Step-by-step explanation:
It is given that,
Radius of the circular road, r = 62 m
Speed of the car, v = 15 m/s
(a) We need to find the force that causes the centripetal acceleration of the briefcase when it is stationary relative to the car. The centripetal acceleration is given by :



The centripetal acceleration is caused due to static frictional acting along the curve of the road. The briefcase starts sliding when the centripetal force exceeds the frictional force.