# Mr_Nick

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1. ## Contact force situation

Hi bpj1138, yes it says that there should always be a solution in the frictionless case (which Im currently [trying] to implement) but not always for the friction case.
2. ## Contact force situation

Thanks for you're reply. Think I understand. It was just that in Coutinho's book, it said that after contact forces had been resolved, you can integrate to the end of the time step and no interpenetration will have occured. I guess he meant to add 'at that particular point on the body'? If that's the case I can see also why its necessary to use polytopes, as with the smooth sphere it would seem that in the above mentioned case that the simulation would be stuck in an infinite loop (because in the above case no state changes are made, the ball goes on to penetrate the plane, exactly as in the previous step, and then the system backs up the ball to the time of contact, and repeats the resolution - if that makes sense?). Either way thanks a lot think Ive had a big conceptual spoon up.
3. ## Contact force situation

Hey Inferiarum, wow thats a really good point they do use convex polyhedra whereas I was using a perfect sphere. Guess Ill have to bite the bullet and implement a general intersection test for trimeshes. Im thinking either V-clip or GJK.. any recommendations? Still, even if it were a polytope, the force required still wouldn't be enough in one go, it would take 1 or more iterations of the response algorithm to prevent penetration. Does that sound right? (I was previously under the impression that once the force was calculated, it would at least prevent penetration until the end of the time step). Anyway thanks alot Inferiarum
4. ## A-Level Mathematics: Looking for a Distance Learning Provider

Yeah thats basically what I meant. I know what its like being rusty but you'll soon be fluent in it. As I said, look at the Edxcel (or other body) specs as they briefly summarise the subjects covered so you can choose what modules you'd like to study, as well as what modules are compulsory. I just saw on the Edxcel website they have an 'ask the expert' service now. Also they may have distance/evening learning courses for the maths. Cambridge learning mite also be worth a look. The titles of the books Edxcel specifies are the same as the module names so you can't go wrong. It's difficult for info to actually stick if you just work through summary booklets. You're own notes will be more useful to you, and the act of writing stuff down in itself really helps stuff stick. If I remember, the pure/core modules cover a lot on calculus; especially for calculating areas, volumes (& a zillion other things). If you like maths, its really interesting. The decision maths modules are probably the most straight forward to learn. Everything is explained in an algorithmic way. So thats more about memorising stuff. I did 1 year of a maths/physics/comp science degree, and it was stuff related to the decision maths that came up in comp science, so yeah it would certainly help improve programming. I dont think the mechanics modules introduce vectors until module 3. Before that its more distance & angle. At least with self taught, you can pick and choose what modules to do exams in Jan & May, as well as spread the A-level out if need be. Its much easier to revise if you do say 2 exams in Jan, and 4 in May. You can book exams through a company called 'Pearson VUE'. They have places all over the UK. When I did, I basically had a whole room to myself (apart from once when there was a hot girl ) @BenS1, the only place Ive seen you can do further maths is if you went to a 6th form college. Otherwise you'll have to go down the self-taught route (which only requires slightly more self-organisation than distance learning). Also if you're interested, you can do 3 A-levels in maths; maths, further maths, further maths additional. Is it possible you could do a 1-year foundation (distance) degree?
5. ## Contact force situation

Hi, since no one has replied (though Im greatful for the number of views), I thought I'd clarify the situation. A ball is in contact with a plane. At the contact point, the relative velocity along the contact normal is zero. The ball's centre of mass (COM) has accn of -g. The ball is spinning at such a rate that at the contact point, the centripetal accn cancels out the gravitational accn. So the calculated contact force to prevent penetration of the point is zero. Obviously, this will not prevent the ball penetrating the plane. I've now looked at the equations by Baraff, Eberly and Coutinho and all get this result of zero contact force. What am I doing wrong? Do I ignore centripetal accn (even though the above authors don't)? Is it simply an unavoidable situation that I'll to code for? Could anyone do an example calculation for me?

7. ## A-Level Mathematics: Looking for a Distance Learning Provider

Hey stitchs, I went with the National Extension College (nec.ac.uk) a few years back. I had a tutor I could talk to on the phone and would send coursework for marking every month or two. It wasn't cheap but they sent all the required books. I did A-level maths. The following year though I thought the cost of using a distance learner wasnt worth it, so when I did further maths I just looked at the exam board's website for the modules and bought the Heineman books that covered each one (they were excellent). If you prefer working from a book then I recommend it as its a lot cheaper. The exam board was Edexcel and were fine. When it comes to exams you'll want to book early to be safe. The NEC (did/does) have a list of places where you can take your exams. Goodluck!
8. ## Resolving contact force to prevent penetration and LCP

Hello, Im working through "Dynamic Simulations Of Multibody Systems" by Murilo Coutinho. Ive got a ball rigid body (B1) and a plane. I've been trying to setup the LCP problem to calculate the force require to prevent pentration. The problem is a = AF + b a >= 0 F >= 0 aF = 0 where a = relative acceletaion at the contact point (to solve) A = a constant (roughly the inverse mass at the contact point) F = relative force at the contact point (to solve) b = current relative acceleration at contact point + relative velocity x contactFrame time derivate. The ball has a gravity force of -10 units, and mass = 1 so -10 acceleration too. When there is no rotation, all works fine. But when there is rotation, the contact point on the ball gets a centrepetal acceleration so its accelertaion becomes -9. So it calculates a force of +9 to apply at the contact. But the body still has an accn of -10, so applying this force won't prevent penetration. I could just calculate the force required to prevent penetraion but that would violate the aF = 0 constraint. I've been googling all over the place the past 2 days and im nearly bald from tearing my hair out so Id be really really grateful for any help. I know someone posted a similate comment (Bastian1978) but it doesnt help me maintain the constraints which I need so I can extend the system to sovling multiple simultaneous contacts. Formule are: b = B1.netForce/B1.mass + (B1.inertia^-1 * B1.Torque + B1.angualMomentum X B1.W) X r1 + B1.W X (B1.W X r1); + (B1.V + B1.W X r1) * (coordFrame time derivate) A = (I * B1.invMass) - (x1 * B1.inertia^-1 * x1); where: r1 = contactPoint - B1.centre of mass x1 = r1.skewSymmetric I = identity matrix The final term of 'b' is zero as the contactFrame remains stationary.