Question

Eric has a mass of 70 . He is standing on a scale in an elevator that is accelerating downward at 1.7 . What is the approximate reading on the scale?
A. 0
B. 570N
C. 690N
D. 810N

Answer 1

Answer:

B)

Explanation:

The value the scale shows is the reaction force to the normal force (they are equal by Newton's 3rd Law) that the scale exerts on Eric.

The forces on Eric are his weight (downward) and this normal force (upward), so we can write the net force over him as (also using Newton's 2nd Law):

$F=W-N=ma$

which means

$N=W-ma=mg-ma=m(g-a)$

and for our values this is:

$N=mg-ma=(70kg)(9.8m/s^2-1.7m/s^2)=567N$