# Chapter 13: Vector-Valued Functions and Motion in Space - Section 13.6 - Velocity and Acceleration in Polar Coordinates - Exercises 13.6 - Page 775: 4

$v=a \cos t u_r +a(1+\sin t) e^{-t} u_{\theta}$ $a=-a(\sin t +(1+\sin t) e^{-2t})u_r-ae^{-t} (1+\sin t -2 \cos t) u_{\theta}$

#### Work Step by Step

The velocity and acceleration in terms of $u_r$ and $u_{\theta}$ can be computed as: $v=r_t u_r+r \theta_t u_{\theta}$ and $a=(\dfrac{d^2r}{dt^2}-r\theta^{.2})u_r+(r\theta^{..} +2r^{.}\theta^{.})u_{\theta}$ Now, $\dfrac{d \theta}{dt}=\theta^{.}=e^{-t}$ and $\theta^{..}=-e^{-t}$ Thus, $v=a \cos t u_r +a(1+\sin t) e^{-t} u_{\theta}$ Next, $a=(\dfrac{d^2r}{dt^2}-r\theta^{.2})u_r+(r\theta^{..} +2r^{.}\theta^{.})u_{\theta}$ So, $a=-a(\sin t +(1+\sin t) e^{-2t})u_r-ae^{-t} (1+\sin t -2 \cos t) u_{\theta}$

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