Speed and force

Under the action of another body a body can change its speed or shape; bodies interact with each other. The measure of the action is called force, denoted F, with the unit newton (N). A force is defined by its direction, magnitude and point of application. The Earth attracts all bodies; this is the force of gravity F = m · g, where g ≈ 9.8 N/kg (in calculations 10 N/kg is often used). For example, the Earth pulls a 1 kg body with about 9.8 N. Types of force: gravity, elastic force (a stretched spring), friction (resistance to motion along a surface), and electric and magnetic forces. Force is measured with a dynamometer. Friction is sometimes useful (walking, braking) and sometimes harmful (machine parts wear), so we reduce it by lubrication.