Inertia is a property of matter — specifically, its tendency to resist any change in velocity. An object with more mass has more inertia. It takes more force to get it moving, more force to stop it, and more force to change its direction.
In AP Physics 1, inertia is measured by mass. A 10 kg block has five times the inertia of a 2 kg block — meaning the same force produces one-fifth the acceleration. That's exactly what Newton's Second Law quantifies: a = ΣF / m.
If the net external force on a system is zero, the velocity of that system's center of mass remains constant. That's the whole law. Two scenarios both satisfy it:
A book sitting on a table. Net force is zero — normal force balances gravity. The book stays put because nothing is changing its velocity from zero.
A hockey puck gliding on frictionless ice. Net force is zero — nothing is pushing or slowing it. It keeps moving at the same speed in the same direction indefinitely.
Translational equilibrium is the condition where the vector sum of all forces on a system equals zero. The system is not necessarily at rest — it could be moving at constant velocity. The defining equation is:
This means every force in one direction is perfectly cancelled by forces in the opposite direction. The net force vector — when you add all the individual force vectors together — points nowhere, because they all cancel.
A 5 kg lamp hangs from the ceiling by a single cable. The lamp is at rest. Find the tension in the cable and state whether the system is in translational equilibrium.
The First Law applies independently in each dimension. A system can be in equilibrium horizontally — constant vₓ — while simultaneously accelerating vertically. The two axes don't talk to each other.
Adjust forces in each direction independently. Notice how a system can be in equilibrium in one direction while accelerating in another.
A 3 kg block is pushed horizontally at constant velocity across a rough floor while gravity and the normal force act vertically. Show that Newton's First Law applies in both directions, but for different reasons.
Newton's First Law only works when you observe it from an inertial reference frame — a frame that is itself not accelerating. In an inertial frame, an object with no net force truly moves at constant velocity (or stays at rest). If your frame is accelerating, you'll see objects appear to accelerate for no reason, which violates the First Law.
The classic non-inertial example: a passenger in a braking car feels themselves thrown forward, even though nothing is pushing them. From outside (inertial frame), the car decelerated but the passenger's inertia kept them moving forward — no mystery. From inside the braking car (non-inertial frame), it looks like an invisible force pushed them, which is why Newton's First Law seems to break down.