The fundamental concept of mechanics is force.

• A force is a push or a pull.

• A force acts on an object.

• A force requires an agent.

• A force is a vector.

A force can be a contact force or a long-range force.

• Contact forces occur at points where the environment touches the object.

• Contact forces disappear the instant contact is lost. Forces have no memory.

• Long-range forces include gravity and magnetism.

Forces can be displayed on a free-body diagram. You’ll draw all forces—both pushes and pulls—as vectors with their tails on the particle. A well-drawn free-body diagram is an essential step in solving problems, as you’ll see in the next chapter.

A net force causes an object to accelerate with an acceleration directly proportional to the size of the force. This is Newton’s second law, the most important statement in mechanics. For a particle of mass $m$,

《 LOOKING BACK Sections 1.4, 2.4, and 3.2 Acceleration and vector addition

Newton’s first law—an object at rest stays at rest and an object in motion continues moving at constant speed in a straight line if and only if the net force on the object is zero—helps us define what a force is. It is also the basis for identifying the reference frames—called inertial reference frames—in which Newton’s laws are valid.

Kinematics describes how an object moves. For the more important tasks of knowing why an object moves and being able to predict its position and orientation at a future time, we have to know the forces acting on the object. Relating force to motion is the subject of dynamics, and it is one of the most important underpinnings of all science and engineering.