# mechanics - polar coordinates

• Apr 26th 2012, 08:57 AM
qwerty31
mechanics - polar coordinates
A particle of mass m moves under the influence of a central force which attracts a particle with force of magnitude F(r) = m/(r^5). Assuming that the particle moves on a circular orbit of radius R where R = constant, find the period of particles motion.

Any help would be lovely, thanks!
• Apr 26th 2012, 09:46 AM
ignite
Re: mechanics - polar coordinates
$F(r)=\frac{m}{r^5}$
Since force is in the radial direction,we only have centripetal acceleration which would be $\frac{1}{r^5}$
$\Rightarrow \omega^2R=\frac{1}{R^5} \Rightarrow \omega=\frac{1}{R^3}$
$\Rightarrow T=\frac{2\pi}{\omega} \Rightarrow T=2\pi R^3$
• Apr 26th 2012, 12:03 PM
qwerty31
Re: mechanics - polar coordinates
Quote:

Originally Posted by ignite
$F(r)=\frac{m}{r^5}$
Since force is in the radial direction,we only have centripetal acceleration which would be $\frac{1}{r^5}$
$\Rightarrow \omega^2R=\frac{1}{R^5} \Rightarrow \omega=\frac{1}{R^3}$
$\Rightarrow T=\frac{2\pi}{\omega} \Rightarrow T=2\pi R^3$

thank you very much, also i have another question....I was given the differential equation (d^2u/dphi^2) + 4/9u = 0 and told to show that the particle will eventually move along the line phi = 3pi/4...any idea how to start?
• Apr 26th 2012, 12:06 PM
ignite
Re: mechanics - polar coordinates
Please post entire question giving details like what is u and 'phi' and initial conditions.
• Apr 26th 2012, 12:16 PM
qwerty31
Re: mechanics - polar coordinates
Quote:

Originally Posted by ignite
Please post entire question giving details like what is u and 'phi' and initial conditions.

This is the one, I can do all of it aside from the last part