Question

A book sitting on a table is moved horizontally. Describe the
relationship of unbalanced forces (the applied force and
friction).
They are opposite in magnitude and direction
They are equal in magnitude and opposite in direction
They are equal in magnitude and acting in the same direction

Answer 1

Frictional force and Applied force has same “magnitude” and “opposite” direction.  

Option: B  

Explanation:  

When a book is moved horizontally by applying “force” on the book, the frictional force is opposed to the book by the table. Here, this “frictional force” is opposing the book has the same force what we applied on the book but this frictional force and the applied force are opposite in direction. Always the “frictional force” is opposite to the “applied force” which stops the object to move. For example, if a force applied leftward to the object the frictional force is acted on the right side of the object.

When two objects are in contact they experience a "frictional force". This "frictional force" acts opposite to the force applied on to move the object.

Formula for "frictional force" is $\mu\times N$

Where, $\mu$ is coefficient of friction and N is normal force.