A pitcher throws a ball. It leaves the hand and keeps moving toward the plate. Nothing touches it from behind.
A common answer says the force of the throw stays with the ball and carries it. Newton's first law can judge that claim, and every claim like it.
This is a judging lesson. Each claim says that ongoing motion needs an ongoing force. Two facts from earlier lessons do all the judging.
The first fact is Newton's first law: an object keeps a constant velocity unless a nonzero net external force acts on it.
The second fact is what a force is: an interaction between two objects. Every real force names the object that exerts it.
Here is the first claim: the thrown ball keeps moving because the force of the throw stays with it.
Look for the exerting object. After the release, the hand no longer touches the ball, and no other object acts on the ball in the forward direction.
So no forward force acts on the flying ball. By Newton's first law it needs none: its velocity continues on its own. The claim is not correct.
One honesty note. A real surface always rubs a little, and real air resists a little, so a real glide is nearly constant, not perfectly constant. The claims are about what keeps the motion going, and the judging is the same.
A curling stone slides down the ice at a nearly constant velocity. Arlo says: a forward force must still be acting on the stone to keep it sliding. Is he correct?
Saskia says: after a bowling ball leaves her hand, no forward force acts on it, and it keeps moving down the lane anyway. Is she correct?
Now change one thing: a box slides over a floor and slows down. A new claim: friction from the floor acts backward on the box and changes its velocity.
The floor touches the box and exerts that force, and the box's velocity really is changing. This claim is correct.
That flip is the whole boundary. Keeping a velocity needs no force at all. Changing a velocity needs a net force. Judge every claim by which of the two it is about.
One more case. A car cruises at a steady 25 m/s on a flat road. The claim: the forward force on the car must be larger than the resistive forces, or the car would stop.
The car's velocity is constant, so the net force on it is zero. The forward force exactly balances the resistive forces. The claim is not correct.
The forward force on the cruising car is still needed, but it is needed to cancel friction and drag, not to maintain the velocity. With nothing to cancel, like the flying ball, no forward force is needed at all.
Nils kicks a soccer ball, and it rolls a long way over level grass. Nils says: the ball keeps rolling because the force of my kick stays in the ball. Is he correct?
A sled glides over level snow and very slowly loses speed. Judge this claim: the sled slows because the force that started it moving is running out.
Look for the exerting object. No object travels with the sled holding a stored force; a force exists only while two objects interact.
The sled's velocity is changing, so some net force must act on it.
The snow touches the sled, and friction from the snow acts backward on it.
So the sled slows because friction acts on it.
The claim is not correct: friction from the snow slows the sled; no starting force is stored in the sled.
A rider steps off a skateboard, and it coasts down the sidewalk, slowing. Which statement is correct?
An ice skater glides in a straight line at a nearly constant velocity. Which statement is correct?
A delivery van travels a flat, straight road at a steady 20 m/s. Which statement about the forces on the van is correct?