Which statement about how light travels is true? a light waves need a medium to travel, and they travel at the same speed even if the light source is moving. b light waves can travel in a vacuum and travel at a constant speed even if the light source is moving. c light waves can travel in a vacuum and will travel faster if the light source is moving forward. d light waves need a medium to travel, and they travel faster if the light source is moving forward.

As you are trying to move a heavy box of mass m, you realize that it is too heavy for you to lift by yourself. there is no one around to , so you attach an ideal pulley to the box and a massless rope to the ceiling, which you wrap around the pulley. you pull up on the rope to lift the box. use g for the magnitude of the acceleration due to gravity and neglect friction forces. once you have pulled hard enough to start the box moving upward, what is the magnitude f of the upward force you must apply to the rope to start raising the box with constant velocity? express the magnitude of the force in terms of m, the mass of the box.