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 arran up:ka0j,ylpc l :cb8z*hi*w3ged ve uc+ q7t3+ o-zww-sg mf-cj)ggrtically, with the lower cannon pointing upward (towards the upper cannon) and the upper c c ww+ -gg-+z gt-om3 7j)uqcfsannon 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 long after the cannons fire do the projectiles collide?
A. $2.2\,\mathrm{s}$
B. $2.5\,\mathrm{s}$
C. $3.6\,\mathrm{s}$
D. $6.7\,\mathrm{s}$
E. $8.0\,\mathrm{s}$
参考答案: B
本题详细解析:
We can use relative velocity. The projectiles are moving toward each o- hloh yf19pb8jvgrm,m v3p r6, j9-ffther. Their relativelmf j 9hroff,-8g, j916vy rpmvh3 -pb speed is $v_{rel} = v_{lower} + v_{upper} = 25\,\mathrm{m/s} + 55\,\mathrm{m/s} = 80\,\mathrm{m/s}$. Since gravity accelerates both projectiles downward equally, it does not affect their relative velocity.
The time to collide is $t = \frac{\text{distance}}{v_{rel}} = \frac{200\,\mathrm{m}}{80\,\mathrm{m/s}} = 2.5\,\mathrm{s}$.
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