Un cuerpo se desliza por una pendiente que forma un ángulo de 30° con la horizontal, sabiendo que no existe roce calcular su aceleración con que desciende aceleración de gravedad 10 m/s^2

The radiation of the fire causes the saucepan to heat up. the conduction of the saucepan then to distribute the heat over the saucepan and to give heat to the water. the convection of the water distributing the heat within the water, till all the water is boiling

Because they're in different places for this the temperature varies.