A puck gets one hit at an ice rink, then slides on its own. The ice is extremely slippery. The puck keeps going at a steady velocity, with nothing driving it forward.
What does it take to change a velocity, and what happens when nothing does?
You saw that an object tends to keep its current velocity. Newton's first law makes that tendency exact.
An object keeps a constant velocity unless a nonzero net external force acts on it.
Staying at rest counts: rest is a constant velocity of zero.
The word net matters. Forces may act and still leave the velocity alone, when they cancel.
A book resting on a table has two forces on it: its weight and the normal force from the table. The two balance, so the net force is zero, and the book's velocity stays at zero.
The gliding puck is the moving case. Friction from the slick ice is tiny, so the net force on the puck is close to zero, and its velocity stays as it is.
Almost no friction is not quite none. The real puck loses speed very slowly, because that tiny friction is still a force on it. On perfectly frictionless ice, it would keep its velocity exactly.
No forward force is needed to keep the puck going. A net force is needed only to change a velocity.
Deep in space, no net force acts on a drifting wrench. What does its motion do?
The net force on a parked motorcycle is zero. What does the motorcycle do next?
This whole statement is called Newton's first law.
Clara says: 'On ice with almost no friction, a moving puck would keep its velocity without any forward force.' Is Clara correct?
Far from any planet, a space probe coasts at 3 km/s with its engine off and no net force on it. What is its velocity an hour later?
Newton's first law: an object keeps a constant velocity unless a nonzero net external force acts on it.
No net force acts on the probe.
So its velocity does not change.
Still 3 km/s, in the same direction.
A parked trailer has zero net force on it, and it is at rest. What does Newton's first law say it will do?
Rest is a constant velocity of zero.
With zero net force, that velocity stays constant.
It stays at rest until a nonzero net force acts on it.
Which condition is needed for an object's velocity to change?
Far from any planet, a cargo capsule drifts at 8 m/s with no net force on it. What is its velocity one minute later?
A crate at rest has a 90 N force leftward and a 90 N force rightward on it, and nothing else horizontal. What does the crate do?