Force is a push or pull that causes or prevents acceleration; Newton's second law states F = ma. The SI unit is the newton (N): 1 N is the force that accelerates 1 kg by 1 m/s². The kilogram-force (kgf) is the weight of 1 kg at standard gravity (9.80665 m/s²) — a non-SI unit still found in older engineering manuals and some industrial contexts. The pound-force (lbf) is the gravitational force on 1 avoirdupois pound at standard gravity = 4.44822 N.
Rocket thrust is specified in kilonewtons (kN) or meganewtons (MN). The Space Shuttle Main Engine produced about 1,860 kN each; three engines combined generated 5,580 kN of thrust. Structural engineering uses force in kN or MN for load calculations on beams, columns, and foundations. The dyne (10⁻⁵ N) is a CGS unit surviving in older scientific literature and surface tension measurements. Your approximate weight in newtons equals your mass in kg multiplied by 9.81.
| 1 lbf | = 4.44822 N |
| 1 kgf | = 9.80665 N |
| 1 kN | = 224.809 lbf |
| 1 dyne | = 0.00001 N |
| Weight of 1 kg | = 9.807 N |
| 1 ton-force | = 8,896 N |