Physical adsorption is a process in which atoms or molecules from a gas or liquid phase adhere to the surface of a solid material. It is a relatively weak type of adsorption compared to chemical adsorption, as it involves only intermolecular forces and does not involve the formation of chemical bonds between the adsorbate and adsorbent.

The force involved in physical adsorption is van der Waals force. Van der Waals forces are the attractive forces that exist between atoms or molecules due to temporary fluctuations in electron distribution. These forces arise from the interaction between permanent dipoles (permanent dipole-dipole interaction), the attraction between permanent dipoles and induced dipoles (dipole-induced dipole interaction), and the interaction between two induced dipoles (London dispersion forces).

Van der Waals forces play a crucial role in physical adsorption because they allow the adsorbate molecules to be attracted to the surface of the adsorbent material. The adsorbate molecules are held onto the surface through intermolecular forces, which can be weak enough to allow for desorption under certain conditions (e.g., changes in temperature or pressure).

The other forces listed in the options, such as gravitational force, magnetic force, and electromagnetic force, do not play a direct role in physical adsorption. Gravitational force is the force of attraction between two masses and is generally negligible at the molecular scale. Magnetic force and electromagnetic force are related to the interaction between charged particles and are not significant in physical adsorption, which involves nonpolar or weakly polar molecules.

In summary, physical adsorption primarily involves van der Waals forces, which are attractive intermolecular forces between molecules. These forces allow for the adsorption of gas or liquid molecules onto the surface of a solid material.