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**1. Homework Statement**

A square of edge length 19.0 cm is formed by four spheres of masses m1 = 5.10 g, m2 = 3.00 g, m3 = 1.20 g, and m4 = 5.10 g. In unit-vector notation, what is the net gravitational force from them on a central sphere with mass m5 = 3.00 g?

**2. Homework Equations**

G*m_1*m_2/d^2

**3. The Attempt at a Solution**

First, I calculated the distance, d, use the edge length they give you and the pythagorean theorem to solve for a diagonal in the square. I divided this answer by two for the distance between the center particle and any of the 4 particles at the extremities.

Next, I ran the formula above for each of the 4 particles to solve for the forces they exert. However, I'm confused as how to take the difference between the opposing forces to get two net forces, and calculate the forces in the x and y directions by using trig. For example, I'm unsure whether to subtract F_54 from F_51 or vice versa?