The more pressure a basketball has inside it, the less its surface dents during a bounce and the more of its original energy it stores in the compressed air. Air stores and returns energy relatively efficiently during a rapid bounce, so the pressurized ball bounces high. But an underinflated ball dents deeply and its skin flexes inefficiently. Much of the ball's original energy is wasted in heating the bending skin and it doesn't bounce very high. In general, the higher the internal pressure in the ball, the better it will bounce.

However, the ball doesn't bounce all by itself when you drop it on a flexible surface. In that case, the surface also dents and is responsible for part of the ball's rebound. If that surface handles energy inefficiently, it may weaken the ball's bounce. For example, if you drop the ball on carpeting, the carpeting will do much of the denting, will receive much of the ball's original energy, and will waste its share as heat. The ball won't rebound well. My guess is that you dropped the ball on a reasonably hard surface, but one that began to dent significantly when the ball's pressure reached 12psi. At that point, the ball was extremely bouncy, but it was also so hard that it dented the surface and let the surface participate strongly in the bouncing. The surface probably wasn't as bouncy as the ball, so it threw the ball relatively weakly into the air.

I'd suggest repeating your experiment on the hardest, most massive surface you can find. A smooth cement or thick metal surface would be best. The ball will then do virtually all of the denting and will be responsible for virtually all of the rebounding. In that case, I'll bet that the 12psi ball will bounce highest.

Flies travel at modest speeds relative to the air that surrounds them. Since the outside air is nearly motionless relative to the ground (usually), a fly outside the van is also nearly motionless. When the fast-moving van collides with the nearly motionless fly, the fly's inertia holds it in place while the van squashes it.

But when the fly is inside the van, the fly travels about in air that is moving with the van. If the van is moving at 70 mph, then so is the air inside it and so is the fly. In fact, everything inside the van moves more or less together and from the perspective of the van and its contents, the whole world outside is what is doing the moving--the van itself can be considered stationary and the van's contents are then also stationary.

As long as the fly and the air it is in are protected inside the van, the movement of the outside world doesn't matter. The fly buzzes around in its little protected world. But if the van's window is open and the fly ventures outside just as a signpost passes the car, the fly may get creamed by a collision with the "moving" sign. Everything is relative and if you consider the van as stationary, then it is undesirable for the van's contents to get hit by the moving items in the world outside (passing trees, bridge abutments, or oncoming vehicles.

I'd suggest finding a hollow rubber ball with a relatively thin, flexible skin and putting different things inside it. You can just cut a small hole and tape it over after you put in "the stuff." Compare the ball's bounciness when it contains air, water, shaving cream, beans, rice, and so on. Just drop it from a consistent height and see how high it rebounds. The ratio of its rebound height to its drop height is a good measure of how well the ball stores energy when it hits the ground and how well it uses that energy to rebound. A ball that bounces to full height is perfect at storing energy while a ball that doesn't bounce at all is completely terrible at storing energy. You'll get something in between for most of your attempts--indicating that "the stuff" is OK but not perfect at storing energy during the bounce. The missing energy isn't destroyed, it's just turned into thermal energy. The ball gets a tiny bit hotter with every bounce.

You won't get any important quantitative results from this sort of experiment, but it'll be fun anyway. I wonder what fillings will make the ball bounce best or worst?

During a bounce from a rigid surface, the ball's surface dents. Denting a surface takes energy and virtually all of the ball's energy of motion (kinetic energy) goes into denting its own surface. For a moment the ball is motionless and then it begins to rebound. As the ball undents, it releases energy and this energy becomes the ball's new energy of motion.

The issue is in how well the ball's surface stores and then releases this energy. The ideal ball experiences only elastic deformation--the molecules within the ball do not reorganize at all, but only change their relative spacings during the dent. If the molecules reorganize--sliding across one another or pulling apart in places--then some of the denting energy will be lost due to internal friction-like effects. Even if the molecules slide back to their original positions, they won't recover all the energy and the ball won't bounce to its original height.

In general, harder rubber bounces more efficiently than softer rubber. That's because the molecules in hard rubber are too constrained to be able to slide much. A superball is very hard and bounces well. But there are also sophisticated thermal effects that occur in some seemingly hard rubbers that cause them to lose their stored energy.

Bouncing is related to elasticity. Any object that stores energy when deformed will rebound when it collides with a rigid surface. As long as the object is elastic, it doesn't matter whether it's hard or soft. It will still rebound from a rigid surface. Thus both a rubber ball and a steel marble will rebound strongly when you drop them on a steel anvil.

But hardness does have an important effect on bouncing from a non-rigid surface. When a hard object collides with a non-rigid surface, the surface does some or all of the deforming so that the surface becomes involved in the energy storage and bounce. If the surface is elastic, storing energy well when it deforms, then it will make the object rebound strongly. That's what happens when a steel marble collides with a rubber block. However, if the surface isn't very elastic, then the object will not rebound much. That's what happens when a steel marble collides with a thick woolen carpet.

A dead ball, a ball that doesn't bounce, is one with enormous internal friction. A bouncy ball stores energy when it collides with a surface and then returns this energy when it rebounds. But no ball is perfectly elastic, so some of the collision energy extracted from the ball and surface when they collide is ultimately converted into heat rather than being returned during the rebound. The deader the ball is, the less of the collision energy is returned as rebound energy. A truly dead ball converts all of the collision energy into heat so that it doesn't rebound at all.

Most of the missing collision energy is lost because of sliding friction within the ball. Molecules move across one another as the ball's surface dents inward and these molecules rub. This rubbing produces heat and diminishes the elastic potential energy stored in the ball. When the ball subsequently undents, there just isn't as much stored energy available for a strong rebound. The classic dead "ball" is a beanbag. When you throw a beanbag at a wall, it doesn't rebound because all of its energy is lost through sliding friction between the beans as the beanbag dents.

It's true that the earth's surface is moving eastward rapidly relative to the earth's center of mass. However, that motion is very difficult to detect. When you are standing on the ground, you move with it and so does everything around you, including the air. While you are actually traveling around in a huge circle once a day, for all practical purposes we can imagine that you are traveling eastward in a straight line at a constant speed of 950 mph relative to the earth's center of mass. Ignoring the slight curvature of your motion, you are in what is known as an inertial frame of reference, meaning a viewpoint that is not accelerating but is simply coasting steadily through space.

You'll notice that I keep saying "relative to the earth's center of mass" when I discuss motion. I do that because there is no special "absolute" frame of reference. Any inertial frame is as good as any other frame and your current inertial frame is just as good as anyone else's. In fact, you are quite justified in declaring that your frame of reference is stationary and that everyone else's frames of reference are moving. After all, you don't detect any motion around you so why not declare that your frame is officially stationary. Since the air is also stationary in that frame of reference, flying about in the air doesn't make things any more complicated. You are flying through stationary air in your old stationary frame of reference. The only way in which the 950 mph speed appears now is in comparing your frame of reference to the rest of the earth: in your frame of reference, the earth's center of mass is moving westward at 950 mph.

Yes. The bouncier the golf ball, the farther it will go after being struck by a golf club. While we normally think of a bounce as occurring when a ball hits a stationary object, it's also a bounce when a moving object hits a stationary ball. The golf ball bounces from the golf club and the more bouncy the golf ball is, the faster and farther it will travel.

A properly inflated soccer ball bounces well when you drop it on a hard floor because the ball stores energy by compressing the air during the bounce and the air returns this energy quite efficiently during the rebound. An under inflated soccer ball doesn't bounce so well because it stores energy by bending its leather surface during the bounce and the leather doesn't return energy very efficiently during the rebound. The same result holds true when you kick a ball rather than dropping it on the floor. Whether a moving ball hits a stationary surface or a stationary ball hits a moving surface, the ball is still bouncing from a surface. When you kick a ball with your foot, the ball is bouncing from your foot and a properly inflated ball will bounce more efficiently from your foot than an under inflated ball. The properly inflated ball will rebound at a higher speed and will travel farther.

When you lift a ball off the floor, you transfer energy to it. This energy is stored in the gravitational force between the ball and the earth and is called gravitational potential energy. When you release the ball, its weight makes it accelerate downward and its gravitational potential energy gradually becomes kinetic energy, the energy of motion. When the ball hits the floor, both the ball's bottom surface and the floor's upper surface begin to distort and the ball's kinetic energy becomes elastic potential energy in these two distorted surfaces. The ball accelerates upward during this process and eventually comes to a complete stop. When it does, most of the energy that was initially gravitational potential energy and later kinetic energy has become elastic potential energy in the surfaces. However, some of the original energy has been converted into thermal energy by internal frictional forces in the ball and floor. The distorted ball and floor then push apart and the ball rebounds into the air. Some or most of the elastic potential energy becomes kinetic energy in the ball, and the rising ball then converts this kinetic energy into gravitational potential energy. But the ball doesn't reach its original height because some of its original gravitational potential energy has been converted into thermal energy during the bounce.

Yes to both questions. When a basketball collides with the floor, the ball's kinetic energy--its energy of motion--is temporarily stored as elastic potential energy in two objects: the ball and the floor. The fractions of the collision energy stored in the basketball and the floor depend on how far each of them dents--the more one dents, the larger the fraction of the collision energy it receives. How well the basketball rebounds from the floor depends on how much of the collision energy returns to the ball during the rebound. Some of the stored energy in each dented surfaces is converted to thermal energy and is lost from the bouncing process. A hardwood floor is very springy and returns its share of the collision energy efficiently. A properly inflated basketball is also very springy. Thus when a firm basketball bounces on a good hardwood floor, it bounces well. But if the basketball is underinflated, its surface bends too far so that it receives most of the collision energy and internal friction in the ball's skin wastes most of that energy. The ball bounces weakly. And if you try to bounce the ball on a soft carpet, the carpet dents easily, receives most of the collision energy, and wastes most of it as thermal energy. Again, a weak bounce.

I suspect that cool storage will prolong the life of a tennis ball in an opened can. That's because the ball's bounciness depends on its retaining air inside its rubber shell. As the ball loses air by diffusion through the rubber, it loses its ability to bounce high. Diffusion is a thermally activated process in which the individual air molecules move between the rubber molecules and migrate through the material. At lower temperatures, the air molecules will move much more slowly through the rubber and the pressure inside the ball will stay high for a longer time.

The sweet spot. Hitting someone with the bat is very similar to hitting a ball. When you hit a ball with the sweet spot of the bat, the bat slows down and begins to rotate slowly. The slowing is good because it means that some of its kinetic energy has been transferred to the ball. The rotation is bad, because it means that the bat has put energy into rotation (spinning objects have kinetic energy). If the ball hit the bat's center of mass, the bat wouldn't rotate and the transfer of energy would be better; except for one new problem: the bat would begin to vibrate and that vibration would use energy. By hitting the ball on the sweet spot, you keep the bat from vibrating and wasting some of its energy. The transfer of energy and momentum to the ball is maximized. The same occurs when hitting any other object, including a person.

The speed of the ball's rebound from the stationary bat (let's adopt the bat's inertial frame of reference for the moment) depends on the speed at which the ball and bat approach one another. The faster the ball approaches the bat, the higher the ball's rebound speed will be. Since a fastball approaches the bat faster than a slow ball, the fastball also leaves the bat at a higher speed and is more likely to fly out of the outfield for a home run. You can even consider the case in which the batter tries to bunt and holds the bat stationary. A fastball will approach the bat faster and will bounce back faster than a slow ball will. If the pitch is fast enough, the rebounding ball could conceivably fly past the outfield for a home run, too.

As the ball hits a wall and stops, it transfers its forward momentum to the wall. The ball pushes the wall forward for a certain time and thus provides a forward impulse to the wall. As the ball rebounds from the wall, it also pushes the wall forward for a certain time and thus provides an additional forward impulse to the wall. The ball ends up traveling in the opposite direction from that which it had initially, so its momentum points in the opposite direction. This reversal of momentum required an enormous transfer of forward momentum to the wall; so large that the ball actually ended up with a negative amount of forward momentum (which is equivalent to a positive amount of backward momentum).

When a ball bounces from a rigid surface, the ball's surface distorts inward and then pops back outward. During the inward motion, the ball stores energy--pushing its surface inward takes energy. During the outward motion, the ball releases that stored energy. But not all the energy invested in the ball emerges as useful work. Some of that energy is turned into thermal energy and never reappears. A properly inflated basketball returns a good fraction of the energy it receives while other balls may not. In fact, a bowling ball bounces pretty well from a hard surface such as cement. But when it hits a softer surface such as wood, the wood receives much of its energy and wastes that energy as thermal energy.

Some objects can deform elastically, storing energy in the process, while others can't. The surface of a rubber ball is made up of long, flexible molecules called polymers that can bend and stretch without breaking. As the ball's surface dents during an impact, these polymer molecules move about and begin to exert forces on one another (storing energy in the process). As the ball rebounds, these molecules release their stored energy and push the ball back into the air. An egg, on the other hand, is made of hard, crystalline material that shatters during the deformation. Whole rows of atoms and molecules rip apart from one another and are unable to return. The egg doesn't store the impact energy. Instead, it turns that energy into thermal energy. The shell just crumbles.

The frame of reference from which you observe the situation doesn't cause the rebounding ball to move quickly, but it does help you to understand why the ball rebounds so quickly. Instead of describing the ball bounce from the rising board, let's look at the ball bouncing from a horizontally moving bat. That way, we won't have to worry about gravity--we can pretend it doesn't even exist for a moment. Let's begin from the fan's inertial frame of reference as a pitched ball heads toward a bat at home plate. As the ball approaches the bat, the bat approaches the ball. Both objects are moving, which makes things complicated. So we'll now shift to the bat's frame of reference for a while. In this frame of reference, the bat is stationary and the ball is approaching at high speed. (This rapid approach speed reflects the fact that the two objects are each moving toward the one another in the fan's reference frame.) The ball now bounces from the bat. Because it approached the bat at such a high speed, the ball rebounded at a high speed, too--it heads away from the bat at high speed. Now we'll shift back to the fan's reference frame. The ball is still going away from the bat at high speed, but now we must notice that the bat itself is heading toward the outfield at a high speed, too. So the ball must really be heading toward the outfield fast--it's outrunning the bat toward the outfield. And that is the case. The ball heads toward the outfield at a much higher speed than it had when it was heading toward the bat originally. In the fan's frame of reference, there is a large transfer of energy from the bat to the ball

If the wall doesn't move at all, no. Work requires both a force and a movement in the direction of that force. But in reality, the wall will certainly move at least a short distance. When it does, it moves in the direction of the force on it and the ball is doing work on the wall.

If the ball bounces from the sweet spot, the two push on one another hard. The ball slows to a stop and then reverses its direction, rebounding from the bat at high speed. The bat accelerates in the opposite direction, and begins to rotate slightly about its center of mass. This rotation is just right to keep the bat's handle from accelerating either toward or away from the ball. That's why the hit feels so clean and neat. The handle doesn't accelerate. The force from the ball on the bat also doesn't cause the bat to vibrate, because the sweet spot is a vibrational node.

Your right. The dead ball transfers less momentum to the baseball than the lively super ball does. That's because the dead ball transfers momentum only one, essentially coming to a stop on the baseball's surface. The bouncy ball transfers momentum twice because it also pushes on the baseball as it rebounds. Overall the baseball receives more momentum (and also more energy) from the super ball than from the dead ball. The dead ball turns much of the collision energy into thermal energy.

As you bounce off the ropes, you exchange momentum with the ropes (and the earth). As a result, you normally reverse your momentum and head back into the ring. When you hit your opponent, you begin to exchange momentum with him/her. If you hit your opponent feet first and jump backward, you will reverse your direction of travel again and your opponent will receive an enormous amount of forward momentum. All of this transfer of momentum means that your personal momentum will change often but the total momentum of the earth and its population won't change. That momentum will just be rearranged amount the various objects.

If you observe the world from an inertia frame of reference--meaning that you aren't accelerating--then all of the laws of physics will apply properly to the objects you see. Energy will be conserved during activities, momentum will be transferred between objects without being created or destroyed, and so on. So it's true that any inertial frame of reference will do. However, there is often a "best" reference frame from which to observe a situation. A good example of this is the situation in which a ball bounces from a bat. The best inertial reference frame from which to watch that bounce is the frame of the moving bat. In that special inertial reference frame, the bat doesn't move and the ball bounces off the stationary bat.

The table never pushes up on the ball harder than the ball pushes down on the table. That would violate Newton's third law and is just not the way our universe works. As the ball strikes the table, the two objects dent. The ball dents most and has work done on its surface--the table pushes the surface inward and work is force times distance in the direction of that force. The ball stores this work/energy as a deformation of its elastic surface and a compression of the air inside the ball. The ball then rebounds from the table as this stored energy reemerges as kinetic energy in the ball. Throughout the bounce, the upward force that the table exerts on the ball is much larger than the ball's downward weight. As a result, the ball accelerates upward the whole time. It starts the bounce heading downward and finishes the bounce heading upward.