Question

Lucy Lightfoot stands with one foot on one bathroom scale and her other foot on a second bathroom scale. Each scale reads 325 N. What was Lucy's weight?

Answer 1

We can find Lucy's weight from the Newton's Second Law of Motion:

$$\sum F_y = 0,$$ $$N_1 + N_2 - W_{Lucy} = 0,$$

here, $W_{Lucy}$ is Lucy's weight, directed downward, $N_1, N_2$ are the normal forces (or the scale readings) directed upward.

Then, from this equation we can find Lucy's weight:

$$W_{Lucy} = N_1 + N_2 = 325 N + 325 N = 650 N.$$

Answer:

$W_{Lucy} = 650 N.$