Take the sine of the of the force's given angle. For example, a force approaching at 45 degrees would require a calculation as follows: sin (45).
Simplify the resulting answer, rounding as necessary. Trigonometric functions often yield several decimal places of accuracy. Rounding to the nearest thousandth is often practiced when a high degree of accuracy is needed. Lesser approximations, such as to tenths or whole numbers, are often allowed in classroom settings.
Multiply the magnitude of the force -- measured in Newtons (N) -- by the result from the trigonometric function evaluation. This quantifies the scale of the force being applied. For example, if the original vector force was described as 30N at 45 degrees, the result of √2 / 2 from the trigonometric function is multiplied by 30N, resulting in 15√2N. This is the overall force, longitudinally, from the original vector force given.