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#21 LookingForLag

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Posted 07 March 2008 - 04:30 PM

Well I did a Google search and found this link that explains it a little. http://www.tsn.ca/sh...d=2907&hubname=

#22 matt42s

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Posted 07 March 2008 - 07:50 PM

Matt

Let’s presume that increased drag slows the boat down. That means that the pulling force via the rope decreases. I still canot understand why the waterskier accelerates when the waterskier moves at an angle relative to the direction of the boat.

LoL

I cannot understand your point. What causes the skier to accelerate when angling away from the center line of travel?

Jeff.

Jeff, Yes the forward force decreases, but the total force increases - as the skier is using the increased drag to accelerate him 'sideways'. Once he is moving forwards and sideways, his total velocity can be calculated with Pythagoras and can be easily seen to be higher that the boats simple forward speed. You need a paradigm shift in your thinking - how about yachts? do you understand how a yacht sails into the wind?

#23 matt42s

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Posted 07 March 2008 - 08:24 PM

Jeff, it is when the skier is angling back towards the centerline that they are accelerated.

No - the acceleration is when the skier is angling away from the boat. His forward velocity is decreasing (along with the boat), but his sideways velocity is increasing at a much higher rate. His total velocity is the sum of his sideways and forward velocities. (pythagoras) When the skier turns back toward the boats direction of travel, the tow rope goes slack and the skier starts to decrease total velocity - but most of his sideways velocity has been converted into forward velocity. As soon as he turns, his new forward velocity is greater than the boats.

#24 Golf-Guru

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Posted 07 March 2008 - 08:28 PM

Good old force vectors at work.

#25 msilent_one

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Posted 07 March 2008 - 08:37 PM

Not sure if this makes sense but I think its all about "constants". The boat and skier will travel at the same (constant) speed when: the boat travels in a straight line at constant speed pulling the skier directly behind it with a rope that doesnt change in length. (this is like the natural state) The boat will keep travelling at the same speed throughout this whole thing. When the skier angles away from the boat he will be decelerating and the rope will slacken even at the slightest depending on the angle. Naturally the boat which is going straight will try to pull the skier back in line directly behind it to go back to that "natural state" where both are travelling together in a straight line. The skier then angles back and because the boat is still moving forward, the skier has to travel a greater distance to get back behind the boat and so is accelerated to catch up until it is back to the "natural state" directly behind the boat. The skier can actually keep going past "Straight" due to the momentum but he will no longer be accelerating until the boat once again tries to pull him back to "natural state" Hope it makes some kind of sense... ;)

#26 jeffmann

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Posted 07 March 2008 - 08:47 PM

LfL Thanks for the reference. I still have a hard time understanding this concept. The article states-: "A car traveling in a circle at a constant speed is changing the direction it is moving and is therefore accelerating." I cannot understand why the car is accelerating if it travels at a constant speed. You also state that the waterskier accelerates when he comes back to the centerline. Are you sure? I though that acceleration happens when he moves away from the centerline - because the tow rope has a "fixed" length and if the tow rope attached to the boat moves forward at a constant speed, and the skier is moving sideways away from the center + also moving forward at the same speed as the boat, the skier must be traveling a greater distance per unit time, and therefore must be traveling faster. F869 I cannot remember which DVD discusses the waterskier - it was related to a discussion of the release action. I am enjoying the Tomasello DVDs. I am on number 7. Matt - can you refer me to a Pythagoran equation that discusses speed rather than the relationship between side lengths in a triangle? Thanks. Jeff.

#27 DGLaville

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Posted 07 March 2008 - 09:48 PM

Look up centripetal acceleration. It should answer your questions.

#28 Toolish

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Posted 07 March 2008 - 10:03 PM

I cannot understand why the car is accelerating if it travels at a constant speed.

Speed does not have a direction, velocity and acceleration do. Driving in circles at constant speed you have a constant speed but the velocity is changing all the time due to a constant acceleration towards the middle of the circle. Just remember any time something changes speed or direction it has accelerated.

#29 LookingForLag

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Posted 08 March 2008 - 12:30 AM

LfL

Thanks for the reference.

You also state that the waterskier accelerates when he comes back to the centerline. Are you sure? I though that acceleration happens when he moves away from the centerline – because the tow rope has a “fixed” length and if the tow rope attached to the boat moves forward at a constant speed, and the skier is moving sideways away from the center + also moving forward at the same speed as the boat, the skier must be traveling a greater distance per unit time, and therefore must be traveling faster.

Jeff.

Jeff, I think I was incorrect when I stated that. I believe the skier would be able to accelerate whether they were moving towards or away from the centerline as they are able to generate a force on the tow rope in either direction, and that force is what causes the acceleration.

#30 matt42s

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Posted 08 March 2008 - 02:15 AM

Matt – can you refer me to a Pythagoran equation that discusses speed rather than the relationship between side lengths in a triangle?
I can't refer you anywhere, to me they're the same thing - but speed/velocity just has a time variable on both sides of the equation. This is probably hard if you don't have high school physics. How about this, picture a remote control hovercraft on an ice rink (low friction) 2 fan blowers on the hovercraft, the main fan at the back can propel it forward at a maximum speed of 20kph. A smaller thruster fan on the side of the hovercraft can propel it sideways at 5kph. If both fans are operating at full force, the hovercraft is moving at the square root of 20 squared plus 5 squared which is equivalent to the square root of 425 = 20.61kph

#31 msilent_one

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Posted 08 March 2008 - 02:33 AM

soon as you angle the skis away from the straight line, the boat moving forward will try and pull you back straight. if you dont pull on the line you will fly forward face first into the water. The pulling on the rope and the angle of the skis will keeps you from going back into the centre line. Because the distance has increased there will be tension on the rope accelerating the skis to the point where you are momentarily faster than the boat which is when the rope slackens. When the rope slackens you will be slowing down cos the boat is going at a constant speed. You will slow down to the point where the boat is again going faster than you are. The boats pull will tighten the rope and try to pull you into the centre. If you dont pull on the rope you will fly forward face first. so you pull on the rope and angle the ski in to keeping the rope tight and when you do the force will accelerate you towards the centre till you reach boat speed or even a little faster which allows you to cut across the other way. If you cut across the other way you will be slowing down and the rope will again slacken.




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