Atruck is traveling horizontally to the right. when the truck starts to slow down, the crate on the (frictionless) truck bed starts to slide. in what direction could the net force be on the crate?

a. straight up (the normal force).
b. horizontal and to the left.
c. straight down (because of gravity).
d. horizontal and to the right.
e. no direction. the net force is zero.

To solve this problem it is necessary to apply Newton's first law which warns that every body remains in continuous motion or at rest until an external force acts on it.

From the statement it is said that there is no friction on the crate, then the sum of forces in the horizontal direction will be zero. Here as the truck is slowing down, there is not net horizontal force on the crate, it means that the crate is at rest.

The correct answer is e. No direction. The net force is zero.

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