Types of forces

In early mechanics we meet a few fundamental forces, each with its own law fixing magnitude, direction and sense. Knowing them by heart makes it possible to correctly draw the free-body diagram of an object, the first step of every dynamics or statics problem.

• Weight $\vec{P} = m\vec{g}$: the force with which the Earth attracts every object towards itself. It points vertically downward and has magnitude $|\vec{P}| = m\,|\vec{g}|$, with $|\vec{g}| \approx 9{,}81\;\text{m/s}^2$.
• Normal reaction (or normal force) $\vec{R}$: the force with which a supporting surface pushes on the resting body, always perpendicular to the surface. Its magnitude is not fixed a priori, but adjusts just enough to prevent the body from penetrating the constraint.
• Tension in a rope $\vec{t}$: always directed along the rope, pointing outward from the object to which the rope is attached (a rope can pull, not push). In an ideal rope (inextensible and of negligible mass) the tension is transmitted unchanged from one end to the other.
• Elastic force of a spring $\vec{F}_m$: has magnitude $|\vec{F}_m| = K\,|\ell - \ell_0|$, where $K$ is the spring constant and $\ell_0$ is its rest length. It is always directed so as to restore the spring towards its rest length: it pulls back if the spring is stretched, pushes back if it is compressed (Hooke’s law).
• Friction $\vec{F}_a$: opposes relative motion, or the tendency towards relative motion, between two surfaces in contact. A distinction is made between static friction, which acts as long as the body stays still and can take values up to a maximum, and kinetic friction, which acts when the body is already sliding.

We shall look more closely at all these forces, with their respective quantitative laws, in the chapter devoted to forces and dynamics.

Collegamenti

Argomenti: Statica ed equilibrio Concetti: Forza peso · Forza normale · Tensione · Forza elastica e legge di Hooke · Attrito statico · Attrito dinamico Oggetti: Molla · Fune ideale

Esercizi collegati: Esercizio svolto — Il Gorilla tira una cassa · Esercizio svolto — Scatola con molla: zona di equilibrio · Esercizio svolto — Scala appoggiata a un muro liscio