Mechanics, in physics, the study of forces and their effect on matter. Mechanics is divided into two main branches—statics and dynamics.

Statics is the study of the balance of forces needed to keep a body in equilibrium. A practical application is in architectural engineering, in which it is necessary to calculate all stresses to which load-bearing supports will be subjected.

Dynamics is the study of motion and the forces that produce it. Dynamics is sometimes divided into kinetics and kinematics. Kinetics deals with the effects of forces, such as gravity or electromagnetic force, on the motion of matter. Kinematics studies motion, but ignores the forces that produce it. Studies of velocity and acceleration are parts of kinematics. An important practical application of dynamics is in mechanical engineering, which is concerned with the design and construction of machinery.

Mechanics and its branches are divided into separate fields according to the matter involved. For example, celestial mechanics is concerned with astronomical bodies. Fluid mechanics is the mechanics of gases and liquids. The term hydromechanics, narrowly speaking, is used to refer to the branch of fluid mechanics concerned with liquids; more generally, however, hydromechanics also includes gases in situations where they behave like liquids.

The term classical mechanics refers to the mechanics established by Sir Isaac Newton in the late 17th century. Quantum mechanics refers to the mechanics based on the quantum theory, as developed by Max Planck, Albert Einstein, and others in the 20th century.

The basic laws of classical mechanics were stated by Newton. They are:

• A body at rest remains at rest, and a body in motion continues to move in a straight line at a uniform velocity (speed), unless it is acted upon by some external force. (The principle stated by this law is called inertia.)
• Change in the motion of a body is in proportion to, and in the direction of, the force causing the change.
• The action of every force is accompanied by an equal action (called a reaction) in the opposite direction.

Force is commonly measured in newtons or poundals, units that are defined in terms of the acceleration (change in velocity) that the force causes when applied to a body of a given mass. A newton is the force that will change the velocity of a one-kilogram mass by one meter per second for every second the force is applied (one meter per second per second); a poundal is the force that will give a one-pound mass an acceleration of one foot per second per second.

A body, whether in motion or stationary, tends to oppose the action of a force in proportion to its mass. Thus a body with a mass of two kilograms requires a force twice as great as that needed to give a body with a one-kilogram mass the same acceleration.