Objectives Define force. Apply Newton’s second law to solve problems. Explain the meaning of Newton’s first law. Vocabulary force free-body diagram net force Newton’s second law Newton’s first law inertia equilibrium A force is a push or pull exerted on an object. Forces can cause objects to speed up, slow down, or change direction as they move. A force exerted on an object causes the object’s velocity to change; that is, a force causes an acceleration. The symbol $$F$$ is a vector and represents the size and direction of a force, while $$F$$ represents only the magnitude. When considering how a force affects motion, it is important to identify the object of interest, called the system. Everything around the object that exerts forces on it is called the external world. Contact Forces: A contact force exists when an object from the external world touches a system and thereby exerts a force on it. Field Forces: Field forces are exerted without contact (e.g., gravitational force). Forces result from interactions; thus, each force has a specific and identifiable cause called the agent. Free-body diagrams represent the forces acting on a system; apply the particle model and represent the object with a dot and represent each force with an arrow. The length of each arrow should be proportional to the size of the force. Always draw the force arrows pointing away from the particle, even when the force is a push. Use the symbol $$F$$ with a subscript label to identify both the agent and the object on which the force is exerted.

Force

A push or pull exerted on an object that causes an acceleration or change in velocity.

System

The specific object or collection of objects of interest when analyzing how forces affect motion.

External world

Everything surrounding a system that exerts forces on it.

Contact force

A force exerted on an object only when touching it directly.

Field force

A force exerted on an object across a distance without physical contact, such as gravity.

Agent

The specific, identifiable cause of an exerted force.

Free-body diagram

A physical model representing an object as a dot and depicting each exerted force as an arrow pointing away from the dot.

Net force

The vector sum of all individual forces acting upon an object.

Newton's first law

The law stating that an object at rest remains at rest, and an object in motion continues at a constant velocity, unless acted upon by a nonzero net force.

Inertia

The tendency of an object to resist any change in its state of motion.

Equilibrium

The state of an object where the net force acting on it is equal to zero.

Newton's second law

The physical principle stating that an object's acceleration is directly proportional to the net force applied and inversely proportional to its mass, written as $F_{\text{net}} = ma$.