Pourquoi doit-on toujours préciser le référentiel dans lequel est étudié le mouvement d'un système?

Which feature indicates that mars resides in the inner region of the solar system

The difference between a red shift and a blue shift has to do with wavelength frequency. t or f

Asolid round steel rod 6.25 mm in diameter and 375 mm long is rigidly bonded to the end of solid square brass rod 25 mm on a side and 300 mm long. the geometric axes of the bare are along the same line. an axial tensile force of 5.4 kn is applied at the extreme ends of the assembly. for steel e = 200 gpa and for brass e = 90 gpa. determine the total elongation for the assembly.