What minimum force would you need to apply in order to do work against gravity without accelerating the skater? There is no friction and the skater starts at height 5m.

An insulated 40 ft^3 rigid tank contains air at 50 psia and 120°f. a valve connected to the tank is now opened, and air is allowed to escape until the pressure inside drops to 25 psia. the air temperature during this process is kept constant by an electric resistance heater placed in the tank. determine the electrical work done during this process.

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?

Achlorine atom has 17 protons and 18 neutrons. what is its mass number? what is its atomic number?