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Name: Carol
Status: educator
Grade: 9-12
Location: IL
Country: USA
Date: Summer 2011



Question:
Why do we feel dizzy when we ride on a merry go round that goes pretty slowly, but we feel nothing while riding on the earth that travels in a circle at about 1,000 miles an hour?

Replies:
Hi Carol,

As a pilot who flies aerobatics, I deal with motion sickness sometimes. From my understanding, it has to do with a disconnect between what your body is feeling and what your eyes are seeing. On a merry-go-round, your inner ear (where our sense of balance comes from) tells our brain that we are moving, by sensing an apparent force tending to pull you away from the center of rotation. But if you are looking at your other friends on the merry-go-round, they are not moving relative to you and so your vision is not sensing relative motion. These senses conflict and ultimately lead to a sense of disorientation and other symptoms of motion sickness.

So it is understandable then why we do not get motion sickness just standing on the earth, regardless of speed. We do not visually sense any relative motion. Also, the pull of gravity on us is constant (by the way, the apparent centrifugal force from the rotation of the earth is also constant and in fact negligible compared to the acceleration of gravity). Our inner ear then is only telling us that we're being pulled by gravity as normal but not accelerating in any direction, which agrees with our visual information. Hence our senses are all in agreement and no motion sickness.

Regards,

John C Strong


Carol,

Dizziness is more from rate of rotation (revolutions per second) than from speed (meters per second). On the earth, we rotate once per day. On a moderate merry-go-round, five revolutions per minute is common.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


On Earth your head and inner ear are in a state of balance between gravity and inertia. The fluid in you inner ear rests at its usual position so you do not feel any acceleration as you spin with the earth.

On a merry go round there is no force pulling your head sideways. The fluids in you inner ear are pushed to the outside of the turn and you know you are spinning, even with your eyes closed.

For the same reason if you are on a jet plane approaching the airport with your eyes closed, you cannot feel the plane turning. The pilot tips the wings so that the forces on your body are balanced and you cannot detect the turn with any senses but your eyes,(assuming a good pilot and smooth air).

Hope this helps.

R. W. "Bob" Avakian
Instructor
Arts and Sciences/CRC
Oklahoma State Univ. Inst. of Technology


Carol, the key factors you feel are angular velocity and acceleration (centripetal force), not the tangential linear velocity that your question includes. Angular velocity is how fast you rotate and acceleration relates to how much change in movement you feel.

Your angular velocity on even a slow merry go round (1 revolution per minute) is still much, much higher than on the rotating earth (1 revolution per day). And, the acceleration you feel on a merry go round (a radius of a few feet) is much higher than on earth (with a radius of thousands of miles). The linear velocity you feel is relative -- if you travel at the same speed as another object, it does not feel like you are moving. Such is the case with you an the earth -- you both move along at basically the same speed, and so you don't perceive the motion.

Specifically, there are little canals in the ears that influence your sense of balance, and respond to the rotation of your head -- and rotating faster (which is not the same as moving linearly) affects that balance.

Hope this helps,
Burr Zimmermann



I found a detailed explanation of this process from a reliable source. I am passing this along to you rather than recreating the same explanation. Go to

http://www.madsci.org/posts/archives/1999-10/939275753.Ns.r.html

for a detailed explanation.

Vince Calder


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