1.39: Past-paper question 5

The force $\displaystyle F$ newtons between two magnetic poles is given by the formula

where $\displaystyle r$ is the distance, in centimetres, between the poles.

The distance between the two poles is increasing at a constant rate of 0.7 cm/s

Find the rate of change of $\displaystyle F$, in newtons/s to 3 significant figures, when the distance between the poles is 2.8 cm