Fictitious forces

If a reference frame is non-inertial, then fictitious forces may be invoked in order to explain events that occur inside it. Such "forces" are not caused by pushes or pulls on objects in the frame, but by an acceleration of the frame itself. The objects continue to move inertially; they do not accelerate relative to a stationary observer, although they would appear to accelerate according to an observer attached to the accelerating frame. Think of a helium balloon floating in an elevator. When the elevator accelerates upward, the balloon accelerates toward the floor. To an observer inside the elevator, it would appear that a force is pushing the balloon downwards, when in fact the balloon is at rest. The "force" that the elevator passenger observes is therefore fictitious.

A familiar example of a fictitious force is the "force" that pushes you forward when the driver of the car you are sitting in slams on the brakes. We are accustomed to thinking of the car as an inertial reference frame, since its walls do not normally move relative to the passengers; this often leads us to the erroneous conclusion that this acceleration is caused by a force pushing the objects inside the car forward. However, the real force is the force over the road causing the car (and objects attached to it) to accelerate backward. An object that is partially attached (such as seatbelted passenger) will experience strain due to different parts of their body experiencing different amounts of force. Do not be lulled into a sense of complacency, however: while the "force" pushing your body forward is fictitious, the force of the decelerating car striking your forehead is quite real and painful.

Even the Earth is not a true inertial reference frame, because of its rotational acceleration. This rotation provides every object on the Earth's surface with a certain inertia which would carry it tangentially into space if not for gravity (or some other force) holding it down. As a result of this inertia, a person who jumps at the equator will be able to jump slightly higher'—'and will be a few tenths of a percent lighter'—'compared to a person at one of the poles. When viewed from a rotating reference frame, this effect will appear as an added "centrifugal force" pushing the person off the ground, even though in fact it is only their greater inertia carrying them. We do not normally notice this fictitious force, because it is small in relation to our weights. However, if the Earth were many times lighter or rotating many times faster, we would certainly notice. The European Space Agency takes advantage of this effect by launching its rockets from French Guiana, near the equator where this rotational effect is largest.

Another fictitous force is the Coriolis effect which produces cyclonic winds in the Earth's atmosphere due to different accelerations at different latitudes. Since the Earth's (eastward) rotation is greatest near the equator, a northbound wind from the equator will travel east relative to slower-moving air masses it encounters on the way. Likewise a southbound wind in the northern hemisphere will fall behind and drift westward. As a result, air will not merely fall into a region of low pressure, but will be deflected into a counter-clockwise circular path. In the southern hemisphere, the direction of deflection is clockwise. It is a common misconception that the Coriolis effect is responsible for the direction of spiraling drains

Fictitious forces are important to general relativity.