mechanics

1.有d graph 既x-axis 係t二次,y-axis係v既二次,點做

Just take t^2 or v^2 as a quantity. For example, let T=t^2 or V = v^2.
If the plot is a straight line, than V against the the x-axis quantity, or the y-axis quantity against T, follows the linear relationship y = mx + c, where m is the gradient and c is the y-intercept.

2.有個人坐在熱氣球裏,拿著一塊石頭,熱氣球正在上升,他將塊石頭放手掉下來(drop),咁石頭既u係咪0?

Clearly, it is not. When the stone is in the hand of the person, it rises with the same speed of the balloon. If the stone is released, its initial velocity equals to that of the rising balloon.