Question

A 130lb woman stands in an elevator on a scale as the elevator just begins to move upward from the first to second floor. Which choice could properly describe her weight, contact force, and acceleration at this instant? Select one:

A. a. weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is zero
B. b. weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is downward
C. c. weight = 130 lbs downward, contact force = 130 lbs upward, acceleration is upward
D. d. weight = 140 lbs downward, contact force = 130 lbs upward, acceleration is downward
E. e. weight = 130 lbs downward, contact force = 140 lbs upward, acceleration is upward
F. f. weight = 130 lbs downward, contact force = 140 lbs upward, acceleration is downward

Answer 1

Answer: E. e. weight = 130 lbs downward, contact force = 140 lbs upward, acceleration is upward.

Explanation: taking into account the second Newton law,we have the following:

N-W= m* a where N and W are contact and weight forces

as the velocity is changing in that instant the acceleration must be different of zero. So as the mass move upward, the contact force must be higher that the weight force.