It seems that this is not only related to physics ... grapefruit and grapefruit peel are mainly connected by cellulose. The shape of grapefruit is roughly abstracted into a sphere, and then you can assume the tensile strength of a cellulose ... that is, when you apply a force vertically to the surface of grapefruit, the force required to destroy the cellulose may need to be calculated ... Guess it may be related to the thickness and density of the fiber. Water content and so on ~ Then you can assume that the cellulose distribution under the grapefruit peel is uniform, then you need to calculate the average diameter of a grapefruit and calculate the surface area, and the rest … you need to establish a suitable physical model to discuss the painless peeling of grapefruit …