If this car is to roll without slipping on the ground,

如果这辆车在作无滑动的滚动
can you see that by the time it finishes one full revolution,

你能看出来当它转过一周之时
when it's come over here,

当它运动到这里时
the center will have moved the distance

圆心移动的距离
equal to circumstance of that tire.

就等于轮胎的周长
So, every part of the tire touches the ground;

轮胎表面每一部分都接触到地面
then, that comes down and that comes down,

然后这个下来那个下来
and when it goes all the way to this end,

当它走到这时
it has also finished one full revolution.

它也正好转了一圈
For example, if every part of that circle was colored with some chalk,

举个例子如果用粉笔把这个环涂上颜色的话
then when it finished the full revolution,

那么当它完成转动一圈
every part of the circle would have touched the road

环的每一部分就会与路接触