We use only one variable at a time to find the accurate result. We want to see how the result of experiment changes everytime with a single variable.
-- You've GOT the kinetic energy of the bullet.
-- That's the amount of work that has to be done to stop it.
-- work = (avg force) x (penetration) and they just gave you the average force.
-- Did you think it would be this easy ?
Answer:
The description is true because the plane has to be moving order for it to fly. :)
Refraction is caused by any wave crossing the boundary out of one substance and into another one. The two different substances may be no more than two parcels of air at different temperatures.
The sun is actually plasma but let us say that it is actually a Gas ball, as are all stars. All stars are essentially made out of gasses.