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## Homework Statement

What is the magnitude of the velocity at t=4.00s?

I would like to see if my approach and answer is correct.

## Homework Equations

Position: r = {-30cos([itex]\frac{\pi}{10}[/itex]t)

*i*+ 30sin([itex]\frac{\pi}{10}[/itex]t)

*j*- (7t)

*k*} ft

## The Attempt at a Solution

I took the first derivative to get the velocity and my equation came out to be:

r' = {3[itex]\pi[/itex]sin([itex]\frac{\pi}{10}[/itex]t)

*i*+ 3[itex]\pi[/itex]cos([itex]\frac{\pi}{10}[/itex]t)

*j*- (7)

*k*} [itex]\frac{ft}{s}[/itex]

And after plugging in t=4, my velocity vector equation was: [(8.96)

*i*+ (2.91)

*j*- (7)

*k*] [itex]\frac{ft}{s}[/itex]

Then my velocity magnitude was: 11.74 [itex]\frac{ft}{s}[/itex]