When is facilitated diffusion necessary? a) when the molecules are too large b) when the molecules are insoluble c) when the direction of movement is opposite to natural diffusion c) none of the above

Facilitated diffusion is necessary when B. the molecules are insoluble. Insoluble molecules need help to diffuse across membrane, this is also called passive transport. 

It depends on the surface of the levels. the rougher the surface, the greater is the frictional force. for rougher surfaces, you would need to exert greater force to move the object. the concept is that, you must overcome the frictional force in order to bring the object into motion. this is the reason why we add lubricants to the joints so they could easily move. this answer was on