phy d-t graph

無意思
番番原來位置 再向相反方向行

Q: 我想問d-t graph 個area有咩意思???
The is no physical meaning for the area under a displacement-time curve, as the area represents [displacement x time] which doesn't carry any physical interpretation.

Q: 同埋如果d-t graph個條線由+ve經過0再去-ve咁係咩意思???
If the curve decreases from a certain (+ve) value to zero, this indicates that the object is returning to the origin (starting reference point), as the displacement from the origin is gradually diminished.
When the curve cuts the time axis, this shows the 'time' that the object is at the origin. Further increase of the displacement in the -ve direction indicates that the object is going away from the origin on the other side, i.e. not on the same side as the object is before the curve cuts the time-axis.
You could also look at the gradient of the curve to determine the direction of velocity.