I was told today in a lecture that there is such a thing called an inertia force that occurs only when an object is accelerated by a force. I was also told it is equal in magnitude and opposite in direction to the force accelerating the object. Obviously you can feel this in a car - when you suddenly accelerate forwards, you feel a force pushing you into your seat. But if the force accelerating you and the car is equal and opposite (direction) to the inertia force pushing you into your seat, why don't these forces on your body cancel out?! Obviously they don't as you are still ultimately accelerating forwards with the car, but why is it that they don't cancel eachother out?