A person stands on a scale inside an elevator accelerating upward with acceleration a. The scale reads:

A person stands on a scale inside an elevator accelerating upward with acceleration a. The scale reads:

• A. m(g + a)
• B. m(g - a)
• C. m g
• D. m a

Answer: (A)

When you’re in an elevator accelerating upward, the scale reading reflects the normal force that the scale exerts on you. That normal force must do two things at once: support your weight and provide the upward acceleration. Apply Newton’s second law to the person: the upward normal force N minus the downward gravitational force mg equals the mass times the upward acceleration a. So N − mg = m a. Solve for N: N = m(g + a). Since the scale reads that normal force, the reading is m(g + a). This makes sense: you feel heavier when the elevator pushes you upward faster, and lighter if the elevator were moving downward or decelerating upward. If the elevator weren’t accelerating (a = 0), the reading would be mg.

When you’re in an elevator accelerating upward, the scale reading reflects the normal force that the scale exerts on you. That normal force must do two things at once: support your weight and provide the upward acceleration.

Apply Newton’s second law to the person: the upward normal force N minus the downward gravitational force mg equals the mass times the upward acceleration a. So N − mg = m a. Solve for N: N = m(g + a).

Since the scale reads that normal force, the reading is m(g + a).

This makes sense: you feel heavier when the elevator pushes you upward faster, and lighter if the elevator were moving downward or decelerating upward. If the elevator weren’t accelerating (a = 0), the reading would be mg.