Final answer:
The normal force is exerted by a surface perpendicular to the surface, supporting the weight of an object, and simple friction is always proportional to the normal force, which is parallel to the surface.
Step-by-step explanation:
The normal force is the force exerted by a surface, perpendicular to the surface itself, which supports the weight of an object resting on that surface. In the context of friction, when friction behaves simply, the normal force is related to the amount of frictional force that can be exerted between two surfaces. Specifically, simple friction is always proportional to the normal force, and frictional force is always parallel to the surface of contact and opposite the direction of motion, thereby making option (a) the correct answer: Normal force is perpendicular to the surface; friction is parallel.
Example of Normal Force and Friction
If a crate with a mass of 100 kg is sitting on a level surface, the normal force is equal to its weight, which is calculated by multiplying its mass by the acceleration due to gravity:
w = mg = (100 kg) (9.80 m/s²) = 980 N. This 980 N is the normal force exerted by the floor on the crate, which in turn determines the maximum possible frictional force.