# Newton's deffinitions on centripetal force.

Will someone explain to me what is ment by Newton's deffinitions VI-VIII? I dont understand exactly what they mean. What are the three qualitys of centripetal force he is talking about (I want to know what they are, not just there names).

Arrummzen
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Commented:
"Newton's deffinitions VI-VIII?" I have never heard of these, centrifugal force is usually described from his laws. What are the definitions you are talking about?
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Author Commented:
In his book Principia Newton begins by laying out some deffinitions. (its in the begining of the book, I dont fully understand VI-VIII)
They can be found here -
http://members.tripod.com/~gravitee/definitions.htm

Arrummzen
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Commented:
First of all, Newton's terminology differs from our modern-day usage. What he calls centripetal force, we generally call central force. We usually reserve centripetal force for rotational systems, but Newton uses it to describe any kind of central force. Also, what he refers to as motion, we would call momentum.

Def. VI: absolute quantity of a centripetal force

This describes the property of a central force intrinsic to the source. It depends only on the source itself, and is independent of anything else. For gravity, the absolute quantity is proportional to the object's mass. For Coulomb's law, the absolute quantity is proportional to charge.

Def. VII: accelerative quantity of a centripetal force

This is the acceleration of a given object due to a central force from a given source. It is dependent on the distance between the objects, and is a property of the field generated by the source.

Def. VIII: motive quantity of a centripetal force

The motive quantity is what we would call the actual "force". It depends on both the source and the object being acted upon, as well as their relative positions. Thus, the motive quantity is proportional to both the absolute quantity and the accelerative quantity, as in F=ma.

Hope that helps,

-Seth
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Author Commented:
I understood the first 5, its 6-8 that I needed the help with.

What does "intrinsic" mean?
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Commented:
Arrummzen,
Look at them this way. Consider a weight on a piece of string that you are swinging round in a circle.

DEFINITION VI.  The heavier the weight the more force you feel on the string. ie proportional to the mass.

DEFINITION VII.  For given speed of the weight then the longer the string the less force you feel on the string. Shorten the string and keep the weight's speed the same then the more force you feel. ie proportional to 1/(string length).

DEFINITION VIII.  Is a combination of VI and VII. ie the faster the weight rotates and the shorter the string then the more force you feel, and visa versa. He is just saying VI and VII both apply together.

GfW
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