Xander's friend Corey has a skateboard that he rides at Newton's Skate Park. The combined mass of Corey and his
skateboard is 60 kg. At the top of his jump, the net force acting
on him is 30 Newtons. What is his acceleration at that moment?

im doing test rn im going with 2 but ill give answer if itss wrong,   : )

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.