Un niño lanza piedritas a la ventana de su amigo, que está a 4.5m de altura y a 5m de distancia horizontal desde la pared. Si quiere que las piedritas toquen la ventana sólamente con una velocidad horizontal (vy = 0), qué velocidad tendrían las piedritas al tocar la ventana?

If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.

Planck decided to assume that atoms could only vibrate at certain frequencies that were whole numbers eg 2f, 3f and so on. others used to think that atoms vibrate continuously

Athermometer to measure the temperature of course