本题目来源于试卷: 2012美国US F=MA物理竞赛，类别为 美国F=MA物理竞赛
[单选题]
Two cannons are arranged vertica lus, v 8clqkgk0y8c7,q bc6.jc*9uqp 5)arr qlly, with the lower cannon pointing upward (towards the upper cannon) and the upper cannon pointing downward 59 prb*j uc6qqa)rqc. (towards the lower cannon), $200\,\mathrm{m}$ above the lower cannon. Simultaneously, they both fire. The muzzle velocity of the lower cannon is $25\,\mathrm{m/s}$ and the muzzle velocity of the upper cannon is $55\,\mathrm{m/s}$.  How far beneath the top cannon do the projectiles collide?
A. $31\,\mathrm{m}$
B. $67\,\mathrm{m}$
C. $110\,\mathrm{m}$
D. $140\,\mathrm{m}$
E. $170\,\mathrm{m}$
参考答案: E
本题详细解析:
From problem 6, the collision time is oawj/, , jrd3r$t = 2.5\,\mathrm{s}$. We can find the distance the top projectile travels downward in this time.
Using $g = 10\,\mathrm{m/s^2}$ (as per instruction).
$s = v_0 t + \frac{1}{2}at^2 = (55\,\mathrm{m/s})(2.5\,\mathrm{s}) + \frac{1}{2}(10\,\mathrm{m/s^2})(2.5\,\mathrm{s})^2$
$s = 137.5 + 5(6.25) = 137.5 + 31.25 = 168.75\,\mathrm{m}$.
This is closest to $170\,\mathrm{m}$.
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