请问一道物理题(语言为英语)

题目

请问一道物理题(语言为英语)
(11 points possible)
An object is moving along a straight line parallel to the x-axis.Its position as a function of time is given by:
x(t)=20 m−14 (m/s) t+2 (m/s2) t2
where the position x is in (m) and the time t is in (s).
(a) What is the object's velocity at t=0 s,2 s,and 5 (enter the x-component of the velocity in m/s.Do not enter the units in the box.)
有七小题,但是第一题我就无法理解到底是要怎样
个人理解是一个物体沿X轴直线运动.x(t)=20 m−14 (ms) t+2 (ms2) t2是x位置的方程式.x的单位是米,t的单位是秒.
题a)是求物体在0秒,2秒,5秒时的速度
可是这要怎麼求,如果0秒是初速度的话那不就是原题的14吗,但是显示错误.

答案

对X求导,x'=-14+4t,0s,v=x'=-14;;;;;;;;;;2s,v=x'=-6;;;;;;;;;;;5s,v=x'=6.注意正负.