Which statement is true?
a. velocity and speed both have units of m/s.
b. velocity is a scalar, while speed is a vector.
c. velocity has a magnitude, but speed does not.
d. speed always includes a direction as well as a magnitude.

The two stars in a certain binary star system move in circular orbits. the first star, alpha, has an orbital speed of 36 km/s. the second star, beta, has an orbital speed of 12 km/s. the orbital period is 137 d. a) what is the mass of the star alpha? b) what is the mass of the star beta?