Final answer:
When the fan speed on a fan cart is reduced on a frictionless surface, the cart will decelerate because the force propelling it forward has lessened, but it will continue to move forward at a decreasing speed.
Step-by-step explanation:
If you change the fan speed from High to Low while the fan cart is moving forward on a frictionless surface, the fan cart will decelerate. This occurs because the force generated by the fan, which is pushing the cart forward, is reduced when the fan speed decreases. According to Newton's first law of motion, an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. In this case, reducing the fan speed decreases the thrust force, but since there is no friction to oppose the motion, the cart will continue to move forward, albeit at a decreasing speed until it reaches a new constant velocity or is acted upon by another force.