一物体在空气中由静止下落，若物体下落时受到的空气阻力与它的速度平方成正比，即f=kv2．已知当物体的速度达到40m/s后就匀速下落，此时空气阻力______重力（填“大于”、“等于”或“小于”）；当它的速度为10m/s时，则物体下落的加速度为______．
当物体匀速下落时，重力与空气阻力平衡，大小相等，
即有 mg=kv12 ①
当物体的速度为10m/s时，空气阻力大小为kv22．由牛顿第二定律得
mg-kv22=ma ②
将①代入②得
mg-
•mg v 21
=mav 22
得到 a=g（1-
）=9.375m/s2v 22 v 21
故答案为：等于；9.375m/s2
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| Tom and Sam are twin brothers. They like playing soccer. They live 1 a big house 2 a beautiful garden. The house is 3 the sea. They often go to the sea and swim 4 it. There are four 5 in 6 family. They are parents, Tom and Sam.. Their father is a worker, 7 their mother is a teacher. 8 is Sunday today. Tom and Sam don't go to school. Their parents are not at work. They go to a park by car. Now they are in the park. The two 9 are swimming in the river and their parents are sitting 10 a tree. Look, they are talking and laughing. A bird is flying in the sky. | |||
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