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

試想一下，以東為正方向時(shí)，一輛車在向東勻速行駛，那么在這個(gè)時(shí)候，它的velocity是正的（不知道velocity的可以看一下我前面發(fā)的筆記）。加速時(shí)，acceleration為正的，但是當(dāng)物體減速時(shí)，accelaration就是負(fù)的了，那這個(gè)時(shí)候，accelaration就是向著負(fù)方向的西邊了，所以說這個(gè)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 (見上圖中的B點(diǎn)).

? ? ? ??At the highest point (見上圖中的B點(diǎn)), 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.

那今天分享就到這里了，謝謝閱讀