Is hydroplaning a form of sliding friction?

Not exactly. Sliding friction refers to the situation in which two surfaces slide across one another while touching. In hydroplaning, the two surfaces are sliding across one another, but they aren’t touching. Instead, they’re separated by a thin layer of trapped water. While hydroplaning still converts mechanical energy into thermal energy, just as sliding friction does, the lubricating effect of the water dramatically reduces the energy conversion. That’s why you can hydroplane for such a long distance on the highway; there is almost no slowing force at all.

Dan Barker, one of my readers, informed me of a NASA study showing that there is a minimum speed at which a tire will begin to hydroplane and that that speed depends on the square root of the tire pressure. Higher tire pressure tends to expel the water layer and prevent hydroplaning, while lower tire pressure allows the water layer to remain in place when the vehicle is traveling fast enough. As Dan notes, a large truck tire is typically inflated to 100 PSI and resists hydroplaning at speed of up to about 100 mph. But a passanger car tire has a much lower pressure of about 32 PSI and can hydroplane at speeds somewhat under 60 mph. That’s why you have to be careful driving on waterlogged pavement at highway speeds and why highway builders carefully slope their surfaces to shed rain water quickly.

If you walk up 10 steps, one by one, do you exert the same amount of energy if you walk up the same set of 10 steps two by two? How are energy and effort related, or are they?

Ideally, it doesn’t matter how many steps you take with each step—the work you do in lifting yourself up a staircase depends only on your starting height and your ending height (assuming that you don’t accelerate or decelerate in the overall process and thus change your kinetic energy, too). But there are inefficiencies in your walking process that lead you to waste energy as heat in your own body. So the energy you convert from food energy to gravitational potential energy in climbing the stairs is fixed, but the energy you use in carrying out this procedure depends on how you do it. The extra energy you use mostly ends up as thermal energy, but some may end up as sound or chemical changes in the staircase, etc.

If ball bearings create no friction, why do bearings have bearing grease as an essential ingredient?

Actually, some bearings are dry (no grease or oil) and still last a very long time. The problem is that the idea touch-and-release behavior is hard to achieve in a bearing. The balls or rollers actually slip a tiny bit as they rotate and they may rub against the sides or retainers in the bearing. This rubbing produces wear as well as wasting energy. To reduce this wear and sliding friction, most bearings are lubricated.

How do anti-lock brake systems work?

If you brake your car too rapidly, the force of static friction between the wheels and the ground will become so large that it will exceed its limit and the wheels will begin to skid across the ground. Once skidding occurs, the stopping force becomes sliding friction instead of static friction. The sliding friction force is generally weaker than the maximum static friction force, so the stopping rate drops. But more importantly, you lose steering when the wheels skid. An anti-lock braking system senses when the wheels suddenly stop turning during braking and briefly release the brakes. The wheel can then turn again and static friction can reappear between the wheel and the ground.

How can a ball create thermal energy or “get hotter”?

When a ball bounces, some of its molecules slide across one another rather than simply stretching or bending. This sliding leads to a form of internal sliding friction and sliding friction converts useful energy into thermal energy. The more sliding friction that occurs within the ball, the less the ball stores energy for the rebound and the worse the ball’s bounce. The missing energy becomes thermal energy in the ball and the ball’s temperature increases.