This may be a Newton's Law question, but not sure?

Question

What forces act on a frictionless air puck as it moves across a table at a constant speed in a straight line?

Answer 1

No forces act on it

Apart from the initial force that gave it motion in the first place, if it is frictionless and at constant speed then there are no forces acting on it. It will retain its kinetic energy for ever.

This is much like throwing on object in space - assuming it has no collisions it will move in a straight line for ever.

Answer 2

Gravity acts downward, a force of air pressure acts upward to cancel it out.

In Newtonian mechanics, "constant speed in a straight line" is the *state of nature*: it is "what nature would do anyway." Forces are our way of describing deviations from that expectation. So when someone says "it moves across a table at a constant speed in a straight line," they're saying that "the forces must all balance out; there must not be a net tendency for it to go one way or another."

However, we know that on Earth, things tend to fall. So we know that the gravitational force still exists. It must be balanced out by a force going up -- and that force must come from the cushion of air that it's riding on, because that's the only thing that it's touching.