What is meant by the MA of a simple machine is 3?
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-- The two negative charges both attract the positive charge at the origin.

-- The -4C charges are horizontally symmetrical, so there's no net horizontal force on the +3C at the origin.

-- However, the -4C both pull it down.

-- So the net force on the +3C at the origin is directed straight down.

The arrow labelled ' C ' represents the net force on it.