The force F newtons between two magnetic poles is given by the formula
F = \displaystyle\frac{1}{500r^{2}} where r m is their distance apart.
Find the rate of change of the force when the poles are 0.2 m apart and the distance between them is increasing at a rate of 0.03 ms^-1"
\displaystyle\frac{dF}{dt} = \frac{dr}{dt}*\frac{dF}{dr}
dr/dt = 0.03 ms^-1 and df/dr = -1000r^-3.....where r = 0.2m so don't i just do for df/dr, -1000*(0.2^-3) and then multiply it by 0.03???
