A 2-kg block hangs from a spring balance calibrated in Newtons that is attached to the ceiling of an elevator (Figure). What does the balance read when? 

(a) The elevator is moving up with a constant velocity of 30 m/s; 

(b) The elevator is moving down with a constant velocity of 30 m/s; 

(c) The elevator is ascending at 20 m/s and gaining speed at a rate of 10 m/s²?

            From t = 0 to t = 2 s,

The elevator moves upward at 10 m/s. Its velocity is then reduced uniformly to zero in the next 2 s, so that it is at rest at t = 4 s. Describe the reading on the balance during the interval 0 < t < 4 s.

2kg