Question

A 615 N student standing on a scale in an elevator notices that the scale reads 645 N. From this information, the student knows that the elevator must be moving A. downward B. upward. C. You cannot tell if it is moving upward or downward. D. Moving at constant rate up or down F. None of the above. E. Stationary

Answer 1

Answer:

B. upward

Explanation:

When a person is going up in an elevator the force exerted on a person is

$F=mg+ma\\\Rightarrow F=m(g+a)$

Where,

m = Mass of person

g = Acceleration due to gravity

a = Acceleration of the elevator

This force only exists till the elevator is accelerating. After the acceleration stops only acceleration due to gravity acts on the body.

Hence, the elevator is moving upwards if the scale reads more than actual.

Answer 2

Answer:

B) Upwards

Explanation:

As per newton's law we know that

$F_{net} = ma$

here we know that

$F - F_g = F_{net}$

also we know that

$F = 645 N$

$F_g = 615 N$

now we know that

$645 - 615 = \frac{615}{9.81} a$

so we have

$30 = \frac{615}{9.81}a$

$a = 0.48 m/s^2$

so here we can say that elevator is moving upwards with constant acceleration of a = 0.48 ms/s/s