In Figure, a square of edge length 20.0 cm is formed by four spheres of masses m1 = 5 g, m2 = 3 g, m3 = 1 g,
and m4 = 5 g.
(a) [10 points] Draw a force diagram on a central
sphere.
(b) [20 points] What is the net gravitational force on a
central sphere with mass m5 = 3g due to four masses?
(c) [20 points] Suppose the central sphere is removed.
What is the total potential energy of the system of 4
masses now?

no. dot product   =0 when vecs at right angles

bacteria that live on the ocean floor are sustained by phytoplankton. phytoplankton, microscopic marine photosynthetic organisms, have a vastly significant role to play not only in the marine food web of which they’re part of, but also on a more global scale. despite their infinitely small size in comparison to other marine organisms, these tiny creatures occupy an immensely important ecological niche: they are the foundation of the marine food web, and as primary producers, play key roles in supporting all other organisms in the marine environment, as well as in the regulation of the earth’s climate through the sequestration of carbon, oxygen production, and other related processes. phytoplankton account for roughly half of all global primary productivity; therefore, their significance extends far beyond the marine environment alone. there is an intriguing sense of irony in the realization that these tiny living beings that often live out their existence unnoticed and undetected by the rest of the world, have such a far-reaching impact on the lives of virtually all other living organisms on the planet, particularly on those in the marine environment. the world that we have become accustomed to has been and is continuously shaped by the workings of these miniscule yet vital “plants of the sea”.

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