Physics questions.
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Shaper
Member # 247
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written Saturday, December 3 2005 20:01
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Hey, this is a question I'm sure is going to be on my Physics final. But I can't figure it out exactly. Can you have velocity to the west and acceleration to the East? I think that was how my teacher posed the question. Another problem I've come across is: Say you have a box that is 50 kg being pulled along a floor at a 20 degree angle with friction equal to 30N. Is the Efx= Tcos(20)-Fk=ma, or does the mass work into that somewhere. It seems logical that mass doesn't work in the X direction, but I just can't remember. This is first year physics so no advanced stuff. -------------------- The Knight Between Posts. Posts: 2395 | Registered: Friday, November 2 2001 08:00 |
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Lifecrafter
Member # 1468
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written Saturday, December 3 2005 20:07
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Yes, you can have West velocity with East acceleration. It's just slowing down. Technically, deceleration is acceleration. As for your second question, I got to the end of the first sentence, and my brain said "Oh, come on, is the work really worth it?" So I don't have an answer to it. -------------------- "We can learn a lot from crayons. Some are short, some are dull, some are sharp, some are tall. Some have funny names and they are all different colors, but they all learn to live in the same box." "Happy is the man that has wisdom and gets discernment. For having wisdom as gain is better than having silver as gain and having wisdom as produce is better than gold itself" Proverbs 3:14-3:15 The horrible part about life is, you'll never get out of it alive. Currently boycotting: AngelFire, GameFAQ's, Macintosh PC's Posts: 818 | Registered: Tuesday, July 9 2002 07:00 |
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The Establishment
Member # 6
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written Saturday, December 3 2005 20:11
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1) Yes. This would imply that you are moving west but decelerating. Think of acceleration as the rate at which your velocity is changing. If you are speeding up, it is positive and in the same direction; if you are slowing down, it is negative and in the opposite direction. 2) Well, the best way to do this is to draw out the forces in what is called a Free-Body Diagram. I suggest reading up on these as it will allow you to theoretically allow you to solve any mechanics problem. If I am picturing this correctly, you have a box of 50 kg on a flat surface being pulled by an external force (T) 20 degrees with respect to the surface. Friction (f) always acts in the opposite of the direction of motion, it will always oppose an applied force. The equations I get are: F_x = Tcos(20) - f = ma_x F_y = N - mg = 0 F_x = forces in x direction F_y = forces in y direction T = external/applied force f = friction force m = mass of block a_x = acceleration in x-direction N = normal force exerted by the ground upon the block g = gravitation accelration 9.81 m/s^2 Also note that f = mu*N where mu is the coefficient of friction. Hope this helps. -------------------- Your flower power is no match for my glower power! Posts: 3726 | Registered: Tuesday, September 18 2001 07:00 |
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...b10010b...
Member # 869
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written Saturday, December 3 2005 23:25
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A question that came to me while driving on a particularly boring stretch of highway: I'm driving along a flat road, and at one point I notice that from where I'm sitting, the vertical distance between the front of my own car's bonnet and the bottom of the wheels of the car in front of me appears to be exactly half the vertical distance between the front of my car's bonnet and the horizon (that is to say, the former subtends half as broad an angle on my retina as the latter). What further information do I need in order to work out the horizontal distance between myself and the car in front, and how would I go about calculating it? [ Sunday, December 04, 2005 00:28: Message edited by: Thuryl ] -------------------- The Empire Always Loses: This Time For Sure! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Warrior
Member # 3480
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written Sunday, December 4 2005 00:40
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The infomation you need is the horizontal distance between you and the car in front. :cool: Posts: 169 | Registered: Wednesday, September 24 2003 07:00 |
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...b10010b...
Member # 869
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written Sunday, December 4 2005 02:03
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Really? I'd have thought it'd be possible to work it out based on the height of my eye level above the ground, the height of my car's bonnet above the road, the horizontal distance between my eye and the end of my car's bonnet and possibly the size of my eye and the curvature of the Earth. [ Sunday, December 04, 2005 02:04: Message edited by: Thuryl ] -------------------- The Empire Always Loses: This Time For Sure! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Infiltrator
Member # 4637
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written Sunday, December 4 2005 02:33
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quote:The scenario and the shadows are good hints for your brain, to calculate that distance. -------------------- Visit the Blades of Avernum Center and the Beta Testing Center -------------- "Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ." Colossians 2:6-9 Posts: 483 | Registered: Tuesday, June 29 2004 07:00 |
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...b10010b...
Member # 869
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written Sunday, December 4 2005 04:45
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Well, sure, if you want to do things the sensible way. :P -------------------- The Empire Always Loses: This Time For Sure! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Post Navel Trauma ^_^
Member # 67
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written Sunday, December 4 2005 05:53
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Knowing the curvature of the earth would make for a more accurate calculation, but I think the error from not knowing would be less than the errors in all the other factors. I'd say the easiest-to-do way would be to measure the height of your eyes while driving, and the distance from you to the closest point on the road that you can see from your driving position (measured while stopped, if you have any sense). Then, you can use trigonometry in some way I can't be bothered working out exactly to get your answer. [ Sunday, December 04, 2005 05:57: Message edited by: Khoth ] -------------------- Barcoorah: I even did it to a big dorset ram. desperance.net - Don't follow this link Posts: 1798 | Registered: Thursday, October 4 2001 07:00 |
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The Establishment
Member # 6
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written Sunday, December 4 2005 08:19
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Don't forget about the curvature of spacetime caused by you and your car! You will be off by 10^-30%!! [ Sunday, December 04, 2005 08:19: Message edited by: *i ] -------------------- Your flower power is no match for my glower power! Posts: 3726 | Registered: Tuesday, September 18 2001 07:00 |
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Post Navel Trauma ^_^
Member # 67
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written Sunday, December 4 2005 11:13
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Just accelerate until you feel a bump. Then you'll know that the answer is zero. -------------------- Barcoorah: I even did it to a big dorset ram. desperance.net - Don't follow this link Posts: 1798 | Registered: Thursday, October 4 2001 07:00 |
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The Establishment
Member # 6
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written Sunday, December 4 2005 11:46
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Not quite zero, there is the pesky interatomic distance thing worry about. -------------------- Your flower power is no match for my glower power! Posts: 3726 | Registered: Tuesday, September 18 2001 07:00 |
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Master
Member # 4614
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written Sunday, December 4 2005 12:43
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No, no. I'm sure the intermingling of atoms from each can will cause a negative or effective zero distance. -------------------- -ben4808 Posts: 3360 | Registered: Friday, June 25 2004 07:00 |
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Infiltrator
Member # 4637
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written Sunday, December 4 2005 13:13
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quote:Anyway, the horizon's distance is not an objective data, as your visual accuracy is different from others'. People who can see further than you, will see the horizon more ahead than you. (Except if the fistance between you and the other car is short, so the error could be ignorable). -------------------- Visit the Blades of Avernum Center and the Beta Testing Center -------------- "Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ." Colossians 2:6-9 Posts: 483 | Registered: Tuesday, June 29 2004 07:00 |
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Law Bringer
Member # 335
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written Sunday, December 4 2005 13:42
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Accelerate fast enough and the intermingling of cars as well as the spray of bits of car in all directions will ensure that the uncertainty in your location and that of your victim are much greater than any of these pesky little phenomena. —Alorael, who would rather find the distance to another car by knowing how fast he's going and how long it takes to get from where he is at a given time to where the other car was at that time. Posts: 14579 | Registered: Saturday, December 1 2001 08:00 |
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Shaper
Member # 247
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written Sunday, December 4 2005 23:15
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Why do velocities in fluids effect pressures in a tube system? V1 >V2 implies P2 >P1 why? :confused: -------------------- The Knight Between Posts. Posts: 2395 | Registered: Friday, November 2 2001 08:00 |
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Apprentice
Member # 6009
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written Monday, December 5 2005 00:28
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The total pressure of a fluid is defined by the Bernoulli integral: Pt (total pressure) = P (static pressure) + density*g*height (elevation pressure) + (density*v^2)/2 (dynamic pressure) For a given fluid, 'total pressure' is constant, unless you're putting energy into the system. 'Static pressure' is what we normally think of as 'pressure' (that is, the forces exerted by a substance on a surface). However if you have a fluid with the a constant 'total pressure' (and at the same height, so you can ignore the effect of elevation pressure due to the lack of gravitational effects), clearly when you increase velocity, there has to be a corresponding decrease in the static pressure. If that helps. Posts: 18 | Registered: Friday, June 24 2005 07:00 |
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Electric Sheep One
Member # 3431
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written Monday, December 5 2005 00:48
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This is a surprisingly good question. Surprising, that is, until you come to appreciate that most questions about hydrodynamics are good. It's a very tough subject on several levels. The answer in terms of hydrodynamics is really that it is the pressure change that has caused the velocity change, and this is why the two go together. A pressure difference accelerates or decelerates a fluid. So to understand why V_1 > V_2 implies P_2 > P_1, think of a ball rolling into a bumper, and slowing down as it compresses the spring. When the ball has gone a little further and slowed down some, the spring is also more compressed. What makes it easy to get confused is that fluid velocity can in turn affect pressure. A net spreading pattern of fluid velocity makes for a drop in fluid density, which lowers pressure. A net convergence raises density and pressure. So figuring out what velocities and pressures are everywhere in say, a pipe, is very subtle and difficult. It's a vicious circle that you have to settle all round. (Welcome to the world of coupled nonlinear partial differential equations, where dead numbers go if they were bad.) To solve these problems, at least approximately, people often use tricks to deduce what a steady velocity pattern might have to look like. Some of these tricks are clever; some are just knowing the final answer and talking fast enough to give a false impression of understanding why it comes out that way. Whether your trick is clever or deceptive, once you have an idea of the velocity pattern, then you can use the relation between pressure and velocity to work out what the pressures must have been, to maintain such a pattern of velocities. This is reasoning backwards, in comparison to physical causality, where it is the pressure change that has caused the velocity change. But if you did somehow get the velocity pattern right, then this working backwards will indeed give the right answer for the pressures. All of that is the hydrodynamic answer. There is a deeper answer, in principle, in terms of the motion and collisions of the particles that compose the fluid. I haven't gotten around to thinking this through completely myself, though. And I've never heard anyone else give a convincing answer, in terms of air molecules, to questions like why an airfoil generates lift. I'm sure it can be done, and I'm almost sure that it has been done; but very few physicists know about it. [ Monday, December 05, 2005 00:54: Message edited by: Student of Trinity ] -------------------- We're not doing cool. We're doing pretty. Posts: 3335 | Registered: Thursday, September 4 2003 07:00 |
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...b10010b...
Member # 869
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written Monday, December 5 2005 01:03
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By the way, just to give a biological perspective, the relation between pressure and velocity in a fluid has some important clinical consequences related to the circulatory system. If a region of an artery is narrowed (due to atherosclerosis, for example), fluid will have to travel faster to get through that region, and pressure will drop. This can cause the artery to intermittently collapse and reopen as blood pressure varies throughout the cardiac cycle. Conversely, in an aneurysm, a region of a blood vessel is widened. Blood moves more slowly in this region and is therefore at higher pressure. This pressure on the wall of the blood vessel may cause it to stretch, increasing the size of the aneurysm and therefore further increasing the pressure. As a result, an untreated aneurysm is likely to grow over time until eventually it bursts. And no, my biophysics lecturer couldn't give a good explanation of why pressure was inversely proportional to velocity either. [ Monday, December 05, 2005 01:09: Message edited by: Thuryl ] -------------------- The Empire Always Loses: This Time For Sure! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Infiltrator
Member # 4637
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written Monday, December 5 2005 04:07
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quote:I can't agree 100%. Atherosclerosis causes an arthery to be narrowed, making the artery thickier, reducing its flexibility and elasticity AND increasing peripheral vascular resistance. The result is an increase of the blood pressure and lower blood flux velocity. The general elasticity loss can cause a slower response of the baroreceptors (don't know the english name, but they sense and regulate arterial pressure), making them less capable of modulating arterial pressure. "pressure was inversely proportional to velocity", that's correct, but the velocity is what is lowered, not pressure. That's why older people usually have HAT [=high arterial tension=higher pressure] (age is one of the risk factors of atherosclerosis). -------------------- Visit the Blades of Avernum Center and the Beta Testing Center -------------- "Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ." Colossians 2:6-9 Posts: 483 | Registered: Tuesday, June 29 2004 07:00 |
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...b10010b...
Member # 869
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written Monday, December 5 2005 05:12
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quote:Atherosclerosis increases the mean arterial pressure, true. But the pressure in an individual artery that's narrowed is actually lower than it would be if it weren't. Basically, which of our explanations is correct depends on whether you're considering an individual blood vessel or the circulatory system as a whole. -------------------- The Empire Always Loses: This Time For Sure! Posts: 9973 | Registered: Saturday, March 30 2002 08:00 |
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Infiltrator
Member # 4637
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written Monday, December 5 2005 06:03
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quote:I understand what you mean, but I don't understand how the pressure decrease in an individual artery will translate in a whole system increase (specially if you consider that atherosclerosis affects the whole vascular system, not just specific single arteries, although specific problematic arteries and vassels exist)... A tighter passage and increased vascular resistance in that artery can only, logically, produce higher pressure and lower velocity, not the opposite. The fact that the why can't be explained should make you doubt about that opposite. Let's say you're an old man (age is a risk factor), and you suffer from atherosclerosis (not in a single arthery, but in the whole system, more in specific areas and less in other, but all). There would be many artheries and vassels where, as you say, the pressure would drop and the velocity increase. How could that translate in a general higher pressure and lower velocity? -------------------- Visit the Blades of Avernum Center and the Beta Testing Center -------------- "Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ." Colossians 2:6-9 Posts: 483 | Registered: Tuesday, June 29 2004 07:00 |
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La Canaliste
Member # 5563
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written Monday, December 5 2005 07:30
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quote:It depends on what you mean by "why". *rushes in where biophysics lecturers and anges fear to tread* Explanation 1: low level fluid mechanics. The fluid slows down, so its kinetic energy drops. The energy has to go somewhere, so it is converted to pressure. Explanation 2: an analogy. Imagine traffic flow. If the traffic is flowing fast and smoothly, then it is well spaced out (no not like Thuryl on a Saturday night) and you may be able to "see" that the pressure is low. If it now slows down, as at a toll, the vehicles have to be a lot closer together. In fact the increase in "pressure" at a toll causes the road to bulge. There are many other levels of explanation. Like I said, it depends on what you mean by "why". -------------------- I am a mater of time and how . Deep down, you know you should have voted for Alcritas! Posts: 387 | Registered: Tuesday, March 1 2005 08:00 |
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Infiltrator
Member # 4637
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written Monday, December 5 2005 07:48
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quote:Aham. That explanation doesn't explain that "why". It just confirms what I said. -------------------- Visit the Blades of Avernum Center and the Beta Testing Center -------------- "Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ." Colossians 2:6-9 Posts: 483 | Registered: Tuesday, June 29 2004 07:00 |
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Law Bringer
Member # 335
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written Monday, December 5 2005 08:35
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Imagine people instead of cars. People moving forward tend to just move forward. People inching forward at a crawl are more likely to fidget, shift, bump into each other, get angry, and bang into things. The banging into things is pressure. —Alorael, who acknowledges that this is a terrible way to explain pressure. It's a good picture physically and emotionally, though! Posts: 14579 | Registered: Saturday, December 1 2001 08:00 |
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