You lift a 6.5-kg backpack from a chair that is 0.47 m above the floor (hc) to a desk that is 0.75 m above the floor (hd). a. When the backpack is on the desk, what is the backpack-Earth system’s gravitational potential energy relative to the floor (GPE1)?
b. When the backpack is on the desk, what is the backpack-Earth system’s gravitational potential energy relative to the chair (GPE2)?
c. How much work was done by gravity as you lifted the backpack from the chair to the desk?

Because air contracts as it cools, the air pressure inside a freezer is typically lower than on the outside. why do ice cubes inside a freezer tend to shrink over time? the ice sublimes, and then the water vapor deposits as ice crystals on the sides of the freezer. the ice melts, and then the liquid freezes as ice crystals on the bottom of the freezer. the ice dissolves oxygen from the air, forming a denser crystalline matrix. the ice reacts chemically with carbon dioxide in the air, forming gaseous carbon compounds.