Fundamental Principles of Force, Motion, and Dynamics

Force

A cause that can produce changes in the motion or shape of an object.

Action-at-a-Distance Forces

Forces whose effects are observed without the bodies needing to be in physical contact with each other.

Hooke's Law

The deformation of an elastic body is directly proportional to the force that produces it.

Scalar Quantities

Quantities defined by a number and a unit.

Vector Quantities

Magnitudes defined by a magnitude, direction, sense, and point of application. They are represented by a vector, which is an oriented segment in space.

Net Force

The single, resultant force equivalent to all forces acting on a particular body.

Equilibrium Conditions

A body is in equilibrium if the resultant of the forces acting on it is zero.

Static Equilibrium

A situation that occurs when the forces acting on a body at rest are balanced. The body remains at rest indefinitely.

Dynamic Equilibrium

A situation that occurs when the forces acting on a moving body are balanced. The body continues with uniform rectilinear motion without changing its velocity.

• If a body is at rest: Static Equilibrium.
• If a body is in motion: Dynamic Equilibrium.

Historical Perspectives on Motion

Aristotle: On Why Bodies Move

He believed it is necessary to exert a force to keep a body in motion. If no force is applied, the body will return to a state of rest.

Galileo: On Why Bodies Move

He proposed that:

• Movement is the interaction between bodies.
• A force is needed to put a body in motion, but not to maintain its movement.
• There are two natural states: rest and uniform rectilinear motion (U.R.M.).

Free System

A system is considered free when it is in equilibrium, meaning the resultant of the forces acting on the system is zero.

Friction

The force that arises from the interaction between two surfaces in contact, opposing the sliding of one over the other.

Newton's Laws of Motion

Newton's First Law (Law of Inertia)

Every body tends to maintain its state of rest or uniform rectilinear motion unless a net external force acts upon it.

Newton's Second Law (Fundamental Law of Dynamics)

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its inertial mass.

Newton's Third Law (Principle of Action and Reaction)

When one object exerts a force on a second object (the action force), the second object simultaneously exerts a force of equal magnitude and opposite direction on the first object (the reaction force). These two forces act on different bodies.

Impulse

A force acting over a very short time interval.