Your problem is that you are too much tied to your experience that tells you that you need force to keep something moving. But that was the ingenious approach by Newton to figure out what would happen if there is a single object in vacuum that does not interact with anything.

Your question with pushing an object is related to Newton 3. You, in fact, apply a force on the object that results in a force from the object to you. Since you are much heavier and you also can use additional force to keep you at rest, you will more or less stay in position. The object will then act with a force on the table. If not compensated by other forces such as friction on the ground, this results in the table to move. The same force acts back on the object, but with opposite direction. If the net forces acting on your object (your initial force and the force from the table on the object) do not balance, this results in an acceleration following Newton 2 (F=ma).

Note that at rest, the friction is much stronger. You have to apply a larger force to commence with moving the box, while you need much less force to keep it moving at constant velocity. The reason is the smaller friction of gliding objects.

By the way, box on a table is already quite complex. Consider instead a car at rest. In order to accelerate, the car acts via the wheels with a force on the earth in the opposite direction of where you want to go. To move forward, the force is backward, and vice versa. The earth acts with the same force on your car, but in the opposite direction. This pushes your car in the desired direction. In principle, earth gets accelerated, too, but this is negligible due to the ratio of masses.

Finally, I would like to add that Newton 1 yields a definition of a distinct class of reference frames. They do not rotate or accelerate with respect to each other, and all yield that an object is either at rest or moving with constant velocity in the absence of forces. Our earth is not such a frame, even in the absence of gravity, because it is rotating.

Look at it from a different perspective. Why should you assume an object will slow down? The main concept we use to describe an object’s motion is position. We call the rate of change(slope) of position velocity, and we call the rate of change of velocity acceleration. Suppose an object starts at v=0, and we apply a constant force on it to accelerate the object to v=10. So on a graph this looks like a line linearly increasing to 10 and then going straight. Now, if you assume you need a force to increase the velocity up to 10, why wouldn’t you need a force to decrease it? Starting at zero vs starting at v=10 will both be straight lines on a graph. The only way to change velocity is to accelerate it, which requires a force.

Just in case you are confusing velocity with acceleration (very common mistake). Net force = 0 simply means that the object is not changing in velocity. Force is defined as ma, so if f=a=0, there is 0 change in velocity. The problem isn’t that there’s something causing it to keep moving, it’s that nothing is stopping it from moving. You must have a force to change the velocity of an object. Also, all velocity is relative. You could say you’re going 10 m/s. But you could also say everything around you is going at -10 m/s, and you’re going at 0. So how do you stop? You slow down by applying a force so that you move at -10 m/s like everyone else. My point is, velocity being zero is just a convenient starting point, it holds no actual significance because we only care about its change. Whether something is moving or not moving is actually the exact same state. Your velocity is zero on a plane, even though you’re moving. Your velocity is zero on the earth, even though you’re on a rotating object.

Edit: To answer your question: assume the box is moving with some constant velocity v. There must be a net force to change it. That’s it. Force can only affect acceleration, or rate of change of velocity.

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