# physics acceleration

• Oct 9th 2012, 08:44 PM
Louisana1
physics acceleration
A fox is chasing a bunny. The bunny is initially hopping east at 9 m/s when it first sees the fox. Over the next half second the bunny changes its velocity to west at 13 m/s and escapes. What was the bunny's average acceleration (magnitude and direction) during this half-second interval?

______________m/s2

What equation is used and how is this set up?
I've tried drawing this and I have figured out 22m/s2
and he is heading west but the 22 was wrong do I need to do something
with the half second?

• Oct 9th 2012, 08:53 PM
MarkFL
Re: physics acceleration
I would let due east be the positive axis of motion, hence:

$\bar{a}=\frac{\Delta v}{\Delta t}$ where:

$\Delta v=v_f-v_i=-13\frac{\text{m}}{\text{s}}-9\frac{\text{m}}{\text{s}}$

$\Delta t=\frac{1}{2}\text{s}$
• Oct 9th 2012, 08:53 PM
richard1234
Re: physics acceleration
$\vec{a} = \frac{\vec{v_f} - \vec{v_i}}{\Delta{t}}$.

If we set up our coordinate system such that east is positive and west is negative, then

$\vec{a} = \frac{-13 \frac{m}{s} - (9 \frac{m}{s})}{\frac{1}{2} s} = -44 \frac{m}{s^2}$. The - sign indicates that the bunny is accelerating west.
• Oct 9th 2012, 09:03 PM
Louisana1
Re: physics acceleration
I've input -22 and -44 and both were incorrect. Thanks guys I'm still working at it
• Oct 9th 2012, 09:14 PM
MarkFL
Re: physics acceleration
If you are asked for the magnitude and direction, then the magnitude of the average acceleration is 44 m/s² and the direction is west.