# inertia

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posted:
10/14/2012
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```							Inertia
1. Galileo’s law of inertia
Why should a rock fall toward the center of the
earth if the earth itself were spinning through
space ?
A       B
P73

The law of inertia : a body will remain at rest or
continue to move with a constant speed in a
straight line unless acted on by an outside agent .
惯性定律
2. Relative motion
By watching a stone fall , you cannot tell
whether the earth is at rest or moving , why ?
All motion is relative . To describe motion you
need to specify a frame of reference .
Any reference frame that is in straight – line
motion with a constant speed is called an inertia
reference frame .

相对运动、参照系、惯性参照系
P75 - 76
3. Projectile motion
A cannonball falls vertically under the force of
gravity while its inertia keeps it moving
horizontally with a constant speed .
1 2
y (t )  gt
2                P77 - 78
x(t )  v x t
g 2
Its trajectory is a parabola : y  2 x
2v x
惯性是阐明抛体运动的关键。水平的匀速
运动与竖直的落体运动合成为抛体运动。

```
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