Newton's First Law
Constant velocity is the default. Rest is just the special case where the constant happens to be zero. A net force is what changes velocity, not what maintains it. Friction and drag dominate everyday life so completely that motion looks like it needs to be renewed. Take those forces away and a puck on smooth ice, an astronaut in deep space, or a cart on a level track all do the same thing: they keep doing what they were already doing.
The trap is built into intuition. Anything moving must be being pushed, anything stopped can't have a force on it, and net force always points the way velocity points. None of these are true. What stays true is the bookkeeping: list every external force, sum them as vectors, and read off whether velocity is changing.
The work
3 ways in · any order
Lesson
Newton's first law
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Read once, do the worked examples, and finish on the ten-scenario applet at the end. Each scenario isolates one way Newton's first law can mislead the impetus instinct, with fix-the-mistake feedback on every wrong pick.
Diagnostic
10-item topic check
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Ten multiple-choice items mapped to the impetus, sign, and equilibrium-boundary failure modes. Take it cold to surface what's still shaky, or after the lesson to confirm the fix held.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you've missed and grind it on its own. The round is adaptive: two correct in a row clears the misconception and you move on.