Quantities can be vectors, having both size and direction. Examples include force, velocity and distance. A scalar quantity has no given direction, for example volume, mass and speed. When two forces or any other two vectors act at the same time at an angle to each other the effect cannot be calculated by simple addition. This video lesson shows how that sort of problem can be solved either by drawing a scale diagram or using trigonometry.
If we need to see the effect of a force in a particular direction such a the force of wind blowing at an angle across a wall rather than directly onto it, we can resolve or split the total force into two parts at right angles to each other. This is a useful technique in designing structures.