Turn on thread page Beta

    (Original post by atsruser)
    Not sure I'm getting the point, TBH. You now think that the material is *less* advanced than you previously thought?
    There wasn't one really beyond me waffling. You can safely ignore

    (Original post by atsruser)
    The question relies on using the so-called "intrinsic coordinates" of a curve. The idea is that we know how to handle the motion of a particle constrained to circular motion, and we extend this to more general curvilinear motion.

    The physics is as follows: a particle constrained to a smooth curve (say a bead on a wire) knows only about its local environment - it can feel only the forces exerted on it by the wire and external forces at its current location, P.

    At any instant in time, the particle has a velocity tangent to the wire, and feels an component of external force accelerating it in the direction of tangent. Also, it feels a force exerted by the wire normal to the tangent at that point. Locally the wire is curved, and over an infinitesimal distance before and after P, that curvature is constant. So at P, we can model the wire as a circle of radius \rho, chosen to give the same curvature as that of the wire at P. So when the particle is at P, we can write down normal and tangential equations of motion, if we know the curvature, by treating the particle as being on a circle, instantaneously.

    Of course, an instant later the bead is at another point and the tangent direction has changed, and so has the radius of curvature \rho, so we need to be able to write down the equations of motion in a general form, taking into account the fact that the tangent and \rho are functions of the position, and that from the POV of the bead, it's now on a circle with a different curvature.

    To do so, we measure the position of the particle with coords P(s,\psi) where s is the arc length along the curve measured from some origin, and \psi is the angle that the tangent at P makes with the x-axis. These coords are used because it turns out that there are nice formulae relating them to the radius of curvature of the curve at any point - in fact, you can show that \rho=\frac{ds}{d\psi}, and since, if we have the equation of the constraining curve in the form y=f(x) (as we usually do), then also \frac{dy}{dx}=\tan \psi, so with a bit of work we can use the cartesian eqn of the curve to calculate \rho at any point (x,y). [Fun exercise: work out the details yourself].

    Having done all that, we can write down the radial and tangential equations of motion by noting that \dot{s}=v, the tangential velocity, and \ddot{s} is the tangential acceleration so we have, by Newton II:

    F_t = m\ddot{s}
    F_n = m\frac{\dot{s}^2}{\rho}

    by using the results from circular motion, generalised to our situation where we don't have a constant radius of curvature r, but a variable one, namely \rho.
    Thank you for this explanation, I read it earlier but didn't quite have chance to write a response. Looking at radius of curvature is the first thing on my to-do list, after that I intend to look at this post again and figure out how that applies to intrinsic co-ordinates
    • Thread Starter

    (Original post by Euclidean)
    I'd be interested in a thread like this!


students online now


Exam discussions

Find your exam discussion here

Should predicted grades be removed from the uni application process
Useful resources

Make your revision easier


Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here


How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.