Scalar and vector quantities and operations on them
A quantity fully described by its numerical value alone is a scalar: mass, time, path and temperature are scalars. A quantity for which direction matters as well as the value is a vector quantity: displacement, velocity and force are examples. A vector is drawn as an arrow: the length of the segment shows the magnitude and the arrowhead shows the direction. Scalars add by ordinary arithmetic, but vectors must be added taking direction into account. Vectors along one line in the same direction have their magnitudes added; for opposite directions the smaller magnitude is subtracted from the larger. The projection of a vector on an axis is like its “shadow” on that axis: positive if the vector points the same way as the axis, negative if against it.
“Arrow game”: draw arrows of different lengths on the board; pupils say which has the larger magnitude and which way it points. Safety: use no sharp tools.