VCE 物理干货贴之NEWTON'S LAW & momentum(审题仔细),高中生补习联盟Australia微信公众号文章

1. A rocket of mass 0.5kg is set on the ground , pointing vertically up. When ignited, the gunpowder burns for 1.5s, and provides a constant force on the rocket of 22N. The mass of the gunpowder is very small and can be ignored, as can air resistance.

What is the magnitude of the resultant force on the rocket？

After 1.5s, what is the height of the rocket above the ground?

Answer:

Q1此题在题干中MAGNITUDE告诉我们这道题目只需要求力的大小，方向不用考虑。如果一般题目没有MAGNITUDE的存在，同学们在做求力或者加速时候一定要带有方向。

Resultant force

=22-0.5x10

=17N

Q2此题目是基本的牛顿第二定律（NEWTON’S SECOND LAW）F=ma。

a=34m/s

x=1/2 at2

=17x 1.52

=38m

2. A box of mass 30kg is loaded on to a slide, and slides from rest to the floor. The slide is 6.0m long. It provides a constant friction force of 50N on the box. The box reaches the end of the slide at 8.0m/s.

What is the height h between second floor and the ground floor?

The box slides along the (frictionless floor colliding with the spring, which brings it momentarily to rest. The spring has a stiffness constant of 30000N/m. At this point, what is the compression of the spring?

Answer:此题用到了动量（momentum）守恒定律以及转换

mgh=0.5mv2+fx

30x10xh=0.5x30x64+50x6

h=4.2m

Q2 次题干中提到了Frictionless，就是忽略摩擦力，一般题目中如果没有这个词的出现，同学们需要把摩擦力考虑进去。

0.5kx2=0.5mv2

X=0.25m

综上所述，在这次物理干货贴中主要是提醒VCE物理学生们，审题一定要仔细，哪些地方要考虑，哪些地方不用考虑。一般考试或者平时SAC中一定要把关键词圈出来，避免审题疏忽而全盘皆输。

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