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 :
![a=(v^2)/(r)](https://img.qammunity.org/2020/formulas/physics/middle-school/4mqosu85eg8s8aua5nxyoiszib1as2oxxa.png)
![a=((15)^2)/(62)](https://img.qammunity.org/2020/formulas/physics/college/acs1c18qx3h3hot2gdtl5dyo498ang6ft9.png)
![a=3.62\ m/s^2](https://img.qammunity.org/2020/formulas/physics/college/24gjdt650cqecejutqhl0w7r07ax96n0jb.png)
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.