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## Homework Statement

A particle moving along the x axis has a position given by

x = 24t - 2.0t

^{3}m, where t is

measured in s. What is the magnitude of the acceleration of the particle at the instant

when its velocity is zero?

## Homework Equations

v=dx/dt

a=dv/dt=d

^{2}x/dt

^{2}

## The Attempt at a Solution

v = dx/dt = 24 - 6t

^{2}= 0 =>t

^{2}= 4 => t= ± 2

a = dv/dt = d

^{2}x/dt

^{2}= -12t

=> a = -12 (2) = -24 m/s

^{2}

or a = -12(-2) = 24 m/s

^{2}

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the right answer is a = 24 m/s

^{2}

but why??