Define first order phase transition

Newton's law of universal gravitation states that the force f between two masses, m1 and m2, is given below, where g is a constant and d is the distance between the masses. find an equation that gives an instantaneous rate of change of f with respect to d. (assume m1 and m2 represent moving points.)

What is the role of the ocular lens?

The material that keeps its new shape after it is stretched is called?