Punto Materiale

Il punto materiale è un oggetto astratto usato per descrivere un corpo le cui dimensioni sono piccole rispetto all’ambiente circostante. Le sue dimensioni possono quindi essere trascurate e può essere assimilato ad un punto geometrico. Il punto materiale differische da uno geometrico perché nel primo caso vi sono associate altre prorpietà fisiche, come massa, carica elettrica, quantità di moto, energia e così via.

punto materialeEsempio

Se ci trovassimo sopra un aereo, potremmo vedere ad esempio la barca in figura come un minuscolo puntino in mezzo ad una distesa di acqua enorme. Noi però sappiamo perfettamente che quel “puntino” è una barca: un oggetto piccolo rispetto alla vastità del mare.

A cosa serve?

Immaginate di voler descrivere il moto di un corpo, come un pianeta, nel campo gravitazionale del Sole. Una corretta descrizione non potrebbe prescindere dalla sua forma, dalla sua distribuzione di massa e così via. Prendiamo proprio come esempio la Terra. Il diametro terrestre è di circa 12000 Km, Questo vuol dire che la sua massa può variare (e varia) entro questa quantità. Ma la distanza media tra Sole e Terra è di circa 150000000 Km, ossia circa 12 mila volte più grande. Per fare un confronto, se la Terra fosse grande un centimetro e fosse su una linea di porta di un campo da calcio, il Sole si troverebbe oltre l’altra porta. Torniamo all’interazione tra Sole e Terra: pensate ora che la distribuzione di massa di quest’ultima sia rilevante per tale interazione? Esatto, no.

Equilibrio del punto materiale

Un punto materiale, inizialmente fermo, rimane in equilibrio quando la risultante delle forze applicate su di esso è nulla.

Tale principio è un caso particolare del primo principio della dinamica: quanto la risultante delle forze applicate su un corpo è nulla, questo permane nel suo stato di quiete o di moto uniforme.
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