Explain the affects of forces on motion.

A force acting on an object can cause many things to happen to the object, it might change shape, it might start moving, or it might have a chemical reaction. motion is slightly different depending on the planet itself due to gravity (another force) and and the atomic compound of the planet or even how fast it is moving (another form of motion)

hope this helps :)

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.