Imagine pushing a heavy box across the floor at an angle — not straight forward, but slightly downward. Some of your effort moves the box forward, and some presses it into the floor. The force you apply is a single vector, but its effect splits into two independent directions: horizontal and vertical.
That splitting is vector component extraction. Any vector can be seen as the sum of two (or three) simpler vectors pointing along chosen reference directions — usually the coordinate axes. Each of those simpler vectors is a component.
Note
"Component" means "a part of a whole." In vectors, the components are the parts that add up to give the original vector.
The Precise Statement
Given a vector v in a plane, and perpendicular axes x and y, the components of v are its projections onto those axes:
v=vxi^+vyj^
where i^ and j^ are unit vectors along the x and y axes, and vx, vy are scalar components (numbers, possibly negative).
If v makes an angle θ from the positive x-axis, then:
vx=∣v∣cosθandvy=∣v∣sinθ
Component along an axis=(magnitude of vector)×cos(angle between vector and that axis)
Why This Works: The Geometry
Draw a vector from the origin. Drop a perpendicular from its tip to the x-axis — that gives vx. Drop another to the y-axis — that gives vy. The original vector is the diagonal of the rectangle formed by vx and vy. This is the Pythagorean theorem in reverse: if you know the hypotenuse and one angle, trigonometry gives you the legs.
A Concrete Example
A force of 10 N acts at 30∘ above the horizontal.
Fx=10cos30∘=10×23=53≈8.66 N
Fy=10sin30∘=10×21=5 N
So the force vector is 8.66i^+5j^ N.
Watch out
A common mistake: using sin for the horizontal component and cos for the vertical. Check: if the angle is measured from the x-axis, the side adjacent to it is along x — that's cos; the opposite side is along y — that's sin.
Step 1. (a) Tj^−mgj^=maj^: both sides are along j^ only, so comparing the single (y) component directly gives T−mg=ma (this is the tension equation for a lift accelerating upward, e.g. Newton's second law applied to a mass in an accelerating lift).
Step 2. (b) T+F=A+B: resolving all four vectors into x, y, z components and equating each axis separately gives three scalar equations: Tx+Fx=Ax+Bx, Ty+Fy=Ay+By, Tz+Fz=Az+Bz.