Homework Help: Calculate the average velocity of the car

1. Nov 14, 2015

David112234

1. The problem statement, all variables and given/known data

A car is stopped at a traffic light. It then travels along a straight road so that its distance from the light is given by x(t)=bt2−ct3, where b = 3.00 m/s2 and c = 0.130 m/s3 .

Calculate the average velocity of the car for the time interval t= 0 to t= 10.0 s

2. Relevant equations

vav = V2-V1/T2-T1

3. The attempt at a solution

x(t) = 3.00 t^2 - .130 t^3
differentiation \/
v(t) = 6.00t - .39 t^2
a(t) = 6.00 - .78t

V(0) = 6.00 (0) - .39 (0)
v(10) = 6.00 (10) - .39 (10)^2
= 60.0 - 39.
= 21

21-0/ 10-0
21/10
2.1

it tells me its wrong, why?

2. Nov 14, 2015

Astronuc

Staff Emeritus
What are the units of (V2-V1)/(T2-T1) or of velocity/time?

What are the units of distance divided by time?

What are the units of speed or velocity?

3. Nov 14, 2015

BvU

The average velocity is not the same as the change in velocity divided by time (because that is the average acceleration).

4. Nov 14, 2015

David112234

oh right, I forgot, so I tried dividing it my 2 instead of the time interval and i get 10.5, not the answer either

5. Nov 15, 2015

BvU

That's because the velocity doesn't depend linearly on time: $\Delta v\over 2$ doesn't work then.
So all you have to go by in this exercise is the definition of average velocity: $$<\vec v> \equiv {\Delta \vec x \over \Delta t}$$ and with the given information it turns out to be a fairly easy exercise

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