7. A 25 N block is lowered 1.2 m by a 20 N force. a. Draw a free-body diagram of the forces acting on the block.
b. How much work does the force of gravity do on the box? Is this work positive or negative?
c. How much work does the applied force do on the box? Is this work positive or negative?

Asun-sized star will spend most of its lifetime as a: white dwarf red giant protostar main-sequence star

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?

The man of mass m1 and the woman of mass m2 are standing on opposite ends of the platform of mass m0 which moves with negligible friction and is initially at rest with s = 0. the man and woman begin to approach each other. derive an expression for the displacement s of the platform when the two meet in terms of the displacement x1 of the man relative to the platform.