Lessons · Engineering · force, mass and weight
Mass is how much stuff; weight is the pull on it
Weight is a force: W = m × g, with g = 9.81 m/s² in SI and 32.2 ft/s² in US customary.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A motor is listed as 250 kg. The bolt that holds it does not care about kilograms; it holds newtons. Every anchor, hook and beam calculation starts by turning the mass on the nameplate into the force on the structure.
How to think about it
Ask which one the problem gave you. Kilograms and pounds-mass are mass; newtons and pounds-force are weight. Multiply mass by g to get weight, divide weight by g to get mass, and write which g you used.
Worked example
W = m × gWeight is mass times the acceleration gravity would give it.
m = 80 kg, g = 9.81 m/s²A person. The kilograms are mass, whatever the bathroom scale prints.
W = 80 × 9.81 = 784.8 NAbout 785 newtons pulling down, which is what the floor pushes back up with.
US: 64.4 lbm at g = 32.2 ft/s² weighs 64.4 lbf; its mass is 64.4 / 32.2 = 2 slugIn US customary a pound-mass weighs a pound-force on Earth, and the slug is the mass that F = m a works on directly.
Your turn
A 250 kg motor, g = 9.81 m/s². Write its weight.
W = 250 × 9.81 = N
Solve one, graded on the server
The trap
Calling 80 kg a weight. It is a mass. It becomes 784.8 N the moment you ask a bolt to hold it, and a bolt sized on 80 of anything is a bolt sized on nothing.