本题目来源于试卷: 2007美国US F=MA物理竞赛，类别为 美国F=MA物理竞赛
[单选题]
An object moves in two dimens15u2; re tsmymions gw)j m5 e30xzpx)nnq4 according to $$\vec{r}(t)=(4.0t^{2}-9.0)\hat{i}+(2.0t-5.0)\hat{j}$$ where $r$ is in meters and $t$ in seconds. When does the object cross the x-axis?
A. $0.0\,\mathrm{s}$
B. $0.4\,\mathrm{s}$
C. $0.6\,\mathrm{s}$
D. $1.5\,\mathrm{s}$
E. $2.5\,\mathrm{s}$
参考答案: E
本题详细解析:
The object crosses the x-axis when its y-coordinate (tho;zrej+;s mkm pp( :5ye $\hat{j}$ component) is zero.
We set $2.0t - 5.0 = 0$. Solving for $t$ gives $t = 5.0 / 2.0 = 2.5\,\mathrm{s}$.
| 
