## Hunus Group Title Can someone help me understand this? 2 years ago 2 years ago

1. Hunus Group Title

I'm reading a book on classical mechanics and I can't quite catch on to what the author is saying. The scalar product $\vec r \cdot (d\vec r)= r(d\vec r)_r$where $(d\vec r)_r$ is the projection of $d\vec r$ on the vector $\vec r$ This projection is equal to $d\vec r$ the increment of the magnitude of the vector $\vec r$ Therefore $\vec r \cdot (d\vec r)= r*dr$

2. Vincent-Lyon.Fr Group Title

|dw:1335174774073:dw|from the sketch you can see$\vec r \cdot d\vec r= \left| \vec r \right| \left| d\vec r \right| \cos \theta$and$\left| d\vec r \right| \cos \theta=dr$hence:$\vec r \cdot d\vec r= r\:dr$

3. Hunus Group Title

Thank you

4. T.P Group Title

l dont understand the language of the author but this is a vector as you see that it have direction and magnitube. this vector have components Y&X becouse of the angle on the x-axes so l think this language is explaining that