更新1:
2. A hot air balloonist takes off vertically from the ground with an upward a=0.25m/s^2. When she is 8.0m from the ground her husband, realizing that she has forgotten her lunch box, throws it up at 20m/s.
更新2:
a) After what time interval can she catch the lunch box? b) How much time could her husband have spent looking for the lunch box before throwing it up at that same upward speed? (assuming he wanted her to have her lunch).
