You are riding on a roller coaster that starts from rest at a height of 25.0 m and moves along a frictionless track. however, after a bad storm some leaves settled on part of the track causing a 9.4-m length of the track to exert a frictional force of 625 n on the car. to safely make it around the loop, the 50-kg car must have a minimum speed of 7.7 m/s at the top of the loop (point b). how fast should the car be moving initially at point a to ensure that it reaches the top of the loop with the minimum required speed?