1. Friction: This force opposes motion between two surfaces in contact. It arises from the microscopic interactions between the surfaces, including the interlocking of surface irregularities and the attractive forces between molecules. Examples include the friction between your shoes and the ground, or the friction between a brake pad and a wheel.

2. Normal Force: This is the force exerted by a surface on an object that is in contact with it. It acts perpendicular to the surface, and prevents the object from passing through the surface. For example, if you place a book on a table, the table exerts a normal force upwards on the book, balancing the book's weight.

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