How do earth get gravitational force?

The energy equivalent of the rest mass of an electron is

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.)

Derive the formula 2as=v² -u² where the formula have have usual meanings