Answer
a. Mary sees the box as motionless.
b. Chris sees the box accelerate backwards.
Work Step by Step
a. Mary sees the box remain motionless (by Newton’s law of inertia, since there are no forces acting on it) while the truck accelerates out from under the box. There is no friction, so there is no net force on the box and the box will not accelerate.
b. Chris, riding on the truck, is in a non-inertial frame of reference. He sees the box accelerate backward with respect to himself and his frame of reference.
