A nonconservative force is a force whose work depends on the path taken, so it cannot be assigned a potential energy function; it changes the total mechanical energy of a system, typically converting kinetic energy into thermal energy (friction, drag) or adding energy (applied forces, motors).
A nonconservative force is any force whose work depends on the path the object takes, not just where it starts and ends. Drag friction is the classic example. Slide a box straight across the floor versus zig-zagging it to the same spot, and friction does more negative work on the longer path. Because the work is path-dependent, you can't define a potential energy function for a nonconservative force. There's no "friction potential energy."
The practical consequence is the big one for AP Physics C: nonconservative forces change the mechanical energy of a system. Friction and drag drain kinetic energy and convert it to thermal energy. An applied force (like a motor or a push) can pump mechanical energy in. The bookkeeping equation you'll use constantly is W_nc = ΔE_mech = ΔK + ΔU. If the only forces doing work are conservative, mechanical energy is conserved. The moment friction, drag, or an external push shows up, you account for its work explicitly.
This term lives in Topic 3.3 (Conservation of Energy) in Unit 3 of AP Physics C: Mechanics. The entire logic of energy conservation hinges on sorting forces into two bins. Conservative forces get folded into potential energy terms, and nonconservative forces show up as work that changes the total. If you misclassify a force, your energy equation is wrong before you write a single number. Block-on-incline-with-friction, projectiles with air resistance, and "how much energy was dissipated?" questions all depend on recognizing nonconservative forces and tracking their work. It's also the bridge between energy methods and the real world, since real systems lose mechanical energy to heat, and this is the concept that explains where it goes.
Keep studying AP Physics C: Mechanics Unit 3
Conservative Force (Unit 3)
These two are a matched pair that splits every force in mechanics. Conservative forces (gravity, springs) do path-independent work and earn a potential energy function. Nonconservative forces don't qualify, so their work has to be tracked by hand in your energy equation.
Friction (Units 2-3)
Kinetic friction is the poster child of nonconservative forces. It always opposes sliding, so it does negative work on every leg of the trip. Even if an object returns to its starting point, friction's total work isn't zero, which is exactly the path-dependence test failing.
Thermal Energy (Unit 3)
Mechanical energy lost to friction or drag doesn't vanish; it becomes thermal energy. When an FRQ asks how much energy was "dissipated," it's asking for the magnitude of the negative work done by nonconservative forces.
Work-Energy Theorem (Unit 3)
The work-energy theorem (W_net = ΔK) counts work from ALL forces, conservative or not. The conservation-of-energy form just reorganizes it, moving conservative work into ΔU and leaving W_nc on its own. Same physics, two bookkeeping systems.
Nonconservative forces show up two main ways. In multiple choice, expect conceptual stems like "which of the following forces is nonconservative?" or graphs where mechanical energy decreases over time and you have to explain why. In free response, energy-conservation problems with friction or air resistance are a staple. You'll write W_nc = ΔK + ΔU, solve for a final speed or a stopping distance, or calculate the energy converted to thermal energy. A common FRQ move is giving you a track with one rough section and asking where the object finally stops. Watch for the justification trap too. You can only say "mechanical energy is conserved" after stating that no nonconservative forces do work on the system. Skipping that line costs points on derivation and reasoning parts.
Run the round-trip test. A conservative force does zero total work over any closed path, so you can define potential energy for it (gravity, ideal springs). A nonconservative force fails this test. Drag a box in a circle back to where it started, and friction has done negative work the whole way. That nonzero round-trip work is why no potential energy function exists for friction, drag, tension, or applied pushes.
A nonconservative force does work that depends on the path taken, so it cannot be represented by a potential energy function.
Friction, air drag, tension, and applied forces are nonconservative; gravity and ideal spring forces are conservative.
The work done by nonconservative forces equals the change in mechanical energy: W_nc = ΔK + ΔU.
Mechanical energy is only conserved when no nonconservative forces do work, and you need to state that condition to justify using conservation of energy.
Energy "lost" to friction or drag isn't destroyed; it's converted to thermal energy, and its amount equals the magnitude of the nonconservative work.
The round-trip test identifies nonconservative forces: if total work over a closed path isn't zero, the force is nonconservative.
It's a force whose work depends on the path an object takes, like friction or air drag. Because its work is path-dependent, it has no potential energy function, and it changes the system's mechanical energy according to W_nc = ΔK + ΔU.
Yes. Kinetic friction always opposes motion, so it does negative work on every segment of a path. Its round-trip work is never zero, which is the defining failure of the conservative-force test.
No. Total energy is always conserved. Nonconservative forces like friction convert mechanical energy into thermal energy, so the mechanical total drops while the universe's total stays the same. Exam answers should say energy is 'converted' or 'dissipated,' never 'destroyed.'
A conservative force's work depends only on start and end points, so it gets a potential energy function (gravity gives you U = mgh, springs give U = ½kx²). A nonconservative force's work depends on the path, so you have to compute and track its work separately in energy equations.
Generally yes, tension is classified as nonconservative because it has no potential energy function. In many problems, though, tension does zero work (it acts perpendicular to motion, as in a pendulum), so it doesn't change mechanical energy even though it's nonconservative.