Physics engines give games a means of simulating realism in physical terms, and they are a feature in almost all game engines either natively or as a plugin. Unity utilizes the Nvidia PhysX physics engine, a precise modern physics engine that is used in many commercial games in the industry. Having a physics engine means that not only are physical reactions such as weight and gravity possible, but realistic responses to friction, torque, and mass-based impact are also possible.
The influence of the physics engine on objects is known as force, and forces can be applied in a variety of ways through components or scripting. In order to apply physics forces, a game object must be what is known as a Rigidbody object.