What happens when a constant force is exerted on a cart that is initially at rest on an air track?

Answer

The cart will start moving in the direction of the applied force.

Q: What are the laws governing motion? A: The laws governing motion are Newton's laws of motion.
Q: Which of Newton's laws applies to this situation? A: Newton's First Law of Motion.
Q: What is Newton's First Law of Motion? A: Newton's First Law of Motion states that an object at rest will remain at rest, and an object in motion will remain in uniform motion in a straight line, unless it is acted upon by a net external force.
Q: What is the net external force in this situation? A: The constant force that is being exerted on the cart.
Q: What will happen to the cart? A: The cart will start moving in the direction of the applied force.