# mastering physics dumbbell

*label*Physics

*timer*Asked: Nov 18th, 2015

**Question description**

A dumbbell has a mass m on either end of a rod of length 2a. The center of the dumbbell is a distance r from the center of the Earth, and the dumbbell is aligned radially. If r≫a, the difference in the gravitational force exerted on the two masses by the Earth is approximately 4GmMEa/r3. (Note: The difference in force causes a tension in the rod connecting the masses. We refer to this as a tidal force.)Suppose the rod connecting the two masses mis removed. In this case, the only force between the two masses is their mutual gravitational attraction. In addition, suppose the masses are spheres of radius a and mass m=43πa3ρ that touch each other. (The Greek letter ρ stands for the density of the masses.)Write an expression for the gravitational force between the masses m.Express your answer in terms of the variables a, ρ, and appropriate constants.Find the distance from the center of the Earth, r, for which the gravitational force found in part A is equal to the tidal force (4GmMEa/r3). This distance is known as the Roche limit.Express your answer in terms of the variables ME, ρ, and appropriate constants.Calculate the Roche limit for Saturn, assuming ρ=3330kg/m3. (The famous rings of Saturn are within the Roche limit for that planet. Thus, the innumerable small objects, composed mostly of ice, that make up the rings will never coalesce to form a moon.)Express your answer using three significant figures.