People Are Asking Why Do We Feel Acceleration, But Not Constant Speed?

As a science website, it’s easy to get bogged down in questions like “Is there life on Enceladus?” and “Where are all the aliens?”, when people are struggling with more basic (but fun!) questions like “Why can’t we power our cars with magnets?” and “If you fell from a skyscraper on the Moon, would you die or otherwise get badly injured?”

Another basic question people have been asking: why do we feel acceleration, but not velocity? One person recently effectively asked this in the “Physics Is Fun” Facebook group.

“While traveling in a flight which is flying at a height of 30000 feet [9,144 meters], we feel as if we are sitting stationary (unmoved),” the Facebook user wrote. “How come it gives us such a feeling when the aircraft is cruising at 700 [kilometers] per hour [435 miles per hour]?”

The short answer is that you only feel a force when a force is being applied to you. This only happens during acceleration, when the plane’s engines push the plane forward. You feel that push via the seat, as you are accelerating.

When you reach the cruising speed, however, force is not being applied to you via your seat. You are in a state of inertia, or the tendency for objects in motion to remain in motion in a straight line at the same velocity unless another force acts upon it.

            

Since the plane is remaining at the same speed, you carry on at that speed unless something acts upon you, like speeding up or slowing down of the aircraft.

An easy way to think about this is to imagine what would happen if the plane were to suddenly stop while you were not wearing your seatbelt as the flight attendant advised. The answer is that you would continue your journey forward that you’ve been set upon until you are rapidly decelerated to rest by (for example) the chair in front of you. If they were to swing a left or right, of course, you would feel the centrifugal force, which isn’t real, but let’s not complicate things for now.

If you were to feel speed and not velocity of a plane or car, that would be really, really weird, considering that the car is only going along at 110 kilometers per hour (68 miles per hour) while the planet orbits the Sun at 107,000 kilometers per hour (66,500 miles per hour), and the Sun hurtles through the galaxy at 828,000 kilometers per hour (514,500 miles per hour). Let’s just say it’s good thing that we only feel acceleration.

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