# Thread: unsure about definitions

1. ## unsure about definitions

hi
may i know why if g(r(t))=0 or constant then it implies that r'(t) (grad g) = 0 for all values of t?

2. Originally Posted by alexandrabel90
hi
may i know why if g(r(t))=0 or constant then it implies that r'(t) (grad g) = 0 for all values of t?
By the chain rule, $\nabla g(r(t))= r'(t)(\nabla g)$. Since that measures the rate of change of g- and g doesn't change- it is equal to 0.