Resultant = sq. root of 3 sqd. + 4 sqd. = 5
B)
Horizontal component, 3.
Vertical component, 4.
Resultant = sq. root of 3 sqd. + 4 sqd. = 5
B)
Re:Proper Phizz Quizz 14
DW, the thing about the stone and the gun is relevant (or are you thinking about the stone and the feather?).
Assuming the launch is horizontal in both cases, the only vertical forces to consider are gravity and air resistance. Both are unchanged ergo the time taken to fall to the pool will be the same, as will be the vertical speed at impact.
Assuming the launch is horizontal in both cases, the only vertical forces to consider are gravity and air resistance. Both are unchanged ergo the time taken to fall to the pool will be the same, as will be the vertical speed at impact.
Re:Proper Phizz Quizz 14
I just love johnduun\'s answer. If the theory in what Tee and Avid say is correct (and Tee is irritatingly too often correct, but I don\'t know Avid), then johndunn squares the circle for me. 345 built the pyramids and to this day still squares up every kitchen extension being built from Carnsore to Torey and Baltimore to Ballymoney. It would explain the feeling that you hit the pool faster if you don\'t suffer a stall at the point of the drop, and such feelings are never wrong. Avid and Tee are saying the forward movement before the drop doesn\'t count at all at all ,but johndunn is saying it is maintained, and combined with the drop speed, it all adds up, if only a little. He is saying that the combined slanting speed is more than the simple drop speed, but not the full shilling.
I love it, it really has symmetry? I don\'t at all like the simplicity of the explanations of Tee and Avid.
I still say they are all wrong though, johndunn, Tee and Avid!
When a gun shoots there is a single momentary thrust horizontally, so the downward force on the bullet is the same as if someone dropped the bullet, and they hit the ground at the same time, if a mile apart. We are all agreed on that.
But .....
The kayak approaching the lip of the drop is different. It is in the water, being pushed by the water. Just try and backpaddle and you will experience what every wimp does, and I know - its too late. Two inches before the lip the water is doing 3km/hr (almost horizontally), at the lip that doesn\'t slow (45 degrees), and two inches after the drop its still doing 3km/hr (vertically). So is the kayak. Same speed, just a different direction.
The kayak is stuck down into the water and therefore the push isn\'t a once off like with the gun. Its like someone is pushing you, except suddenly the push changes direction. The plunge has initial velocity of 3 and we know it picks up another 4 from its vertical plunge .....
Another but .....
I am so impressed with johndunn and his 345 that I am now going E = 8, that is 3 + 4 + 1 (+1 for landing a bit out from the waterfall, which I also remember, with horror).
DW
I love it, it really has symmetry? I don\'t at all like the simplicity of the explanations of Tee and Avid.
I still say they are all wrong though, johndunn, Tee and Avid!
When a gun shoots there is a single momentary thrust horizontally, so the downward force on the bullet is the same as if someone dropped the bullet, and they hit the ground at the same time, if a mile apart. We are all agreed on that.
But .....
The kayak approaching the lip of the drop is different. It is in the water, being pushed by the water. Just try and backpaddle and you will experience what every wimp does, and I know - its too late. Two inches before the lip the water is doing 3km/hr (almost horizontally), at the lip that doesn\'t slow (45 degrees), and two inches after the drop its still doing 3km/hr (vertically). So is the kayak. Same speed, just a different direction.
The kayak is stuck down into the water and therefore the push isn\'t a once off like with the gun. Its like someone is pushing you, except suddenly the push changes direction. The plunge has initial velocity of 3 and we know it picks up another 4 from its vertical plunge .....
Another but .....
I am so impressed with johndunn and his 345 that I am now going E = 8, that is 3 + 4 + 1 (+1 for landing a bit out from the waterfall, which I also remember, with horror).
DW
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niallmadden
- Posts: 0
- Joined: Sat Oct 26, 2013 9:37 am
Re:Proper Phizz Quizz 14
Hmm. From a standing start, the paddler reached 4 km/hr. So what hight is this waterfall? Might be more of an overfall...
Niall.
Niall.
Re:Proper Phizz Quizz 14
About 3 or 4 inches actually!! Someone else spotted this before. 4km/hr is really slow. 4 metres per second would be a ferocious bang, so I left it at 4 km/hr for simplicity. I need that 4 and that 3 to illustrate the concept.
Thanks to all for replying. The best approach is to consider the energy involved which from the last Phizz Quizz is our old friend Kinetic Energy.
Direction does not matter with energy.
The energy depends on the velocity squared. (ignore mass, constant)
His energy from the seal launch is governed by 4squared=16 units.
His energy from the current is governed by 3squared = 9units.
His combined energy from both fall and current is thus 9+16 = 25units, since we cannot produce energy from nothing (Conservation of Energy), which answers C, D and E do!
To get 25 units of energy his velocity is 5km/hr since 5squared = 25.
This is a very tricky question and many Physics-y people get it wrong.
The result is totally counter-intuitive.
So when you go charging into those waves and your kayak slaps down on the other side (Yeehaw), most of the \"bang\" comes from your horizontal speed\'s energy and not from that little 2foot drop!
More realistically a truck rolling down a hill from a standing start at the top and hitting a wall at say 40km/hr will hit the same wall at 50km/hr if it was doing 30km/hr to start with! Put that in your pipe and smoke it. The answer is B. Sennen
Thanks to all for replying. The best approach is to consider the energy involved which from the last Phizz Quizz is our old friend Kinetic Energy.
Direction does not matter with energy.
The energy depends on the velocity squared. (ignore mass, constant)
His energy from the seal launch is governed by 4squared=16 units.
His energy from the current is governed by 3squared = 9units.
His combined energy from both fall and current is thus 9+16 = 25units, since we cannot produce energy from nothing (Conservation of Energy), which answers C, D and E do!
To get 25 units of energy his velocity is 5km/hr since 5squared = 25.
This is a very tricky question and many Physics-y people get it wrong.
The result is totally counter-intuitive.
So when you go charging into those waves and your kayak slaps down on the other side (Yeehaw), most of the \"bang\" comes from your horizontal speed\'s energy and not from that little 2foot drop!
More realistically a truck rolling down a hill from a standing start at the top and hitting a wall at say 40km/hr will hit the same wall at 50km/hr if it was doing 30km/hr to start with! Put that in your pipe and smoke it. The answer is B. Sennen
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lonanbyrne
- Posts: 1
- Joined: Sat Oct 26, 2013 9:37 am
Re:Proper Phizz Quizz 14
Sennen
Good question and answer, and I’d not made the connection to waves and slamming. Having composed the comments below, but gotten delayed in publication, I submit a late application!
My first thoughts were with Avid/Tee/John - the horizontal and vertical velocities of the kayak can be treated independently. The kayak hits the water with a vertical velocity due to gravity alone and a horizontal velocity equal to its speed down the river, and the resultant is B. Dave’s idea of the water pushing the kayak down the fall could be countered with a picture of bombs falling from a plane – as the bomb-bay doors open the bombs all fall away in a pattern all at the same speed. The kayak and the waterfall keep pace, neither pushing the other, like a barrel over Niagara Falls.
Niall’s question brought me up short – how high is the waterfall? I did the calcs and discovered it was no Niagara Falls. The kayak falls for just over 1/10 sec., and our waterfall is all of 6 centimeters high, or about the draft of a kayak! Niall was right – it’s an overfall.
In the time taken for the kayak to drop this distance, the boat will have moved about 0.8 m distance horizontally. Which brings me back to Dave’s question about what happens when a boat goes over a fall? The boat is not a barrel, but is long and takes about ½ sec. to go over at 3km/hr. As the bow goes out over the fall and starts to drop slowly, the tail is still on the river being pushed by water. The boat is rotating and some of the force of the river on the tail of the boat will be converted into an additional downward velocity over and above that of gravity. The horizontal velocity is correspondingly reduced and the boat\'s fall is steep. Dave’s intuition and experience is correct - the faster the river the harder the landing.
I was in danger of straying here, but you are right. Kinetic energy means the answer is still the same – B. Well done.
Lonan
Good question and answer, and I’d not made the connection to waves and slamming. Having composed the comments below, but gotten delayed in publication, I submit a late application!
My first thoughts were with Avid/Tee/John - the horizontal and vertical velocities of the kayak can be treated independently. The kayak hits the water with a vertical velocity due to gravity alone and a horizontal velocity equal to its speed down the river, and the resultant is B. Dave’s idea of the water pushing the kayak down the fall could be countered with a picture of bombs falling from a plane – as the bomb-bay doors open the bombs all fall away in a pattern all at the same speed. The kayak and the waterfall keep pace, neither pushing the other, like a barrel over Niagara Falls.
Niall’s question brought me up short – how high is the waterfall? I did the calcs and discovered it was no Niagara Falls. The kayak falls for just over 1/10 sec., and our waterfall is all of 6 centimeters high, or about the draft of a kayak! Niall was right – it’s an overfall.
In the time taken for the kayak to drop this distance, the boat will have moved about 0.8 m distance horizontally. Which brings me back to Dave’s question about what happens when a boat goes over a fall? The boat is not a barrel, but is long and takes about ½ sec. to go over at 3km/hr. As the bow goes out over the fall and starts to drop slowly, the tail is still on the river being pushed by water. The boat is rotating and some of the force of the river on the tail of the boat will be converted into an additional downward velocity over and above that of gravity. The horizontal velocity is correspondingly reduced and the boat\'s fall is steep. Dave’s intuition and experience is correct - the faster the river the harder the landing.
I was in danger of straying here, but you are right. Kinetic energy means the answer is still the same – B. Well done.
Lonan