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]
The graph shows velocity as a function colr/ 7kbe;(z of time for a car. What ;joou4ejet 2q9 -q by1was the acceleration joo-eqt;124qbe9j yu at time $= 90$ seconds? 
A. $0.22\,\mathrm{m/s}^2$
B. $0.33\,\mathrm{m/s}^2$
C. $1.0\,\mathrm{m/s}^2$
D. $9.8\,\mathrm{m/s}^2$
E. $30\,\mathrm{m/s}^2$
参考答案: B
本题详细解析:
Acceleration is the slope (gradient) 0 +4(i u2;7oqlysqj 4ejsmxnx of the velocity-time graph.
At $t = 90\,\mathrm{s}$, the graph is a straight line.
We can find the slope using two points on this linear section, for example, $(t_1, v_1) = (60\,\mathrm{s}, 10\,\mathrm{m/s})$ and $(t_2, v_2) = (120\,\mathrm{s}, 30\,\mathrm{m/s})$.
$a = \Delta v / \Delta t = (v_2 - v_1) / (t_2 - t_1) = (30 - 10)\,\mathrm{m/s} / (120 - 60)\,\mathrm{s} = 20\,\mathrm{m/s} / 60\,\mathrm{s} \approx 0.33\,\mathrm{m/s}^2$.
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