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 vertically c )at)drm9wg*tp 8:k6xx )pc r, with the lower cann u-h:oaqj/qc)8b 2 wl ton pointing upward (towards the upper cannon) and the upper oqbuhq) 2/ c:tl8- ajwcannon pointing downward (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 collisio;1nkpw) yfl ;aueo3:f n time is $t = 2.5\,\mathrm{s}$. 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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