Gravitational Pull?

Question

Four 9.0 kg spheres are located at the corners of a square of side 0.64 m. Calculate the magnitude and direction of the total gravitational force exerted on one sphere by the other three.

Answer 1

The resultant gravitational pull exerted on one sphere by the other 3 is simply the vector sum of each of the individual pulls exerted by each sphere (superposition principle).

The gravitational force of attraction exerted by one sphere on the other sphere is:
F = G * m * M / r^2
Where G is the Universal gravitational constant, m and M are the masses of each of the two spheres, and r is the radial distance in between.
The force, F, is along the radial direction pointing towards the other sphere.

Find the force exerted by each of the 3 spheres (remember that force is a vector, it has both magnitude and direction) and then add them together.
Use the symmetry of the system to make the problem easier (although it is still rather simply even if you don't take this into account).
Also, drawing a diagram will make it easier to visualize how things are arranged.