The behavior of objects under the influence of forces is described by Newton's three laws of motion. The first law, sometimes called the law of inertia, states that an object at rest remains at rest, and an object in motion continues with constant velocity, unless acted upon by a net external force. This property of resisting changes in motion is what we call inertia. Newton's second law quantifies the effect of a net force: the net force on an object equals its mass multiplied by its acceleration, written as \(F_{\text{net}} = ma\). Acceleration is directly proportional to the applied force and inversely proportional to the mass of the object. Newton's third law completes the picture by noting that forces always come in pairs: for every action, there is an equal and opposite reaction, with the two forces acting on different objects.
It is important to distinguish between mass and weight. Mass is the amount of matter in an object, measured in kilograms, and is invariant regardless of location. Weight, in contrast, is the gravitational force acting on that mass, given by \(W = mg\), where \(g\) is the local gravitational field strength. Because \(g\) varies from place to place, an object's weight can differ while its mass remains the same. Among the contact forces encountered in everyday life, friction plays a major role. Friction is the force that opposes relative motion between two surfaces in contact. Static friction prevents motion from beginning, while kinetic friction acts once surfaces are sliding, and both are typically proportional to the normal force pressing the surfaces together.
On a universal scale, gravity is described by Newton's law of universal gravitation, which states that every particle in the universe attracts every other particle with a force given by \(F = G m_1 m_2 / r^2\), where \(G\) is the gravitational constant and \(r\) is the distance between the centers of the two masses. The force acts along the line joining the two particles. This single law, together with Newton's laws of motion, explains everything from the falling of an apple to the motion of planets around the Sun.