Liquid helium is stored at its boiling-point temperature of 4.2 k in a spherical container (r = 0.30 m). the container is a perfect blackbody radiator. the container is surrounded by a spherical shield whose temperature is 74 k. a vacuum exists in the space between the container and the shield. the latent heat of vaporization for helium is 2.1 104 j/kg. what mass of liquid helium boils away through a venting valve in one hour?

Um vetor representa um deslocamento cujo módulo é 20 m. se v faz um ângulo de 60 graus com a horizontal, qual é o valor das componentes horizontal e vertical deste vetor?

Name two kinds of practical electric pumps.

Emmy kicks a soccer ball up at an angle of 45° over a level field. she watches the ball's trajectory and notices that it lands, two seconds after being kicked, about 20 m away to the north. assume that air resistance is negligible, and plot the horizontal and vertical components of the ball's velocity as a function of time. consider only the time that the ball is in the air, after being kicked but before landing. take "north" and "up" as the positive ‑ and ‑directions, respectively, and use ≈10 m/s2 for the downward acceleration due to gravity.