A man of mass 75.0 kg stands on a bathroom scale in a lift accelerating upward at 1.20 m/s^2. What does the scale read?

1. Take the person as the system, so the only forces on him are his weight and the upward push of the scale. 2. Newton's second law gives that upward push minus his weight equal to his mass times his acceleration. 3. Rearranged, the push of the scale equals the mass times the acceleration plus the mass times the acceleration due to gravity. 4. Substitute the values: (75.0 kg)(1.20 m/s^2) + (75.0 kg)(9.80 m/s^2). 5. That is 90.0 N + 735 N = 825 N. 6. By Newton's third law the scale reads the force he presses down with, which is the same 825 N. 7. Quoting 90.0 N keeps only the acceleration term and drops his weight entirely. _Source: OpenStax College Physics (CC BY 4.0), Ch 4 "Dynamics: Force and Newton's Laws of Motion", section 4.7 Further Applications of Newton's Laws of Motion_