Imagine pushing on a crate initially at rest so that it begins to move along a floor.

(a) While you are setting the crate in motion, increasing its speed in the desired direction of travel, what is the direction of the vector sum of the forces exerted on it?

(b) Suppose you suddenly stop pushing and the crate slows to a stop. While the crate slows down, what is the direction of the vector sum of the forces exerted on it?

(c) What is the direction of the vector sum of the forces once the crate comes to rest?