According to Newton’s first law (the principle of inertia), if there is no force exerted on an object, the object will move with constant velocity. Consider the following examples of motion. Is the velocity of the object constant? What are the forces acting on each object? In cases in which the velocity is constant, explain why Newton’s first law applies. 

(a) A hockey puck sliding on an ice-covered surface that is horizontal and friction less
(b) A mug of root beer sliding down a bar, whose surface is slippery but not friction less
(c) A car skidding on a flat, desert highway