The object you are targeting is the container.
If the container is sufficiently sealed to where the contents will not fly out, then the effective weight of the container is the combined weight of the container and the items.
If the container is not sealed and the items flew out when it was hurled, then the weight of the container would no longer be > 1 lb.
Keep in mind:
Weight = Mass x Acceleration_Due_To_Gravity
This indicates that weight is synonymous with downward force
Place 3 lbs in a 0.5 lb container. Now place the container on the scale. The scale will read "3.5 lbs" Picking up the container and moving it will require the same strength as picking up all individual items at the same time. So the effective weight of the container is 3.5 lbs.
For easy math, assume a car weighs 2,000 lbs. Now strap 4 adults inside with seatbelts and close all doors. If you put the car on a scale it would now read somewhere around 2,800 lbs. Launching the car suddenly would not cause the people to fly out, so it would still fly through the air with a weight of 2,800 lbs.
Now consider a surf board weighing 5 lbs. Place an adult male on it, and the surf boards effective weight is 205 lbs. Now hurl the surf board forward suddenly, and that person will fall off making the in-flight weight of the surf board 5 lbs.
If you hurled the container and the objects flew out mid-flight, what would happen? The wording of the spell indicates that the object is being magically propelled, so this indicates that once the object (i.e. the container) no longer weighs > 1 lb, it would fall to the ground, dead in its tracks.
I think the answer here is that yes, you can in fact hurl the container full of stuff but the instant enough weight flies out to drop below the 1 lb threshold, the container would fall to the ground. If it were sufficiently sealed, it could be argued that the contents could still potentially shift inside, causing the weight to drop below the threshold. If there is no way for the contents to shift, then it is pretty clear cut that the effective weight of the container is the weight of container + contents.