Originally Posted by

**kevinnwhat** The question I am trying to solve is,

A Particle P moves in the place. The Rectangular Observer computes the magnitude of the speed and acceleration of P by observing the x- and y- coordinates of P (as functions of time) and using these formulas: speed = sqrt( (dx/dt)^2 + (dy/dt)^2 ) and acceleration = sqrt( (d^2x/dt^2)^2 + (d^2y,dt^2)^2 ). The other observer, the Polar Observer, finds it easier and more natural to measure the polar coordinates r and Theta of P(as functions of time), using herself as the origin, of course. The Polar Observer computes dr/dt, dTHETA,dt, d^2r/dt^2, etc. What formula should the Polar Observer use to compute the speed of the particle P? Deduce your answer from the formula for speed displayed above and the relationship among x, y, r, and Theta. Finally, if r and dTheta/dt are constant, derive this formula valid for uniform circular motion = ( (speed)^2 )/(r).

So I know the relationships between x, y, r, and theta as x=rcostheta, y=rsinTheta.

So just plugging them in I figured I would solve for r in both of these equations for x and y, and than take the derivative, but it should be with respect to t, but if it seems if I take the derivative when r = x/cosTheta, than it should be dr/dTheta = x/cosTheta. When it should be dr/dt, Can I just take the derivative and plug them in or do I need to change it to be with respect to t? If so would it looks like r(t) = x(t)cosTheta(t)?

I was thinking if I need dr/dt, than r should be a function of time, so I'm thinking r(t) = t/cost and r(t) = t/sint and than take the derivative of each.