You are designing a new roller-coaster. The main feature of this particular design is a vertical circular loop. At the top of the loop, riders will feel like they are weightless when they are completely upside-down. Explain why such an effect is independent of the mass of the rider. Compute the radius of the loop required for this effect to occur if the coaster start at rest a height of 80m above the ground, speeds up as it descends to ground level, and then enters the loop.(You can assume that the coaster rolls on the track without friction).