A small block is released from rest at a height of $3R$ on a smooth track. The track runs down and then into a vertical circular loop of radius $R$, as shown in the figure. The heights are measured from the bottom of the loop. If $m$ is the mass of the block, the normal force on it from the track at the top of the loop is

Fig. 1: A smooth track that starts at a height 3R and runs down into a vertical circular loop of radius R.