[Physics] Finding car’s average acceleration?

accelerationhomework-and-exerciseskinematicstimevelocity

I am trying to calculate the car's average acceleration in (m/s^2) between 0 and 2.1 s. I need to express this answer using two significant figures, I have been trying the following to get it and it keeps telling me my answer is wrong, what am I doing wrong?

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a = (delta v)/(delta time)
a = (60 – 0) / (2.1)
a = 28.57

Best Answer

What is wrong is the units. You are doing

$$ \frac{(60 \mbox{ miles/hr})}{(2.1 \mbox{ sec})} = 28.57 \mbox{ mph/s } $$

What you need to do is convert the speed into $\mbox{m/s}$

$$ (60 \mbox{ miles/hr}) = ( \frac{1}{3600}60 \mbox{ miles/sec} ) = ( 1610 \frac{1}{3600} 60 \mbox{ meter/sec}) = 26.833 \mbox{ m/s} $$

Now do the acceleration as

$$ \frac{(26.833 \mbox{ meter/sec})}{(2.1 \mbox{ sec})} = ... \mbox{m/s}^2 $$