There are two types of changes: physical change and chemical change. Physical change is defined as the change that changes the physical appearance of the substance and do not affcet the chemical composition of the substance whereas chemical change causes changes in the chemical composition of the susbstance.
The given example shows physical change because solid calcium undergoes sublimation. process in which a solid substance changes into gaseous phase directly without undergoing liquid phase.
Gravity always pulls objects such as a desk, book or person down. Thus, when you jump gravity causes you to land on the ground. Friction, however, doesn't pull objects down
