A box rests on the (frictionless) bed of a truck. The truck driver starts the truck and accelerates forward. The box immediately starts to slide toward the rear of the truck bed. Discuss the motion of the box, in terms of Newton's laws, as seen (a) by Andrea standing on the ground beside the truck, and (b) by Jim who is riding on the truck (Fig. 4-27).
(a) Andrea, standing on the ground beside the truck, will see the box remain motionless while the truck accelerates out from under it. Since there is no friction, there is no net force on the box and it will not speed up.
(b) Jim, riding on the truck, will see the box appear to accelerate backwards with respect to his frame of reference, which is not inertial. (Jim better hold on, though; if the truck bed is frictionless, he too will slide off if he is just standing!)