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completely trivial point. Masses can be measured against each other without using gravity at
all, for example far into space, by comparing their relative accelerations when subject to a
standard force, a push. If one object accelerates at half the rat
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Notice that here persevere in their motions must mean in steady speed straight line motions,
because he is adding the gravitational acceleration on to this.
This is sometimes called The Law of Inertia: in the absence of an external force, a body in
mo
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20.7 Newton Puts Them Together
Newtons major breakthrough was to show that these two different kinds of motion can be
thought of as different aspects of the same thing. He did this by introducing the idea of motion
being affected by a force, then expr
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consider what would happen if the body goes around the circle completely in one second. Then,
if you pick two times one second apart, you would conclude the velocity isnt changing at all, so
there is no acceleration.)
20.5 An Accelerating Body that is
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So we see that, with our definition of acceleration as the rate of change of velocity, which is a
vector, a body moving at a steady speed around a circle is accelerating towards the center all
the time, although it never gets any closer to it. If this
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approximation to that of the circle, but we can improve it by replacing the square by a regular
octagon, with all its points on the circle. Now, this octagon can by divided into eight triangles,
following the same procedure as for the square. The heigh
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making it difficult to see how they could ever move relative to the air at all! Yet they can be,
since they can fall, so the whole idea must be wrong.
Ptolemy did, however, know that the earth was spherical. He pointed out that people living to
the eas