6. (12 pts) A metallic box begins at rest at point 1 at a height of \(5R\) , where \(R\) is the radius of the circular part of the track. The box slides down the frictionless track and around the loop. If the box has a mass of \(8.0 \mathrm{kg}\) , what is the magnitude of the normal force acting on the box at point 2? 讲题 解题