When a swimmer stands on a high diving board, he has 5800J of PE relative to the surface of the water.
What is his PE and KE at each of the following:
a. the instant he steps off the board
b. when he is halfway to the water
c. just before he hits the water

The radioactive isotope of lead, pb-209, decays at a rate proportional to the amount present at time t and has a half-life of 3.3 hours. if 1 gram of this isotope is present initially, how long will it take for 80% of the lead to decay? (round your answer to two decimal places.)