if you exert a force of 500N to walk 4M up a flight of stairs in 4S , how much power do you use?

Answer 1

Power = (Force)*(Vertical distance)/Time
= 2000 J/4 s = 500 Watts

That's about 2/3 Horsepower. Pretty impressive.

Answer 2

work/time = 500 * 4 / 4 = 500 Watts