Calculate the magnitude of particle acceleration

[inflames829]

Homework Statement

A particles position is given by r=42t^2i + 41t^3j + 6k. Calculate the magnitude of its acceleration at time t = 15 seconds

Homework Equations

The Attempt at a Solution

to find the acceleration i found the second derivative which i had as 246t + 84. Then i put 15 in as t and got 3774, but according to the answers i should get 3691. What did i do wrong

 

[Doc Al]

You are dealing with vectors. The acceleration is 246tj + 84i, not just 246t + 84.

 

[inflames829]

but do i still have to find the second derivative using this? Can you give me a hint on how to do this?

 

[inflames829]

ive worked this one out. Thanks for the hint