# Acceleration question

A particle moves along the $x$-axis so that its velocity is given by $v(t)=5+3.1\cos(0.8t)$ for $t\geq 0$. What is the acceleration of the particle at $t=4$.

I differentiated this and got:

$a(t)=-2.48 \sin(0.8t)$

and plugged in $4$ and got

$a(4) = -0.138437$

But none of the answer choices fit the answer, am I wrong or is the question wrong?

Answer choices: $-0.045$, $-0.145$, $-0.181$, $0.145$, $5.145$

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$t$ is expressed in radians!! –  mfl Jun 27 at 2:34
Yes, check your calculations, making sure that your calculator is in radian mode. –  Gahawar Jun 27 at 2:35
It is... I dont know what im doing wrong –  Panthy Jun 27 at 2:35
Check again. I have obtained you result in DEG mode. –  mfl Jun 27 at 2:38
Ok my calculator is weird, i entered it on wolfram and got around 0.145 :) –  Panthy Jun 27 at 2:39

Instructions:

1. Click the start button.
2. Open calculator.
3. Click "view" and select Scientific.
6. Get $0.1447678757...$