Read Microsoft Word - Ball on a Stick Support Notes.doc text version

Ball on a Stick exhibit

get it moving. Heavy things need more force to start moving, so they have more inertia. As you try to balance the ball and stick on your hand, it falls and rotates around your hand. Things that rotate have inertia like other objects. When the ball is up in the air, most of the mass (the ball) is far from your hand (point of rotation). In this position, it has more inertia and rotates more slowly. This gives you more time to use your eye-hand coordination and adjust your hand to save the ball from falling. Want to know more about rotational inertia?

Exhibit message Inertia is the property of an object that resists any change in its state of rotation. If at rest, it tends to remain at rest; if rotating, it tends to remain rotating and will continue to do so unless interrupted. In this exhibit, most of the mass (the ball) is far from your hand (point of rotation). In this position, it has more inertia and rotates more slowly. It is easier to catch when falling. Quick fact Balancing these sticks would be much easier on the moon! Its gravity is only one-sixth as strong as the earth and the rods earth. Balancing the rods would be much easier on the moon. Graphic panel text Is it easier to balance the stick when the ball is in your hand, or up in the air? Try it with your eyes closed or on your fingertip. If something is sitting still, you need to use a force such as a push or a pull to

Which is easier to balance on your finger--a stationary soccer ball or a soccer ball which is spinning? Try it? Now spin a rugby ball on its side as hard as you can! It will start to pin on its side and, after a few turns, will stand up and spin on its end! It is easier to spin the rugby ball on its end (long axis) because its mass is distributed further away from it axis of spin. It is more efficient use of the ball's energy to lift up onto its end and spin, than to stay spinning on its side. A rugby ball can be kicked further it spins around its long axis instead of its short axis. Extra for experts Balance in people is maintained through a number of mechanisms which normally work in harmony to keep us informed about our position and any changes to our orientation. The brain can then send messages to muscles to counteract any change in position in order to maintain balance. Once we have learned to walk, this happens automatically. The main organ of balance is within the inner ear, but this is supplemented by

information from our eyes, the degree of contraction of muscles supporting us, and pressure on the surface of our bodies, particularly the soles of the feet.

Friction causes the fluid to move with the tubes. As soon as your head stops spinning, the canals themselves stop, but the endolymph continues to flow due to inertia. The cupula sensory hairs are bent in the opposite direction, and continue to send signals to the brain indicating motion. The other motion detectors, such as the eyes sense that motion has stopped and send conflicting signals to the brain. The result is dizziness! The semicircular canals signal the need for muscular adjustments to the brain before over balancing occurs. This allows the brain to contract or relax muscles and avoid falling over. The cerebellum is the area of the brain responsible for maintaining balance.

Within the inner ear, embedded in the skull are three cavities ­ the semi circular canals which are only about as big as your thumbnail. They contain a fluid called endolymph and are all connected together to allow the fluid to flow between them. The three canals are at right angles to one another and they work together to inform the brain about different types of motion. The semicircular canals are specifically designed to inform about when the head starts or stops turning. They are generally not responsible for maintaining balance. Inside the cavities are sensory hairs embedded in flaps called the cupulae (singular cupula). When the head moves, the fluid initially stays put due to inertia, bends the cupula and the sensory hairs. This in turn sends a signal along the vestibular nerve to the brain, giving an indication of which way the head is moving. The signals return to `resting' levels if the head turns for around twenty seconds.

When you spin, your eye muscles work hard. They stay focused on one object as your body moves round, then quickly move to focus on another object as the first one goes out of view. The eyes act to `stop the world from turning'. People who twirl round a great deal ­ like ballerinas, use rapid head movements to enable them to focus their eyes on stationary objects for more extended periods. Through training, they are also able to take more notice of the signals coming from the eyes, and less from the semicircular canals. That way they can continue to spin without feeling the effects!

Classroom activity Seeing and balancing Remove any shoes you may be wearing. Stand one foot on a hard surface (such as a concrete or wooden floor) and maintain your balance while keeping your eyes open. How long can you maintain your balance? Repeat this experiment standing on a soft surface (such as sand or grass). Test other students and graph the results. Receptors in our feet sense when we are starting to topple, but this is more difficult to do when the surface is soft. Repeat these experiments while closing your eyes. How long can you maintain you balance on hard or soft surfaces with your eyes closed? Test other students and graph the results. Your eyes play an important role in helping you to maintain balance; they provide visual clues to your brain as you begin to topple, enabling you to correct your position and remain upright. The great tinned food race. Find two cylindrical food tines of the same size--one containing liquid food such as milk or soup, the other containing solid food such as fish. Lay both cans on their side and let them roll down a slope. Which tin reaches the bottom first--the liquid food tin or the solid food tin? The liquid tin rolls faster because the liquid inside does not have to rotate with the tin--it can easily move forward in the direction of the slope. More of its potential energy is used to move in a linear direction. The solid food tin rolls more slowly because the solids inside rotate with the tine. Some of its potential energy is used to start the solids rolling, leaving less energy for moving the tin in a linear direction. Further information More in-depth science about this exhibit m/Balance.htm Dizziness ml Balance and Health Dizziness and Motion Sickness Newton's laws of motion s/newtlaws/newtltoc.html


Microsoft Word - Ball on a Stick Support Notes.doc

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