Question

Let $$\mathbf { u } , \mathbf { v } , \text { and } \mathbf { w }$$ be vectors. Which of the following make sense, and which do not? Give reasons for your answers. $$
a.( \mathbf { u } \times \mathbf { v } ) \cdot \mathbf { w }\quad
b. \mathbf { u } \times ( \mathbf { v } \cdot \mathbf { w } )\ quad
c. \mathbf { u } \times ( \mathbf { v } \times \mathbf { w } )\quad
d. \mathbf { u } \cdot ( \mathbf { v } \cdot \mathbf { w } ).$$

Answer 1

(u × v) • w makes sense, since u × v is another vector, and so (u × v) • w is a scalar.

u × (v • w) does not make sense, since v • w is a scalar, and the cross product is not defined between a vector and a scalar.

(u × v) × w makes sense, since u × v is a vector and so is w.

u • (v • w) does not make sense, since v • w is scalar, and the dot product is not defined between a vector and a scalar.