Which of the following involves an endothermic reaction? a. lighting a laboratory burner b. pens converting sunlight into chemical energy c. burning propane d. using a chemical hand warmer

A100-watt light bulb illuminates a solar cell. the electricity from the solar cell operates a water pump that delivers 1 watt of power. what is the efficiency of the system?

Which governments provide garbage collection services to homes and businesses

Two identical stars with mass m orbit around their center of mass. each orbit is circular and has radius r, so that the two stars are always on opposite sides of the circle. (a) find the gravitational force of one star on the other. (b) find the orbital speed of each star and the period of the orbit. (c) how much energy would be required to separate the two stars to infinity?