# Fictitious Forces ⇔ Constraint Forces? (re: D'Alembert's Principle)

Geremia
Are fictitious forces and constraint forces the same thing?

cmmcnamara
No, fictitious forces are forces which arise from analyzing within a non-inertial reference frame. Constraint forces are those which arise from a geometrical configuration.

For example, a constraint force would be perhaps a normal force exerted by the surface an object rests on such as an inclined plane problem. As long as the object is on the inclined plane, the object's motion is constrained to be along the inclined plane.

For a fictitious force consider that the Earth is in fact a rotating reference frame and therefore non-inertial. This gives rise to the Coriolis force which is needed to correct calculations due to this fact. It is small in effect but needed especially in large-distanced calculations.

Geremia
No, fictitious forces are forces which arise from analyzing within a non-inertial reference frame. Constraint forces are those which arise from a geometrical configuration.
Yes, but how can geometry cause forces? Isn't force a change in momentum? Doesn't force imply movement? If a force isn't doing work, how is it a force?
For example, a constraint force would be perhaps a normal force exerted by the surface an object rests on such as an inclined plane problem. As long as the object is on the inclined plane, the object's motion is constrained to be along the inclined plane.
Yes, but what causes the constraint force, if not inertial effects?

Homework Helper
Yes, but how can geometry cause forces? Isn't force a change in momentum? Doesn't force imply movement? If a force isn't doing work, how is it a force?Yes, but what causes the constraint force, if not inertial effects?
Constraint forces self-adjust so that they are exactly equal and opposite to the force they are opposing. So they do not do work and they do not cause a change in momentum. They balance other forces that would otherwise do work/cause change in momentum.

AM