Gravity force is always downward and it depends how you define the coordinate system. By convention down is negative, but that is only a general rule
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Positive + Negative = Negative Negative + Negative = Positive Positive + Positive = Positive Negative + Positive = Negative
The difference between a positive shear and a negative shear is the direction the beam is distorted into. A force that tends to shear the left portion of the beam upward with respect to the right portion is said to produce a positive shearing force.
The rules for the sign (positive or negative) of the result of a multiplication is the same as division. For multiplication: Positive * Positive --> Positive Positive * Negative --> Negative Negative * Positive --> Negative Negative * Negative --> Positive For division: Positive / Positive --> Positive Positive / Negative --> Negative Negative / Positive --> Negative Negative / Negative --> Positive
a negative x a negative= a positive, a positive x a positive= a positive, a negative x a positive= a negative, and a positive x a negative= a negative. Same thing with division. a negative divided by a negative= a positive, a positive divided by a positive= a positive, a negative divided by a positive= a negative, and a positive divided by a negative is a negative. U see?
positive 49 Because a negative times a negative is a positive. -7*-7=49. positive * positive = positive negative * negative= positive positive * negative = Negative