Sign is taken as – (negative) from pole of a spherical mirror towards object along the principal axis. Follow one sign convention and never get confused. This sign convention is known as New Cartesian Sign Convention. For KCL, incoming currents are taken as positive and outgoing currents are taken as negative at a node/junction. This means the sign is always taken as −(negative) in front of a spherical mirror. The following sign conventions are used by Cartesian: (1) All distances are measured from the pole of the mirror (in the case of the lens from the optic center). In this case you take the lens to be 0, and then, anything to the left is negative (normally the object), and anything to the right is negative. This sign convention is known as New Cartesian Sign Convention. So the thin lens formula equation : $\frac 1 f= \frac 1 {s'}+\frac 1 s $ This will only give you correct results, if you follow the correct sign conventions. Sign Conventions: XYZ Coordinate Offsets. The following symbol is used in the picture above: This mark signifies the Centre of Gravity (CoG), Centre of Rotation (CR) or the Central Reference Point (RP) of the vessel. If you Know your positive sign convention then you can work with it. The sign convention is important in how you define your quantities. But I raised this question to know exactly how this equation found." https://www.khanacademy.org/.../circuit-elements/v/ee-passive-sign-convention The sign convention for dBz/dt data can be explained as follows. Today we will see here the sign conventions for shear force and bending moment diagram in subject of strength of materials with the help of this post. This is done for two reasons. However, the decay curves for this component have historically been plotted as positive and decaying. Let us consider that we have one beam AB and we have assumed one section XX as displayed in following figure. Sign Convention for Spherical Mirror: Cartesian Sign Convention: In the case of spherical mirror all signs are taken from Pole of the spherical mirror, which is often called origin or origin point. Coordinate Offsets in the Ship Geometry . Now we will understand here that what sign conventions will be used here for shear force. "In general, I used to consider different sign convention by other book for analysis, which is different to the one considered in this design book. Sign conventions for shear force. For coincident loop airborne systems, the true dBz/dt response observed at the center of the receiver coil is typically negative and decaying during the off-time. Precisely because signs might also be used the opposite way, a sign convention means that - for the purpose of a certain exercise, for example - a certain direction is chosen as positive. This point has the coordinates X=0, Y=0 and Z=0 in the Ship Geometry.