Desde lo anto de un acantilado de 784 ft de altuta se deja caer una piedra. Dos segundos despues se lanza una piedra hacia abajo, de tal forma que las dos chocan simultáneamente contra el fondo del acantilado, ¿com qué velocidad inicial debe ser lanzada la segunda piedra?

you'd use the equation that establishes a relationship between  energy  and  wavelength, which looks like this

e=h⋅cλ, where

λ  - the wavelength of the photon; c  - the speed of light, usually given as  3⋅108m/s; h  -  planck's constant, equal to  6.626⋅10−34j s

plug the value you were given for the energy of the photon in the above equation to get the wavelength

e=h⋅cλ⇒λ=h⋅ce

λ=6.626⋅10−34js⋅3⋅108ms−1.257⋅10−24j=1.581⋅10−1m

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