15) What is the circle that rolls on a straight line or on the arc of a circle to generate cycloidal curves on its circumference?

16) What is the term for the locus of a point lying on the circumference of the circle which rolls on a straight line?

17) Which is a practical application of cycloidal curve?

18) What is the curve if the generating circle rolls outside the directing circle?

19) What is the curve if the generating circle rolls inside the directing circle?