Constant velocity is motion with unchanging speed and direction, meaning zero acceleration. By Newton's first law, a system moves at constant velocity only when the net force on it is zero (translational equilibrium), the condition described in AP Physics 1 learning objective 2.4.A.
Constant velocity means an object's speed and its direction both stay the same. The object covers equal displacements in equal time intervals, and its acceleration is zero. That second part is the physics payoff. If acceleration is zero, Newton's second law tells you the net force must be zero too.
The CED frames this through translational equilibrium, a configuration where all the forces on a system add up (as vectors) to zero: ∑Fᵢ = 0. Newton's first law says that when the net force on a system is zero, its velocity stays constant. Notice that "constant velocity" includes sitting still. An object at rest has a constant velocity of zero, so the same equilibrium condition covers both cases. One more wrinkle the exam loves: forces can balance in one dimension but not another. A box pushed across a floor can have balanced vertical forces (normal force cancels gravity) while an unbalanced horizontal force changes only its horizontal velocity. Velocity changes only in the direction of the unbalanced force.
Constant velocity lives in Unit 2 (Force and Translational Dynamics) and directly supports learning objective 2.4.A, which asks you to describe the conditions under which a system's velocity remains constant. It's the observable signature of equilibrium. When you see "moving at constant velocity" in a problem stem, you should immediately translate it to "net force = 0" and start balancing forces. The concept also anchors Topic 2.6, since Newton's second law (a = F_net/m) reduces to the first law when F_net = 0. The CED also ties constant velocity to inertial reference frames, which are defined as frames where an observer would verify Newton's first law. So this one term connects equilibrium, dynamics, and the frames physics is allowed to be done in.
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Newton's First Law and Translational Equilibrium (Unit 2)
This is the home base. Newton's first law states that zero net force means constant velocity, and translational equilibrium (∑Fᵢ = 0) is the force configuration that makes it happen. Constant velocity is what equilibrium looks like when you watch the object move.
Newton's Second Law (Unit 2)
The first law is really the second law's special case. Plug F_net = 0 into a = F_net/m and you get a = 0, which means velocity doesn't change. On the exam, "constant velocity" is your green light to set the net force equation equal to zero and solve.
Acceleration (Unit 1)
Constant velocity and acceleration are opposites by definition. Acceleration is the rate of change of velocity, so constant velocity means acceleration is exactly zero. On a velocity-time graph, constant velocity is a flat horizontal line; on a position-time graph, it's a straight line with constant slope.
Air Resistance and Terminal Velocity (Unit 2)
Terminal velocity is constant velocity in disguise. A falling object speeds up until air resistance grows large enough to balance gravity, the net force hits zero, and the object falls at constant velocity from then on. It's a classic exam setup for applying equilibrium to a moving object.
No released FRQ uses "constant velocity" as a term to define, but the phrase shows up constantly in problem stems, and it's doing real work every time. In MCQs, "a block slides at constant velocity" tells you friction exactly balances the applied force, so you can solve for the coefficient of friction or an unknown push. In FRQs, you'll justify claims like "the net force is zero because the velocity is constant" using Newton's first law, or draw a free-body diagram where the force vectors visibly balance. Watch for the dimensional trap the CED calls out directly. Forces can balance in one direction and not the other, so a projectile or a cart on a track can have constant velocity in x while accelerating in y. Read which direction the question is asking about before you balance anything.
Speed is a scalar; velocity is a vector with direction. Constant speed only means the speedometer reading doesn't change, while constant velocity requires the direction to stay fixed too. A car rounding a curve at a steady 30 m/s has constant speed but NOT constant velocity, because its direction is changing. That means it's accelerating and a nonzero net force (centripetal) acts on it. Mixing these up is one of the most reliable wrong-answer traps in Unit 2 multiple choice.
Constant velocity means both speed and direction are unchanging, so acceleration is zero.
By Newton's first law, a system moves at constant velocity if and only if the net force on it is zero, a condition called translational equilibrium (∑Fᵢ = 0).
An object at rest counts as constant velocity (velocity is constantly zero), so "at rest" and "moving at constant velocity" set up the exact same equilibrium equations.
Forces can balance in one dimension but not another, and velocity changes only in the direction of the unbalanced force.
Constant speed is not the same as constant velocity; an object turning at steady speed is accelerating because its direction changes.
An inertial reference frame is one where Newton's first law holds, meaning observers in that frame see zero net force produce constant velocity.
Constant velocity is motion with unchanging speed and unchanging direction, which means zero acceleration. Per learning objective 2.4.A, it happens exactly when the net force on a system is zero, a state called translational equilibrium.
No. It means the net force is zero, not that individual forces are absent. A book sliding at constant velocity still has gravity, a normal force, friction, and a push acting on it; they just cancel out as vectors (∑Fᵢ = 0).
No. Constant speed only fixes the magnitude, while constant velocity also fixes the direction. A car circling a track at a steady 30 m/s has constant speed but changing velocity, so it's accelerating and experiences a nonzero net force.
Yes. Rest is just constant velocity equal to zero, so the same condition applies: net force is zero. That's why AP problems treat "at rest" and "moving at constant velocity" identically when you set up force equations.
Yes, and the CED says so explicitly. Forces may be balanced in one dimension but unbalanced in another, and velocity changes only along the unbalanced direction. A projectile is the classic case, with constant horizontal velocity and vertical acceleration from gravity.