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Two forces applied by the child, allow the system to maintain balance and make the WhirlyGoRound spin. Her kinetic force in shifting of CM away from, and back to, the center of rotation maintains balance and exerts torque (rotational energy), combined with with the balance of centrifugal and centripetal forces as they hold on tight.
A WhirlyGoRound has three main parts, 1) A base and pedestal assembly, 2) a seat with a footrest and handles, and 3) a bearing assembly that allows the seat to rotate on the inclined pedestal. The bearing assembly reduces friction between the pedestal and the seat, allowing the seat to rotate independently of the pedestal with little force applied.
It all revolves around these two centers
The center of mass (CM) is the point at which an object's weight is balanced, also called the center of gravity. If the child’s CM is over the center of rotation (CR) that is, the center of the bearing under the seat and the point around which the seat revolves; there is balance and no rotation.
A child sits stationary on the WhirlyGoRound with her center of mass directly over the center of rotation, so she is balanced and does not rotate.
What happens next depends upon where her center of mass is.
When the child shifts her upper torso, leaning backward a bit, her CM shifts “uphill” and away from the center of rotation. This will begin to cause a rotation as her mass slides “downhill” toward the center of rotation. This initial rotation will be either clockwise or counter-clockwise depending on which way her mass “falls.” Gravity will continue to pull her mass toward the center of rotation.
This rotation is an example of gravitational potential energy (GPE).Because the seat is on an incline, there is GPE stored between the top and bottom of the incline.As the seat rotates and the child’s mass “falls” to the bottom of the incline, the stored GPE is released and becomes Rotational Energy (RE), resulting in rotation of the seat. This change occurs because of the law of conservation of energy.
If the child does not change the location of their CM, the seat would simply rotate 180°. BUT, if the child shifts their CM back toward the center of rotation by pulling herself forward with her core muscles and arms, the kinetic energy of this shift in CM will increase the angular momentum and the child will rotate another 180° completing a full 360° rotation because the force of shifting their CM creates torque increasing the velocity of the rotation.
In order to continue rotating, the child must again shift her CM away from the center of rotation, this force accelerates the rotation another 180°, then the child again shift her CM back toward the CR, accelerating for another 180°. The shift in CM away from CR creates an imbalance while the shift in CM towards CR restores balance.
Here’s the key factor propelling the child to spin: by continuing to shift her mass away from and towards the center of rotation, because of the incline, her mass is effectively raised and lowered, storing and releasing gravitational potential energy (GPE) with every rotation. The GPE, becomes rotational energy due to the law of conservation of energy.
As these shifts in CM and her rotations continue, the child develops an intuitive “feel” for the acceleration she can cause. Faster and shorter shifts away from and back to the CR, result in an increase in rotational velocity, or a rapid spinning of the child with less movement of their body and an increase in kinetic energy.
Spinning creates Centrifugal Force which pushes the CM away from the center of rotation in a direction that is tangential to the rotation. It is countered, or balanced by Centripetal Force, which is the force that keeps her center of mass on a circular path. The child applies this force as a combination of the child’s weight on the seat, grip on the handles and feet on the footrest. Because these two forces are balanced, the child spins with little effort.
DEFINITIONS OF TERMS
Friction is a force that holds back movement of a sliding object.
Center of Mass, also called Center of Gravity, is the point around which the object's mass is evenly distributed.
Center of Rotations is the point around which the object is rotated.
Potential Energy is the stored energy an object has because of its position or state (a kinetic energy waiting to happen).
Gravitational Potential Energy comes from the earth's gravity and is the energy stored in a object based on its height and mass.
Kinetic Energy is the energy and object has due to its motion.
Rotational Energy, or Angular Energy, is a type of kinetic energy whose motion is circular.
Velocity is the rate of motion in a specific direction.
Acceleration is a measure of how much the velocity of an object changes in a certain time (usually in one second).
Rotational Velocity is how fast the angular position or orientation of an object changes with time (or how fast it's spinning).
Torque is is a type of force that measures changes in the rotational speed of an object.
Law of Conservation of Energy says that energy is never created or destroyed, it is only changed from one state to another.
Momentum is a measurement of mass in motion. Any object that is moving has momentum. In physics, momentum of an object is equal to the mass times the velocity.
Centrifugal force is the energy of an moving object trying to go in a straight line when it is in a circular motion. When an object has rotational velocity they want to move in as straight a line as possible. For instance, when spinning, to move in a straight line away from the center of rotation.
Centripetal Force is the force that keeps the object moving in a circular path. It does this by pulling an object toward the center of a circle.
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