- #1

Eclair_de_XII

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- 91

## Homework Statement

"An automobile travels on a straight road for 40-km for 30 km/h. It then continues in the same direction for another 40-km at 60 km/h. (a) What is the average velocity of the car during the full 80 km trip? (Assume that it moves in the positive x direction.) (b) What is the average speed?"

##d_1=40 km##

##d_2=80 km##

##v_1=30 km/h##

##v_2=60 km/h##

## Homework Equations

##v=v_0+at##

##x-x_0=v_0t+\frac{1}{2}at^2##

##v^2 = v^2_0+2a(x-x_0)##

##x-x_0=\frac{1}{2}(v_0+v)t##

##x-x_0=vt-\frac{1}{2}at^2##

## The Attempt at a Solution

Basically, I am trying to solve for time, but I am getting conflicting answers. This is the method I am typically accustomed to:

##(\frac{1 h}{30 km})(40 km) = \frac{4}{3}h##

This is an entirely different result from what I get from equation four on the list.

##(40 km) - (0 km) = \frac{1}{2}(0+30km/h)t##

##80km=(t)30km/h##

##t=\frac{8}{3}h##

As a result, I cannot solve the two parts for this problem, which this book tells me, is 40 km/h for each one. Additionally, I feel the need for a confession... This was the same book I had earlier complained about not having the proper formulae. It was not that it didn't have them; it's just that I was looking in the wrong places. My professor skipped around the chapters for her curriculum. After I had dropped the class, I tried to follow that curriculum, but the formulae I needed were in the chapters she had skipped when I was still taking her class. Anyway, I don't understand why I'm getting such conflicting answers when I try to solve for time.