Why does the disk of a spiral galaxy appear dark when viewed edge on?

Why is the energy that results from a roller coaster's position at the top of a hill referred to as potential energy?

On average 8 bananas make up one pound. the price is four pounds for 5.00. you must buy 25 bananas.what is your cost?

Two kilograms of air within a piston–cylinder assembly executes a carnot power cycle with maximum and minimum temperatures of 800 k and 300 k, respectively. the heat transfer to the air during the isothermal expansion is 60 kj. at the end of the isothermal expansion the volume is 0.4 m3. assume the ideal gas model for the air. determine the thermal efficiency, the volume at the beginning of the isothermal expansion, in m3, and the work during the adiabatic expansion, in kj.