The Physics of a Curveball
The path of a baseball pitch mainly depends on the spin of the ball. Pitchers work for years to perfect the desired spin of each pitch. One of the most effective pitches in baseball is the curveball. To throw this pitch the pitcher must change their release angle and the movement of their hand. The desired spin on a curveball is achieved by a flick of the wrist, like turning a doorknob. When this spin
is achieved the air resistance takes over control. Because of the ball's spin, the seams gather air as the ball rotates. This causes the velocity to be different on either side of the ball. If the ball is spinning from right to left The velocity on the left side of the ball is much faster than the velocity on the right side of the ball. This air resistance causes the ball to be "pushed" to the batter's right. This movement happens throughout the ball's entire flight. Another term used in describing a good curveball is that it "drops off the table". This is because of the air resistance from the different velocities on the top and bottom of the ball. There is a higher velocity on the bottom side of the ball which causes
more stress underneath the ball. This allows the air flowing on the bottom of the ball to break away form the ball sooner. The opposite happens when at the top of the ball. The velocity is going much slower above the ball which causes the air to "hang onto" the ball for much longer. This causes the ball to do most of its curving in the last quarter of the pitch.
The Physics of a Yo-yo
The yo-yo is one of the most popular toys of all time. Although the yo-yo seems fairly simple, there are all kinds of scientific principles at work when a yo-yo is in action. The way a yo-yo works is dependant on the way the string is fixed to the middle of the yo-yo. The early yo-yos had the string tied to the axel. Newer yo-yos have the string looped around the axel allowing the yo-yo to spin freely when the string is all the way out. Before the yo-yo is released it has two kinds of potential energy. The first is the potential to fall for the point where the yo-yoist holds the toy. The second type of potential energy is from the spin that will be created from the string unwinding.
When the yo-yo is released the potential energy turns into kinetic energy. The first potential energy turns into linear kinetic energy. The second kind turns into rotational or angular momentum. When the yo-yo reaches the end of the string the head piece has relatively a lot of angular momentum. In the older model, the yo-yo will start climbing immediately. This is because of gyroscopic stability. Gyroscopic stability is when an object resists changes to its axis of rotation because an applied force moves along with the object itself. This causes the forces on the head of the yo-yo to balance and cause the yo-yo to start climbing up the string. When the axel is free spinning, in the newer design, a small tug is required to allow the yo-yo to start climbing. The new method allows for less friction giving it more kinetic energy.
One of the most popular skills a yo-yoist must know how to do is make the yo-yo "sleep". This is when the yo-yo keeps spinning at the end of the string. This allows the head to keep kinetic 
energy while the yo-yoist makes different shapes or does other tricks with the string. To keep a yo-yo asleep the toy must be thrown straight down and the head must have a lot of angular momentum. Yo-yo creators have been working for a long time to allow the yo-yo to sleep better. The easiest way to do this was to redistribute the weight to alter the moment of inertia. The increased moment of inertia means an increase in the yo-yo's resistance to changes in rotation, meaning the yo-yo can sleep for longer. Another approach to allow the yo-yo to sleep better was to reduce friction. This was accomplished by creating the looped string. Another way was to add bearings to the yo-yo. The approached makes it so that the string never touches the actual axis; it is strung along two tracks of bearings. The two tracks of bearings can either work in tandem to bring the yo-yo up or down or work separately to allow the yo-yo to sleep. This is done by a small tilt in the yo-yo changing the bearing's function.
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