Answer:

Force may be defined as the cause of motion and deformation.  When a force is applied to an object, the object either moves or changes shape or both.  In most cases, it is not possible to detect the deformation by naked eyes at the molecular or atomic level.  Deformation occurs no matter how small.

In Chapter 1, force was defined as the product of mass and acceleration.  Simply F = Ma.  A more useful form of this formula is ΣF = Ma.   ΣF means the sum of forces acting on mass M.  Since forces acting on an object may act in opposite directions, ΣF is also CALLED the net force.

The formula  ΣF = Ma is called the "Newton's 2nd Law of Motion."

 For example, a car moving along a straight and horizontal highway, experiences an engine force FE, while being opposed by an overall frictional force, Ff ( road friction as well as air resistance).  If the car is moving to the right and to the right is TAKEN to be the positive direction, then Fe ACTS to the right and Ff acts to the LEFT.  The net force is ΣF = Fe - Ff .