Question

Distinguish b/w static and dynamic friction. Why is static friction more than dynamic friction

Answer 1

When two or more surfaces are in contact with one another, but are not moving relative to each other (i.e stationary such as a book on a desk) than there is a force that needs to be overcome to allow movement. Following upon the example of the book, imagine trying to slide it across the table, this essentially creates a stress on the surface and there is an underlying molecular explanation case by case for all surfaces in contact of which you can derive this familiar quantity called the coefficient of static friction. Once movement begins, a frictional force is still acting on the book however generally and perhaps always (correct me if I'm wrong) will be lower in magnitude than the corresponding static frictional force. To make sense of this, consider that the book or any object that's being moved upon the surface now has a certain momentum associated with it relative to the frame of the desk or second surface, hence it's "easier" to continue pushing it or applying a force on it.  This frictional force that is acting on the book during movement is the dynamic friction.

When the mass is not moving, the object experiences static friction. Thefriction increases as the applied force increases until the block moves. After the block moves, it experiences kinetic friction, which is less than the maximum static friction.