You are testing a new roller coaster ride in which a car of mass m moves around a vertical circle of radius r. in one test, the car starts at the bottom of the circle (point a) with initial kinetic energy ki. when the car reaches the top of the circle (point b), its kinetic energy is ki/4, and its gravitational potential energy has increased by ki/2. how much work was done on the car by the force of friction as it moved from point a to point b, in terms of m, g and r? what was the magnitude of the friction force (assumed to be constant throughout the motion), in terms of m and g?