Flying West or East. Why is one faster?

In his Minute Physics video, Henry Reich looks at the somewhat paradoxical question of why is it faster to fly west than east. Given that the Earth rotates from west to east, we expect that a plane flying west would get to its destination faster as their destination is moving towards them. Think the person or place you are running to is also moving towards you–the relative speed increases. This does not happen. Instead, planes take longer traveling west along the same route.

Vector Addition

The speeds add when the plane travels to the East (left) but they subtract when the plane travels West (right).

The answer to this question lies in the Earth’s rotation. Reich explains that as the Earth rotates, everything rotates along with it at a speed of 1180 km/h along the Equator. This includes the objects on the ground, as well as the air above. A plane generates lift by moving relative to the air around it, and most planes can travel anywhere between 500 to 900 km/h. This means that an airplane traveling East, along the Earth’s rotation, will travel faster because their speeds add. In this case, the plane is traveling 1180 km/h + 500 km/h = 1680 km/h relative to the ground. If the plane travels in the opposite direction, the speed relative to the ground is less because we subtract the speeds — 1180 km/h – 500 km/h = 680 km/h. We do this because we must take the directions that the Earth and the planes move. In Physics, we know this as vector addition.

Of course, that is the simple answer, and as Reich illustrates in his video, there is far more to the answer. The fact that the air moves, either pushing a plane along or pushing against it, has everything to do with the Earth’s rotation. The prevailing direction of these winds or the jet stream is due to the Coriolis Effect, the apparent deflection of the winds is due to the fact that the Earth rotates. This means that airplane travel times are influenced by the Earth’s rotation. The reason is not as straightforward as we would like.