Velocity 等於0有加速度﹖

Question

velocity 等於0點解會有acceleration﹖係咪因為一件object被拋到最高點，而因為object在空中的關係，所以受到a=g的acceleration﹖但是a的定義是rate of change of velocity，那object在最高點的是否有change of velocity，如果是，又是幾多﹖concept不很清，請各位好心人解答一下～thx^^

當一件obect moves horizontally from rest with increasing v， v -t graph中直線 passes through the origin一直向上，而a-t graph就是一條水平線，when t = 0，v = 0(from rest)，but a已經有一定數值(一條水平線)，所以when v=0，都可以有accelaration，咁樣諗有冇問題﹖

Answer 1

Any object (e.g. a car) that starts to move from rest would have a zero velocity and a non-zero acceleration.

Since acceleration a = (v-u)/t
where v and u are the final and initial velocities respetively, and t is the time interval.

When the object starts from rest, u =0 m/s
thus, a = v/t

hence, as long as v is not zero after time t (if v is zero, that means the object is at rest all the time), then a will have a certain value, i.e. non-zero acceleration.

Answer 2

velocity is the rate of change of displacement, i.e. the change of displacement per unit time. The instantaneous velocity is given by ds/dt - the first time-derivative of the displacement

In the s-t graph, the instantaneous velocity at time = t is actually given by the tangential slope at time =t.