During that final period of time,
his acceleration is
(9 m/s - 5 m/s) / (4 sec) = 1 m/s² .
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It decreases because it gave its momentum to the other car.
The basic relationship between frequency and wavelength for light (which is an electromagnetic wave) is

where c is the speed of light, f the frequency and

the wavelength of the wave.
Using

and

, we can find the value of the frequency:
Scientists use the color of a star to determine its temperature.
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Condensation endothermic or exotérmica