# Why Objects still move under the effect of fictitious force?

As I read in many books and websites, An inertial force is a force that resists a change in velocity of an object. It is equal to—and in the opposite direction of—an applied force.

If the inertial force (P) is equal to the applied force (F), then the net force (P+F) affecting the object is equal to 0. I wonder why object still move although the net force is 0. • Move relative to what reference system? – Qmechanic Oct 12 '16 at 16:56
• To over all system. For example: When I run in a floor, why do I still move although there is an inertial force affects me? – Đặng Minh Hiếu Oct 12 '16 at 17:01
• I really don't understand the question. – Gonenc Oct 12 '16 at 17:03
• I just edited the post In the picture, 2 forces are equal in magnitude, but have opposite direction. Then the total force is 0 Thank you! P/s: sorry I am a beginner, there may be some concept I misunderstand – Đặng Minh Hiếu Oct 12 '16 at 17:10
• There are a couple of misconceptions in your question. Have you checked the wikipedia site on fictitious forces? – Crimson Oct 12 '16 at 19:22

By inertial force I suppose you mean: $F_{inertial}\equiv ma$. In problems involving accelerating bodies, people do apply $-F_{inertial}$ to the body, so that the problem reduces to one of static equilibrium (because net force on the body would then be zero), and so perhaps helps them think better. It is only a ruse. You may think of $-F_{inertial}$ as the force that needs to be applied externally by some means if you wanted the body to achieve static equilibrium. If you have not applied such a counterbalancing force then the body cannot be in equilibrium, but will accelerate.