硝基漆俗称(),是以()为主要成膜物的涂料。
参考答案:喷漆;硝基纤维
_____ teachers are at work now. Be quiet![ ]
A. All of
B. All the
C. All
D. None of
用滑轮组提升重物,当绳的自由端被拉下2m,发现物体升高0.5m,物体的质量为20千克,动滑轮重1N;若不计摩擦阻力,拉绳的力是[ ]
A.19.6N
B.49N
C.49.25N
D.50N