Robittybob1 Posted May 21, 2015 Share Posted May 21, 2015 (edited) You didn't say there was a pair of forces. You said the particle exerted a force on itself. What would the reaction force be? Another force on itself? Momentum is NOT "always conserved". (A common conceptual error). Momentum is conserved when the net force on a system is zero. If an object could start spontaneously moving because it generated a force on itself, momentum would not be conserved. The object would be moving with one velocity, and then spontaneously have a different one, with no external reason. p1 ≠ p2 Momentum not conserved. (Momentum of an object traveling in a circle is not conserved; whatever is exerting the centripetal force is changing it) That is a surprise, The momentum of an object traveling in a circle has angular momentum and that is conserved. I listened to a lecture on that the day before yesterday. If you work out its instantaneous momentum the same amount of momentum is present the next moment but at different direction, true, but that is allowed under angular momentum rules. We allow friction to be part of the centripetal force but that only appears if the object is pushed over another surface. So what is the push force that makes friction into a centripetal force? Edited May 21, 2015 by Robittybob1 Link to comment Share on other sites More sharing options...
studiot Posted May 21, 2015 Share Posted May 21, 2015 (edited) Don't forget that angular momentum is not the same as linear momentum. The dimensions are different. Linear momentum MLT-1 Angular momentum ML2T-1 The extra L is why the old and alternative name for angular momentum is moment of momentum, which some consider more descriptive and less confusing. Edited May 21, 2015 by studiot Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 21, 2015 Author Share Posted May 21, 2015 Don't forget that angular momentum is not the same as linear momentum. The dimensions are different. Linear momentum MLT-1 Angular momentum ML2T-1 The extra L is why the old and alternative name for angular momentum is moment of momentum. Yes but in another way an object rotating about the point of rotation that is not moving laterally might not have any linear momentum at all. Link to comment Share on other sites More sharing options...
studiot Posted May 21, 2015 Share Posted May 21, 2015 Unless parts of a body or system of particles possess linear momentum there can be no "moment of momentum", and even then there may be none because it means that the angular momentum is the sum of the products of all the linear momenta with their lever arm about some axis. Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 21, 2015 Author Share Posted May 21, 2015 (edited) Unless parts of a body or system of particles possess linear momentum there can be no "moment of momentum", and even then there may be none because it means that the angular momentum is the sum of the products of all the linear momenta with their lever arm about some axis. Tell me do you agree with Swansont "(Momentum of an object traveling in a circle is not conserved; whatever is exerting the centripetal force is changing it)"? I see his point, but when it is thought of as angular momentum it is conserved. It would be a bit unusual to say "(The Linear Momentum of an object traveling in a circle is not conserved; whatever is exerting the centripetal force is changing it) Either you are traveling in circle or a straight line you can't be both. Edited May 21, 2015 by Robittybob1 Link to comment Share on other sites More sharing options...
swansont Posted May 21, 2015 Share Posted May 21, 2015 It would be a bit unusual to say "(The Linear Momentum of an object traveling in a circle is not conserved; whatever is exerting the centripetal force is changing it) Either you are traveling in circle or a straight line you can't be both. Nothing unusual about that at all. The nomenclature default is "momentum" = linear momentum. If you mean angular momentum, you say angular momentum. Yes but in another way an object rotating about the point of rotation that is not moving laterally might not have any linear momentum at all. Only if it's at the axis of rotation. L = r X p so in general (classically), an arbitrary object (such as we were discussing) with angular momentum will have linear momentum. In a broader discussion, symmetric object rotating about their symmetry axis will not have net linear momentum, but any component of it will. That is a surprise, The momentum of an object traveling in a circle has angular momentum and that is conserved. I listened to a lecture on that the day before yesterday. If you work out its instantaneous momentum the same amount of momentum is present the next moment but at different direction, true, but that is allowed under angular momentum rules. Different direction means different momentum. It's a vector. Direction matters. Yes, it's allowed, but you need a force present to do it. Luckily, we have a centripetal force present for such an object moving in a circle. We allow friction to be part of the centripetal force but that only appears if the object is pushed over another surface. So what is the push force that makes friction into a centripetal force? No, that's wrong. Friction does not manifest itself only when another force is present, nor does it require motion between objects. For an object on turntable that is rotating but not sliding, the centripetal force is that of friction. Link to comment Share on other sites More sharing options...
StringJunky Posted May 21, 2015 Share Posted May 21, 2015 Nothing unusual about that at all. The nomenclature default is "momentum" = linear momentum. If you mean angular momentum, you say angular momentum. Only if it's at the axis of rotation. L = r X p so in general (classically), an arbitrary object (such as we were discussing) with angular momentum will have linear momentum. In a broader discussion, symmetric object rotating about their symmetry axis will not have net linear momentum, but any component of it will. Different direction means different momentum. It's a vector. Direction matters. Yes, it's allowed, but you need a force present to do it. Luckily, we have a centripetal force present for such an object moving in a circle. No, that's wrong. Friction does not manifest itself only when another force is present, nor does it require motion between objects. For an object on turntable that is rotating but not sliding, the centripetal force is that of friction. Is "centrifuge" a misnomer? Link to comment Share on other sites More sharing options...
studiot Posted May 21, 2015 Share Posted May 21, 2015 Tell me do you agree with Swansont "(Momentum of an object traveling in a circle is not conserved; whatever is exerting the centripetal force is changing it)"? I see his point, but when it is thought of as angular momentum it is conserved. It would be a bit unusual to say "(The Linear Momentum of an object traveling in a circle is not conserved; whatever is exerting the centripetal force is changing it) Either you are traveling in circle or a straight line you can't be both. I have these two images in my head. The first is of a graceful skater spreading her arms wide ashe she travels the ice into a turn. Then bringing her arms in close to her body thus speeding up her rotation rate. Moment of momentum is conserved. The second is of a hairy Orc from Lord of the Rings sharpening a huge knife on an enormous grindstone, sparks flying everywhere. He is sweating profusely from having to work really hard as he presses the knife firmly against the edgge of the stone, maintaining the moment of momentum lost to friction. Link to comment Share on other sites More sharing options...
swansont Posted May 21, 2015 Share Posted May 21, 2015 Is "centrifuge" a misnomer? No, not really. Things move to the outside in a centrifuge. But it's not because of a force that pushes them outward, away from the center. Link to comment Share on other sites More sharing options...
StringJunky Posted May 21, 2015 Share Posted May 21, 2015 No, not really. Things move to the outside in a centrifuge. But it's not because of a force that pushes them outward, away from the center. Right. Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 22, 2015 Author Share Posted May 22, 2015 (edited) .... No, that's wrong. Friction does not manifest itself only when another force is present, nor does it require motion between objects. For an object on turntable that is rotating but not sliding, the centripetal force is that of friction. Yes I was wrong, very wrong. Is friction present at all times even when there is no lateral forces? But we can't call that friction a force can we? I might have to start a thread on friction to begin to understand friction. I had the idea the force will build up till it exceeded the frictional force. So if friction is the centripetal force what force overcomes the frictional force? No, not really. Things move to the outside in a centrifuge. But it's not because of a force that pushes them outward, away from the center. Stuff in a centrifuge tube would be similar to the experiments of objects accelerated in a tube performed by Mike and Robittybob. The masses definitely move along the tube. This is my understanding of a centrifuge: With the centrifuge the denser material displaces any lighter (less dense) material, as happens slowly in a gravitational field but in a centrifuge the G-forces are much greater hence denser material settles out more rapidly. Edited May 22, 2015 by Robittybob1 Link to comment Share on other sites More sharing options...
swansont Posted May 22, 2015 Share Posted May 22, 2015 Yes I was wrong, very wrong. Is friction present at all times even when there is no lateral forces? But we can't call that friction a force can we? I might have to start a thread on friction to begin to understand friction. I had the idea the force will build up till it exceeded the frictional force. So if friction is the centripetal force what force overcomes the frictional force? Why does there need to be another force to overcome it? Stuff in a centrifuge tube would be similar to the experiments of objects accelerated in a tube performed by Mike and Robittybob. The masses definitely move along the tube. Yes, obviously masses move along the tube. That was not the point of contention. I thought we were discussing momentum and angular momentum. Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 22, 2015 Author Share Posted May 22, 2015 Why does there need to be another force to overcome it? Yes, obviously masses move along the tube. That was not the point of contention. I thought we were discussing momentum and angular momentum. Someone else mentioned a centrifuge! Could centrifuge be thought of as the ratio of momentum to angular momentum? Link to comment Share on other sites More sharing options...
swansont Posted May 22, 2015 Share Posted May 22, 2015 Someone else mentioned a centrifuge! Could centrifuge be thought of as the ratio of momentum to angular momentum? A centrifuge is a physical device. I can't parse what "Could centrifuge be thought of as the ratio of momentum to angular momentum?" even means, especially as you apparently aren't clear on what angular momentum is, and how it's related to linear momentum. Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 22, 2015 Author Share Posted May 22, 2015 (edited) A centrifuge is a physical device. I can't parse what "Could centrifuge be thought of as the ratio of momentum to angular momentum?" even means, especially as you apparently aren't clear on what angular momentum is, and how it's related to linear momentum. It was part of a whole paragraph which I had this idea about but I was too tired to check the idea with formulas. But it was still looking promising so I deleted the majority of the paragraph and just left a bit even though I knew it wasn't making a whole lot of sense. I was just too tired to complete it, sorry. But I was looking at any two point masses and calculating the angular momentum between them even though they were not attached in any way but just passing each other in space. The angular momentum is based on the vector component that was perpendicular to the distance between them but the momentum is dependent on the relative velocity component between them. The ratio between the linear and angular has possibly a trig function relationship but I couldn't find what I was looking for just yet. Can you see what I am trying to say? Can you help me on this please? Below is what I had written originally. "Someone else mentioned a centrifuge! Could it be thought of as the ratio of momentum to angular momentum? When the instantaneous momentum exceeds the mass' angular momentum, the object in the centrifuge would go to a larger radius. Now that might sound contradictory but to keep angular momentum constant this would need a force to make the object rotate (move in a circle). Yet that same force would have an effect on its instantaneous velocity too." Edited May 22, 2015 by Robittybob1 Link to comment Share on other sites More sharing options...
swansont Posted May 22, 2015 Share Posted May 22, 2015 But I was looking at any two point masses and calculating the angular momentum between them even though they were not attached in any way but just passing each other in space. Angular momentum between them? What does that mean? The angular momentum is based on the vector component that was perpendicular to the distance between them Vector component of what? Link to comment Share on other sites More sharing options...
imatfaal Posted May 22, 2015 Share Posted May 22, 2015 ! Moderator Note Rob-Bob1 Stop posting rank guesses in the major fora. Questions along the lines of "is a physical object the ratio of two measurements" has no place here. For your guidance the rest of the paragraph doesn't really make it any better. You clearly do not understand momentum or angular momentum - please stick to the very basics which you can work with. There are hard yards that need to be worked for; you cannot just assimilate mechanics - you must do the maths and understand one area before moving to the next. I would recommend Hyperphysics and the Laws of Motion, or if you have the will-power edx.org and Walter Lewin's Physics 8.01. If you complete 8.01 you will know more basic physics than about 50% of those who post on this forum Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 23, 2015 Author Share Posted May 23, 2015 Angular momentum between them? What does that mean? Vector component of what? It is the tangential component of the momentum. Tangential being the direction perpendicular to the radial lines of a circle (curve) The radial line is the shortest distance between them or even more basic just the distance between the two points ( a point on the circumference has a constant distance to the center point in a circle. (they are radii or radial lines maybe.) The angular momentum is based on the vector component that was perpendicular to the distance between them If an object is travelling directly along the radius it is not going to have any angular momentum wrt the center point but it will still have momentum. Link to comment Share on other sites More sharing options...
studiot Posted May 23, 2015 Share Posted May 23, 2015 It is the tangential component of the momentum. Once again see my post2 Link to comment Share on other sites More sharing options...
StringJunky Posted May 23, 2015 Share Posted May 23, 2015 (edited) Imatfaal Lewin is persona non grata at MIT. Edited May 23, 2015 by StringJunky Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 23, 2015 Author Share Posted May 23, 2015 (edited) Once again see my post2 Linear momentum MLT-1 Angular momentum ML2T-1 The extra L is why the old and alternative name for angular momentum is moment of momentum, which some consider more descriptive and less confusing. does m = mass? T-1 = /second or per unit time. L what is this variable? I have seen L being used for angular momentum - what is it in your equation? Finally found what I trying to say: http://en.wikipedia.org/wiki/Angular_momentum#/media/File:Ang_mom_2d.png The angular momentum of the particle m with respect to the origin O is proportional to the perpendicular component v⊥ of the velocity v. Edited May 23, 2015 by Robittybob1 Link to comment Share on other sites More sharing options...
imatfaal Posted May 23, 2015 Share Posted May 23, 2015 Imatfaal Lewin is persona non grata at MIT. oh dear! It is terrible when one's idols turn out to have feet of clay or even worse. Should I burn my letter of congratulation from him for blitzing the class with no mistakes - I nearly TA'd on that edx course but decided my time was too limited and did the advanced mechanics course from MIT instead. I am not going to fall into the trap of defending him on the basis of little knowledge BTW 8.MRevx starts in a couple of days on edx.org . It is much tougher than 8.01 - but still good fun and adopts a nice "synoptic" view to teaching mechanics. Link to comment Share on other sites More sharing options...
studiot Posted May 23, 2015 Share Posted May 23, 2015 (edited) The three basic quantities in mechanics are Mass - symbol M Length - symbol L Time - symbol T All other mechanical quantities can be expressed by combinations of these three. These three are called 'dimensions' and combinations are called 'units'. This makes it easy to compare them and check for consistency. so for instance velocity = distance/time which has the dimensions LT-1 and units of miles per hour or metres per second. volume is L3 and area L2 or cubic metres and square metres. As a little extra: Force is rate of change of linear momentum Torque is rate of change of angular momentum Linear momentum is mass times velocity or mass times (LT-1) or MLT-1 So force the dimensions of force is momentum per unit time or MLT-2 How are we doing? Edited May 23, 2015 by studiot 2 Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 23, 2015 Author Share Posted May 23, 2015 (edited) The three basic quantities in mechanics are Mass - symbol M Length - symbol L Time - symbol T All other mechanical quantities can be expressed by combinations of these three. These three are called 'dimensions' and combinations are called 'units'. This makes it easy to compare them and check for consistency. so for instance velocity = distance/time which has the dimensions LT-1 and units of miles per hour or metres per second. volume is L3 and area L2 or cubic metres and square metres. As a little extra: Force is rate of change of linear momentum Torque is rate of change of angular momentum Linear momentum is mass times velocity or mass times (LT-1) or MLT-1 So force the dimensions of force is momentum per unit time or MLT-2 How are we doing? That helped a lot. Was this a typo? Linear momentum is mass times velocity or mass times (LT-1) or MLT-1 Edited May 23, 2015 by Robittybob1 Link to comment Share on other sites More sharing options...
studiot Posted May 23, 2015 Share Posted May 23, 2015 I'm glad it helped and I see no reason for the discouraging negative so +1. No after checking carefully (since I make too many typos) this is correct. All I have done is said mass times velocity and put in M (for mass) times (the dimensions I worked out earlier for velocity). Is that any clearer? Link to comment Share on other sites More sharing options...
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