# game physics page 28-29

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I couldn't understand the portion of content. On page 28-29 of game physics dave wrote, there is the text following ------------------------------------------------------------ Consider a time-varing vector written in the body coordinate system, ¥î(t) = R(t)s(t) .. digested .. The body observer does not see the change w¡¿¥î because he is rotating with body coordinate system. ------------------------------------------------------------ note: w¡¿¥î = R's But I think that if R, s are written in body coordinate system, ¥î and d¥î/dt must be also in the same. If R is just for the transformation to world space, the body observer can't see the rotation exactly because he must be also being transformed. Moreover, D¥î/Dt = R ds/dt can't be in the body coordinate system. However Eberly said 'the quantity D¥î/Dt represents the time rate of change of ¥î relative to the body coordinates since ds/dt is what the body observer measures.' This means R is also in the same, the body coordinates. So w¡¿¥î = R's must be in the same. But Eberly also said 'the body observer does not see the change w¡¿¥î because he is rotating with body coordinate system.' How could it be? I can't understand. Please somebody, make me understood.

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Hey xdotdt,

I haven't read the book, but I think he means that when an two objects (observer and body) are rotating exactly relative to each other, neither can tell the other one is moving. For example, the earth is rotating, but you don't notice it because you are moving uniformly with it.

Hope that helped.

Keep Programming,
Hobbit

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