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Newton's First Law

▶︎  Watch it animatedinteractive step-through · ~3 min · optional

Newton's first law: an object with zero net force on it keeps the velocity it has. A still object stays still; a moving object keeps moving in a straight line at constant speed. Motion does not require a force to continue. Changes in motion do. Reduce the net force to zero and any object coasts forever. Clearing friction and air resistance is not enough on its own, since gravity, tension, or a spring can still add up to a net force. What everyday surfaces hide is that motion is the default.

NET FORCE AND MOTION NO NET FORCE: CONSTANT VELOCITY ✓ frictionless ice puck normal weight v (constant) Net force = 0. The puck coasts at constant v. NET FORCE: VELOCITY CHANGES level surface cart applied F v (increasing) Net force points along the acceleration, not the velocity.
Left: the puck on frictionless ice has weight and the normal force balancing vertically and nothing acting horizontally. Net force zero, velocity constant. Right: the cart has an applied force, so the net force is non-zero and points along the cart's acceleration. Velocity happens to point the same way here, but it is the acceleration the net force tracks.
Rocket Lab · Open the sandbox →

Two habits do most of the damage. The first calls a moving object's leftover speed a force: a puck sliding across smooth ice keeps going with no horizontal force on it — not a "force of the push," not a "force of motion," nothing. The second lets the net-force arrow chase the velocity arrow. A ball at the top of a vertical throw has zero velocity, yet gravity still pulls it down; a projectile mid-arc, air resistance neglected, has a velocity tangent to its parabola, yet the net force points straight down. The first law fixes the rule: net force and velocity are independent vectors, and net force tracks the change in motion, not the motion itself.

The work

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Lesson
Newton's First Law ›

What Newton's first law says and what it does not. Inertia, how net force and velocity differ, and where they come apart most clearly: a ball at the apex of a throw, a projectile mid-arc, a puck on frictionless ice. Static and dynamic equilibrium. Closes with a ten-scenario skill check on the impetus trap and on net-force direction.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items covering the two main mistakes for Topic 2.4: the impetus reading that a moving object must have something pushing it forward (puck-on-ice and astronaut-in-deep-space style), and the trap of putting the net force along the velocity (ball-at-apex and projectile-mid-arc style). A few items probe the FBD habit of adding phantom forces. Take it cold to see what is still shaky, or after the lesson to confirm it is not.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the mistakes you've missed and drill it on its own. The round is adaptive: two correct in a row clears it and you move on.

Take the diagnostic to identify your misconceptions
Diagnostic first