15 anyone wanna : ( the force, f(newtons), between two objects is inversely proportional to the square of the distance, d(metres), between them. the force is 0.55 newtons when the distance between the objects is 3 metres. work out f(rounded to 2 dp) when d=3.2metres.

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^-3where r = 0.2m so don't i just do for df/dr, -1000*(0.2^-3) and then multiply it by 0.03???