A force is a push or pull that can change an object's motion. Forces are measured in Newtons (N). The net force on an object is the vector sum of all forces acting on it.
Newton's First Law states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force (inertia). Newton's Second Law states F = ma: force equals mass times acceleration. Newton's Third Law states that for every action, there is an equal and opposite reaction.
Friction opposes motion between surfaces. Static friction prevents objects from starting to move, while kinetic friction acts on moving objects. The friction force depends on the coefficient of friction and the normal force.
Gravity is the force of attraction between all objects with mass. Near Earth's surface, gravitational acceleration is approximately 9.8 m/s². The gravitational force between two masses is F = G(m₁m₂)/r², where G is the gravitational constant and r is the distance between the centers of mass.