From Motion to Forces

Measure the motion, get the acceleration, and the second law gives the net force. Subtract the forces you know:

$$a = \frac{\Delta v}{\Delta t}, \quad \sum F = m a$$

An elevator that speeds up on its way up: the cable pulls harder than the weight.

Speeding up on the way up: the cable pulls harder than the weight.

A scale shows the normal force $N$: your apparent weight.