Weight is a force
If weight is a force, it can be worked out from F = m a, the second law itself.
When an object falls under gravitational attraction, the acceleration is the acceleration due to gravity — the g you met in chapter 2. So substitute g in place of a.
Weight
weight = mass × acceleration due to gravity = m g
On the earth g has the value 9.8 m s−2. So an object of mass m has a weight of 9.8 m newtons.
Two examples
Two examples
weight of 1 kg = 1 kg × 9.8 m s−2 = 9.8 N
weight of 3 kg = 3 kg × 9.8 m s−2 = 29.4 N
That is, the force pulling a 3 kg mass towards the earth is 29.4 N.
To make the arithmetic easier
When a problem tells you to take g = 10 m s−2, the weight is just the mass times 10. A man of mass 60 kg has a weight of 600 N.
What does a scale measure?
Saying "my weight is 50 kilograms" is, in physics, incorrect. Fifty kilograms is a mass; the weight that goes with it is 490 N.