Physics Help Forum Calculating average Acc at end of initial acc phase

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Oct 19th 2008, 04:40 PM #1 Junior Member   Join Date: Oct 2008 Posts: 2 Calculating average Acc at end of initial acc phase Alright so here is the question: Using the formula Vf=Vi+(a)(t), calculate the velocity of the cars at the end of their initial average acceleration phase. Assume the acceleration is the average one you calculated above. Answer in m/s and km/h. The average i calculated in the previous one is 4.4m/s or 15.8km/h The biggest problem i have is the fact that velocity is a speed with the amount of distance you have travelled calculated in there. Last edited by simplekin; Oct 19th 2008 at 04:57 PM.
Oct 19th 2008, 05:20 PM   #2
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 The average i calculated in the previous one is 4.4m/s or 15.8km/h
Acceleration has units of $\displaystyle \frac{m}{s^2}$. So if you got an answer with $\displaystyle \frac{m}{s}$ something is wrong.
Anyway, you don't give us what $\displaystyle v_i$ is. Is it $\displaystyle 0\frac{m}{s}$?
I think it would be nice if you could write down the whole problem...
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 Oct 19th 2008, 06:08 PM #3 Junior Member   Join Date: Oct 2008 Posts: 2 alright so here is question #1 Try to imagine cars starting up from a stop light. If you were to examine catrs as they start up, you may find that they seem to accelerate constantly for an average 20m in an average time of 3.0 s. Using the formula d=Vi(t)+ (1/2)at^2 question number 2 is this Using the formula Vf=Vi+at, calculate the velocity of the cars at the end of their initial average acceleration phase. Assume the acceleration is the average one you calculated above. Answer in m/s and km/h.
Oct 19th 2008, 07:13 PM   #4
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Ah ok! It was important because we know that $\displaystyle v_0=0$ and even more important : we know that the initial average acceleration phase lasts 3 seconds.
They say
 Using the formula Vf=Vi+at, calculate the velocity of the cars at the end of their initial average acceleration phase.
but I'd like to precise a bit more the formula into this one : $\displaystyle v_{final}=v_{initial}+a\cdot t_{final}=a \cdot (3s)=4.44\frac{m}{s^2}\cdot 3s=13.33\frac{m}{s}$ which is the answer.
In fact the problem just asked you to plug values into the given formulas.
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 Tags acc, average, calculating, end, initial, phase

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