继续英语物理题A lunar landing craft(mass=11400kg) is about to touch

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继续英语物理题
A lunar landing craft(mass=11400kg) is about to touch down on the surface of the moon,where the acceleration due to gravity is 1.6 m/s*2.At an altitude of 165m,the craft's downward velocity is 18.0 m/s.To slow down the craft,a retrorocker is firing to provide an upward thrust.
Assuming the descent is vertical,find the magnitude of the thrust needed to reduce the velocity to zero at the instant when the craft touches the lunar surface

题目要求当落地时速度恰好为0时其牵引力.可以将这个过程反过来看,看成把一个停止在月球表面上的物体加速到离月球165m时其速度为18m/s的时候,其牵引力为多少,这样就简单多了.
由题,初速度为0的物体,其匀加速直线运动的总距离s=0.5v^2/a.
所以带入上面的数据,得到a=0.5v^2/s=0.982(m/s^2)
而由牛二定律,ma=F-mg,其中月球的g=1.6.
所以F=ma+mg=11400×(0.982+1.6)=29435(N)

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