两个错误的（物理）观念_velocity/acceleration

1.   that acceleration and velocity are always in the same direction ✖️

试想一下，以东为正方向时，一辆车在向东匀速行驶，那么在这个时候，它的velocity是正的（不知道velocity的可以看一下我前面发的笔记）。加速时，acceleration为正的，但是当物体减速时，accelaration就是负的了，那这个时候，accelaration就是向着负方向的西边了，所以说这个acceleration和velocity不一定要是同方向的。

velocity and acceleration are not necessarily in the same direction. When the ball is moving upward, its velocity is positive (upward), whereas the acceleration is negative (down- ward).

    2.     that an object thrown upward has zero acceleration at the highest point (见上图中的B点).

        At the highest point (见上图中的B点), the ball has zero velocity for an instant. Is the acceleration also zero at this point? No. The velocity near the top of the arc points upward, then becomes zero for an instant (zero time) at the highest point, and then points downward. Gravity does not stop acting, so a = –g = –9.80 m􏰀/s^2 even there. Thinking that a = 0 at point B would lead to the conclusion that upon reaching point B, the ball would stay there: if the acceleration ( = rateof change of velocity) were zero, the velocity would stay zero at the highest point, and the ball would stay up there without falling.

那今天分享就到这里了，谢谢阅读