. If the train track gets bumpier in effect with increasing speed, why is it that your car bumps less when you go over a speed bump fast instead of slow?
Actually, if you drive fast over a real speed bump, it's not good for your wheels and suspension. The springs in your car do protect the car from some of the effects of the bump, but not all of them. However, imagine driving over a speed bump on a traditional bicycle—one that has no spring suspension. The faster you drive over that bump, the more it will throw you into the air.
. Is it physically possible for a baseball player to hit a baseball that has been pitched 60 ft away at 90-95 mph? If so, why are the highest baseball records between 3 and 4 out of ten?
If the ball was pitched straight and true, the same way every pitch, good batters could hit every one. There is enough time in the wind-up and pitch for the batter to determine where and when to swing and to hit the ball just right. But the pitches vary and the balls curve. That limits the batter's ability to predict where the ball is going. There aren't any physical laws that limit a batter's ability to hit every ball well, but there are physiological and mental limits that lower everyone's batting average.
. Is the earth a huge magnet? If so, how does it do this without being made out of metal?
The earth is a huge magnet and it is made out of metal. The earth's core is mostly iron and nickel, both of which can be magnetic metals. However, the earth's magnetism doesn't appear to come from the metal itself. Current theories attribute the earth's magnetism to movements in and around the core. There are either electric currents associated with this movement or some effects that orient the local magnetization of the metal. I don't think that there is any general consensus on the matter.
. What keeps the earth stable so that it doesn't get pulled up into the "magnet"?
If you are asking why doesn't the earth itself get pulled up toward a large magnet or electromagnet that I'm holding in my hand, the answer is that the magnetic forces just aren't strong enough to pull the magnet and earth together. I'm holding the two apart with other forces and preventing them from pulling together. The forces between poles diminish with distance. Those forces are proportional to the inverse square of the distance between poles, so they fall off very quickly as the poles move apart. Moreover, each north pole is connected to a south pole on the same magnet, so the attraction between opposite poles on two separate magnets is mitigated by the repulsions of the other poles on those same magnets. As a result, the forces between two bar magnets fall over even faster than the simple inverse square law predicts. It would take an incredible magnet, something like a spinning neutron star, to exert magnet forces strong enough to damage the earth. But then a neutron star would exert gravitational forces that would damage the earth, too, so you'd hardly notice the magnetic effects.
. How do rechargeable batteries get recharged?
You can recharge any battery by pushing charge through it backward (pushing positive charge from its positive terminal to its negative terminal). However, some batteries don't take this charge well or heat up. The ones that recharge most effectively are those that can rebuild their chemical structures most effectively as they operate backward.
. If the battery separates charges even while it's off, how come it doesn't light up when it's off?
The battery stops separating charges once enough have accumulated on its terminals. If the flashlight is off, so that charges build up, then the battery soon stops separating charge and the light bulb doesn't light.
. How are you "shocked"?
Your body is similar to salt water and is thus a reasonably good conductor of electricity. Once current penetrates your skin (which is insulating), it flows easily through you. At high currents, this electricity can deposit enough energy in you to cause heating and thermal damage. But at lower currents, it can interfere with normal electrochemical and neural process so that your muscles and nerves don't work right. It takes about 0.030 amperes of current to cause serious problems for your heart, so that currents of that size can be fatal.
. How come the flashlight works when you switch the batteries but my walkman or gameboy doesn't?
The bulb in a battery doesn't care which way current flows through it. The metal has no asymmetry that would treat left-moving charges differently from right-moving charges. That's not true of the transistors in a walkman or gameboy. They contain specialized pieces of semiconductor that will only allow positive charges to move in one direction, not the other. When you put the batteries in backward and try to propel current backward through its parts, the current won't flow and nothing happens.
. If only electrons move around, why do you keep using positive charges in the demos?
It's useful to describe moving electric charges as a current and for that current to flow in the direction that the charges are moving. Suppose that we define current as flowing in the direction that electrons take and look at the result of letting this current of electrons flow into a charge storage device. We would find that as this current flowed into the storage device, the amount of charge (i.e. positive) charge in that device would decrease! How awkward! You're "pouring" something into a container and the contents of that container are decreasing! So we define current as pointing in the direction of positive charge movement or in the direction opposite negative charge movement. That way, as current flows into a storage device, the charge in that device increases!
. Why are batteries so expensive?
They contain highly purified and refined chemicals and are actually marvels of engineering. It's more surprising to me that they are so cheap, given how complicated they are to make.