Why does the gravitational attraction between the Sun and Mercury vary? Would it vary if the orbit of Mercury were perfectly circular?

According to Newton, two objects attracted by gravity actually both orbit around their common center of mass.  That means:  a the Sun is not orbiting because it's too big and too massive compared with Earth  b the Sun is not orbiting because it's too far from Earth  c the Sun is not orbiting because stars do not orbit  d the Sun is orbiting but we can't clearly see it because it's too big and too massive compared with Earth

Calculate the magnitude of the gravitational force between two 10-kg lead spheres separated by a center-to-center distance of 25 cm.

How do you determine the mass of a planet with just its distance to another object, and the gravitational field intensity?

Why don't you feel a gravitational force from the text book you're reading?

A satellite is orbiting the earth in low orbit (160 km above the surface of the earth). If the universal gravitational constant is 6.67 x 10^-11 m^3/kg x s^2 the mass of the earth is 5.972 x 10^24 kg and the radius of the earth is 6371 km, what speed must the satellite travel in order to maintain the low earth orbit?