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### 3.6 Example 6

Two cars, A and B, start from the same place. A take the direction of the north with a constant velocity of 80 km/h, and B the direction of the east, with an acceleration of 4 m/s2 (start from the rest). After 13 seconds, which is the relative velocity of the car A in reference to B?

Solution with FisicaLab

Erase the content of the chalkboard. And add one element Mobile in X, one element Mobile in Y with constant velocity, one element Relative velocity, and one element Stationary reference system. As show the image below: We assume that the car A take the direction of the positive Y axis, and car B the direction of the positive X axis. And that both cars start from the origin. Now, as we know the end time, in the element Stationary reference system, write:

tf

13

To the element Mobile in Y with constant velocity, we call this car Car A, write:

Name

Car A

yi

0

yf

yf

ti

0

vy

80 @ km/h

To the element Mobile in X, we call this car Car B:

Name

Car B

ax

4

xi

0

vxi

0

ti

0

xf

xf

vxf

vxf

And to the element Relative velocity:

Object 1

Car A

Object 2

Car B

v

v

ang

ang

Where the magnitude and the angle of the relative velocity are unknown data. Now click in the icon Solve to get the answer:

```v = 56.549 m/s ;  ang = 156.861 degrees ;  xf = 338.000 m ;
vxf = 52.000 m/s ;  yf = 288.889 m ;
Status = success.

```

The desired data are v = 56.549 m/s and ang = 156.861 degrees.

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