Inertia is an object’s resistance to changes in its state of motion (starting, stopping, speeding up, slowing down, or changing direction)

If no net external force acts, velocity stays constant (including zero velocity)

Motion does not require a continuous force; changing motion does

Mass quantifies inertia: larger mass → greater resistance to acceleration for the same net force

Described by velocity (speed + direction)

At rest is a special case of constant velocity (v = 0)

The vector sum of all external forces on the object

If net force = 0, forces may still be present but balanced

Balanced forces

Unbalanced forces

Inertial frame

Non-inertial frame

If initially at rest → stays at rest

If initially moving → continues in a straight line at constant speed

Velocity changes

Correction: force is needed to change motion; constant velocity requires no net force

Correction: there may be forces, but they can cancel; motion can persist without a forward net force

Correction: rest and motion depend on the chosen reference frame

Body tends to keep its forward velocity when the car slows rapidly

Seat belt provides an external force to change the passenger’s motion

Passenger’s body tends to remain at rest while the car moves forward

Seat exerts a forward force to accelerate the passenger

Quick pull reduces the time friction acts significantly on dishes

Dishes tend to remain at rest due to inertia

Low friction → small opposing force → puck maintains near-constant velocity longer

Rug moves suddenly; dust tends to remain in its prior state, separating from the rug

Opposes relative motion between surfaces

Often the reason objects stop in everyday life (net force not actually zero)

Opposes motion through air; increases with speed and shape

Can prevent constant-speed motion unless other forces balance it

Frequently balance for objects on a level surface (vertical forces cancel)

Horizontal motion can still change due to friction/drag

First Law describes the special case of zero net force (no acceleration)

Second Law generalizes: net force causes acceleration (∑F = m a)

First Law establishes inertial frames and the baseline behavior absent net force

Is it at rest, moving at constant velocity, or changing velocity?

Examples: gravity, normal force, tension, friction, drag, applied forces

If net force equals zero, conclude constant velocity

If not zero, expect acceleration (then use Newton’s Second Law)

Ask what real forces (friction/drag) might break the net force = 0 assumption