# The Science of Sports

## Center #1 Basketball: Bouncing Balls Experiment

A collision can be thought of as two objects coming into contact. It can be a basketball and floor, or a bug and a windshield.

How collisions work can be explained by Newton’s Third Law of Motion. It states that “for every action, there is an equal and opposite reaction.” This basically means that in the case of a collision between two objects, both objects will experience the same amount of force, but in opposite directions. Take the bug and the car windshield for example. Which feels a greater force? Neither! They both feel the same force, it is just that that force has a much greater consequence for the bug.

So if both the basketball and the floor experience the same amount of force, why doesn’t the floor move? Newton’s Second Law of Motion explains this. It says that the amount of force that it takes to move something will depend on the mass (how much of an object there is) and the acceleration (the rate at which the velocity of the objects is changing). An amount of force that can easily move the small mass of the basketball can’t move the floor. However, if you dropped a bowling ball from about 12 feet up, the floor might shake!

One more step to understanding why balls bounce has to do with elasticity and inelasticity. Every object will deform, or change shape, when any force is applied (the deformation may be so small that you do not notice it!). The energy that it took to squish the object is stored in the object. If it is an elastic object, it will go back to its original shape, and release that stored energy. If it is an inelastic object, it will stay squished, and not release that energy. A basketball is pretty elastic; an egg is an example of something that is inelastic. Even though the egg changes shape when you squeeze it, the energy of your squeeze isn’t stored as energy that can be returned (elastic energy) [note: the energy IS stored in the egg, just in a different form – heat), and it doesn’t bounce back, it breaks!

When an elastic object like a basketball collides with a rigid surface like a floor, it is mashed a little flatter for a moment. An instant later, it snaps back into shape. When it snaps back, it pushes out and against the floor it collided with. That downward pushing action of the basketball on the floor is balanced by a reaction: the floor is pushing upwards on the basketball making it bounce upward, thanks to Newton’s Third Law. [This concept is a very hard one for people to get. If you want to give a similar, more familiar example, ask them to jump. Ask them why they fly into the air. They will invariably say because they pushed with their legs. But their push was not on them, and it was not upwards – tell them to do it again and notice that they are pushing downwards on the floor. By Newton’s Third Law, the reason they move up is because the floor pushes back up on them.]

## Center #2 Football: The Perfect Pass Experiment

Footballs aren’t round. It is tough to stand something up on a narrow point or edge (as demonstrated with the top and coin in the experiment). The top and coin probably kept tipping over until you spun them. A football that is thrown with little or no spin acts much the same way. It will tend to wobble, or even tumble end over end, and it won’t go very far.

However, if the football is spinning as it is thrown, it doesn’t tumble, and it wobbles much less, if at all. Like the top and the coin, if the football has enough spin, it seems strangely more balanced than it was before. Spinning it about its long axis helps to reduce the wobble, and increase the distance the ball is thrown. An axis is an imaginary line running through the object, much like a barbecue skewer, which the object rotates around. If it wobbles, the additional collisions with molecules of air slows it down. A well-thrown football cuts through the air well because it is long and thin and presents less surface for the air to bang into than a tumbling football does

You reduce the air resistance by having the ball travel through the air point first. The spin simply keeps it going point first rather than wobbling, and increasing the air resistance, and hence slowing down the ball.. Newton’s laws of Motion, specifically the first law, which is often stated simply as “objects at rest remain at rest, and objects in motion remain in motion, unless acted on by an outside force.”

## Center #3 Soccer: The Perfect Kick (Just for Kicks) Experiment

Momentum is a measure of how hard it is to slow something down, measured by the amount of stuff in motion (its mass) times how fast it is moving (its velocity). [Note: it is not energy]

If you are hitting or kicking a ball, and you want it to travel far, then you want to give it a lot of momentum. You can do that by increasing your momentum, and then transferring the momentum to the ball through a collision. You can increase your momentum by moving faster. It takes time to get your arm or hand or leg moving, so winding it back helps give you time to build up velocity. You can also increase momentum by increasing the mass in motion. That’s why soccer coaches often tell players to get all of their weight behind a kick for maximum power.

Increasing your momentum is important for hitting or kicking a ball because of the Law of Momentum Conservation that says “in an isolated system, momentum doesn’t just disappear. It can only be transferred from object to object” In a collision, the momentum lost by one object is gained by the other.

So in a collision like your foot smacking a soccer ball, you are moving momentum from your foot to the ball.

## Center #4 Baseball: Swinging Away Experiment

Newton’s Law of motion. The first law states that an object will keep going in a straight line at a constant speed unless it is acted on by an outside force. The ball keeps moving in one direction until the bat forces it to move in another direction. But when you bunt the ball, the bat acts more like a cushion. Instead of jumping off the bat, it just drops a short distance. [For the bunt, the bat is still changing the motion of the ball, so it is still an example of the first law.]

Newton’s First Law: you exert a force, you change the motion. In the case of the upward swing, the spin creates air pressure under the ball, causing it to rise. The opposite occurs when you use a downward strike, creating pressure over the top of the ball, forcing it down.