L’estudi
del moviment, que va començar fa més de 400 anys, va donar lloc al
naixement de la física i a aquesta part s’anomena cinemàtica. La
cinemàtica i la dinàmica (part que estudia les forces en relació al
moviment dels cossos) juntes fan el conjunt de la Mecànica, un dels
pilars de la física de tots els temps.
Moviment: canvi de posició respecte un sistema de referència
Sistema de referència: conjunt d'eixos i un origen respecte al qual mesurem les posicions d'un mòbil.
Trajectòria: conjunt de punts successius per on passa un mòbil.
Tipus de trajectòries:
a) Trajectòries rectilínies: mòbil es mou en línia recta
b)
Trajectòries curvilínies(corbes). Dins de les corbes, n'hi ha de formes
infinites: circulars (com els dels cavallets de les atraccions
infantils), parabòliques ( com les de les pilotes xutades pels
futbolistes), el.líptiques, etc...
Conceptes importants
Posició d'un mòbil:
La posició d'un mòbil en un instant en el cas del moviment rectilini
està determinada per la coordenada X del punt en què es troba.
Per exemple X=3 m ( vol dir que es troba a 3 m de l'origen i cap a la dreta)
X= -3 m (vol dir que és troba a 3 m de l'origen però a l'esquerra)
Desplaçament d'un mòbil (
): El desplaçament d'un mòbil entre dos instants inicial i final, t0 (temps inicial) i tf o t (temps final) , està determinat per la diferència entre la posició final Xf en la qual es troba al temps final tf, i la posició inicial X0 en què es troba en l'instant inicial.![capital delta x equals X subscript f minus X subscript 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La unitat en que es mesura el desplaçament amb el S.I és el metre, m.
El desplaçament és positiu, si el moviment té lloc en el sentit positiu de l'eix, i negatiu, si va en sentit contrari.
Distància recorreguda: La distància recorreguda per un mòbil sobre una recta és el valor absolut o mòdul del desplaçament:
Per tant, la distància recorreguda sempre és positiva.
Exemple
Un motorista es troba inicialment (t0=0) a la posició x0= 23 m, i quan han passat 12 s es troba a la posició xf= 2m. Calcula
a) El desplaçament realitzat.
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=
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b) La distància recorreguda
Velocitat mitjana, Vm
És el quocient entre el desplaçament efectuat i l'interval de temps transcorregut:
El
signe de la velocitat és positiu si el mòbil es desplaça en el sentit
positiu de l'eix X i és negatiu si es desplaça en el sentit contrari.