## I can accept that weight is a force, but it doesn’t seem to follow common sense …

#### I can accept that weight is a force, but it doesn’t seem to follow common sense to me.

It would seem like a force if you had to lift yourself up ladder. Imagine carrying a friend up the ladder; you’d have to pull up on your friend the whole way. That’s because some other force (your friend’s weight) is pulling down on your friend. But when you think of weight as a measure of how much of you there is, then it doesn’t seem like a force. That’s where the relationship between mass and weight comes into play. Mass really is a measure of how much of you there is and, because mass and weight are proportional to one another, measuring weight is equivalent to measuring mass.

## How can an object in space “fall”?

#### How can an object in space “fall”?

Gravity still acts on objects, even though they are in space. No matter how far you get from the earth, it still pulls on you, albeit less strongly than it does when you are nearby. Thus if you were to take a ball billions of miles from the earth and let go, it would slowly but surely accelerate toward the earth (assuming that there were no other celestial objects around to attract the ball—which isn’t actually the case). As long is nothing else deflected it en route, the ball would eventually crash into the earth’s surface. Even objects that are “in orbit” are falling; they just keep missing one another because they have large sideways velocities. For example, the moon is orbiting the earth because, although it is perpetually falling toward the earth, it is moving sideways so fast that it keeps missing.

## Doesn’t weight have resistance to acceleration?

#### Doesn’t weight have resistance to acceleration?

No, weight measures a different characteristic of an object. Mass measures inertia (or equivalently resistance to acceleration). But weight is just the force that gravity exerts on an object. While an object that has great weight also has great mass and is therefore hard to accelerate, it’s not the weight that’s the problem. To illustrate this, imagine taking a golf ball to the surface of a neutron star, where it would weigh millions of pounds because of the incredibly intense gravity. That golf ball would still accelerate easily because its mass would be unchanged. Only its weight would be affected by the local gravity. Similarly, taking that golf ball to deep space would reduce its weight almost to zero, yet its mass would remain the same as always.

## Does air resistance affect a horizontally thrown ball?

#### Does air resistance affect a horizontally thrown ball?

Yes. A ball thrown horizontally gradually loses its downfield component of velocity. For that reason, you must throw a ball somewhat below the 45° angle from horizontal in order to make it travel as far as possible. Actually, the air has even more complicated effects on spinning balls.

## Does a bullet go from 0 to maximum speed instantly?

#### Does a bullet go from 0 to maximum speed instantly?

The bullet accelerates gradually, like everything else. However, the forces that push on the bullet when the gun is fired are extremely large and it accelerates extremely rapidly. It goes from 0 to maximum speed in about a thousandth of a second.

## As the Space Shuttle falls, does it accelerate forever and does it go faster and…

#### As the Space Shuttle falls, does it accelerate forever and does it go faster and faster?

Yes to the first part, no to the second part. Remember that acceleration can change the direction of velocity without changing the magnitude of velocity (the speed of the object). When the space shuttle accelerates, its speed doesn’t change, only its direction of travel. Although it accelerates endlessly, it never goes faster or slower. Actually, if the shuttle’s orbit isn’t circular, its speed does increase and decrease slightly as it orbits the earth in an ellipse, but that’s an unimportant detail. For a circular orbit, the shuttle’s speed is constant but its velocity (speed and direction) is not constant!

## What is deceleration?

#### Are you accelerating when your speed decreases?

Yes! If you are walking east and you come to a stop, it is because you accelerated to the west! By "deceleration" we mean acceleration in the direction opposite our direction of motion. Thus in a car, when you stomp on the brake and decelerate, you are actually accelerating toward the rear of the car (in the direction opposite its direction of motion).

## Does the moon orbit the earth or is it more complicated than that?

#### Your answer to question #1393 is fine for the hypothetical case of the earth orbiting around the moon, but I don’t see how it works for the real case where the moon orbits the earth. What is the real reason for the tides? — DM

There is nothing hypothetical about the earth orbiting the moon; it’s as real as the moon orbiting the earth. The earth and the moon are simply two huge balls in otherwise empty space and though the mass of one is 81 times the mass of the other, they’re both in motion. More specifically, they’re in orbit around their combined center of mass — the effective location of the earth-moon system.

Since the earth is so much more massive than the moon, their combined center of mass is 81 times closer to the middle of the earth than it is to the middle of the moon. In fact, it’s inside the earth, though not at the middle of the earth. As a result, the earth’s orbital motion takes the form of a wobble rather than a more obvious looping path. Nonetheless, the earth is orbiting.

I hope that you can see that there is no reason why the earth should be fixed in space while the moon orbits about it. You’ve been sold a bill of goods. The mistaken notion that the moon orbits a fixed earth is a wonderful example of the “factoid science” that often passes for real science in our society.

Because thinking and understanding involve hard work, people are more comfortable when the thought and understanding have been distilled out of scientific issues and they’ve been turned into memorizable sound bites. Those sound bites are easy to teach and easy to test, but they’re mostly mental junk food. A good teacher, like a good scientist, will urge you to question such factoids until you understand the science behind them and why they might or might not be true.

When my children were young, I often visited their schools to help teach science. In third grade, the required curriculum had them categorizing things into solutions or mixtures. Naturally, I showed them a variety of things that are neither solutions nor mixtures. It was a blast. Science is so much more interesting than a collection of 15-second sound bites.

## Why does water react in a violent and dangerous way when overheated in a microwa…

#### Why does water react in a violent and dangerous way when overheated in a microwave oven? CA

Water doesn’t always boil when it is heated above its normal boiling temperature (100 °C or 212 °F). The only thing that is certain is that above that temperature, a steam bubble that forms inside the body of the liquid will be able to withstand the crushing effects of atmospheric pressure. If no bubbles form, then boiling will simply remain a possibility, not a reality. Something has to trigger the formation of steam bubbles, a process known as “nucleation.” If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit to how hot the water can become.

Nucleation usually occurs at hot spots during stovetop cooking or at defects in the surfaces of cooking vessels. Glass containers have few or no such defects. When you cook water in a smooth glass container, using a microwave oven, it is quite possible that there will be no nucleation on the walls of the container and the water will superheat. This situation becomes even worse if the top surface of the water is “sealed” by a thin layer of oil or fat so that evaporation can’t occur, either. Superheated water is extremely dangerous and people have been severely injured by such water. All it takes is some trigger to create the first bubble-a fork or spoon opening up the inner surface of the water or striking the bottom of the container-and an explosion follows. I recently filmed such explosions in my own microwave (low-quality movie (749KB), medium-quality movie (5.5MB)), or high-quality movie (16.2MB)). As you’ll hear in my flustered remarks after “Experiment 13,” I was a bit shaken up by the ferocity of the explosion I had triggered, despite every expectation that it would occur. After that surprise, you’ll notice that I became much more concerned about yanking my hand out of the oven before the fork reached the water. I recommend against trying this dangerous experiment, but if you must, be extremely careful and don’t superheat more than a few ounces of water. You can easily get burned or worse. For a reader’s story about a burn he received from superheated water in a microwave, touch here.

Here is a sequence of images from the movie of my experiment, taken 1/30th of a second apart:

## I have a thermometer made of a column of fluid containing seven spheres of fluid…

#### I have a thermometer made of a column of fluid containing seven spheres of fluid that rise and fall according to the temperature (commonly known as a Galileo thermometer). How does this work? — LS, Conroe, TX

A Galileo thermometer combines Archimedes’ principle with the fact that liquids generally expand faster with increasing temperature than solids do. Each sphere in the thermometer has an average density (a mass divided by volume) that is very close to that of the fluid in the thermometer. As stated in Archimedes’ principle, if the sphere’s average density is less than that of the fluid, the sphere floats and if the sphere’s average density is more than that of the fluid, it sinks. But the fluid’s density changes relatively quickly with temperature, becoming less with each additional degree. Thus as the temperature of the thermometer rises, the spheres have more and more trouble floating. Each sphere’s density is carefully adjusted so that it begins to sink as soon as the thermometer’s temperature exceeds a certain value. At that value, the expanding fluid’s density becomes less than the average density of the sphere and the sphere no longer floats. The spheres also expand with increasing temperature, but not as much as the fluid.

Here is a picture of a combined Galileo thermometer and simple barometer. In addition to measuring the temperature with floating spheres, this device measures the outside air pressure with a column of dark liquid. It has a trapped volume of air that pushes the liquid (visible at the bottom of the unit) up a vertical pipe when the outside air pressure drops. The owner of this unit would like to know its history and origin, so if you have any information about it, please let me know.