Author: Lou Bloomfield

Why does a body at rest remain at rest and a body in motion remain in motion, in…

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Why does a body at rest remain at rest and a body in motion remain in motion, in the absence of unbalanced force? — AW, Karachi, Pakistan

That observation, known as Newton’s first law of motion, is one of the fundamental characteristics of the universe. I could answer simply that that’s the way the universe works. But a more specific answer is that the universe exhibits translational symmetry—meaning that the laws of physics are the same from your current vantage point as they would be if you shifted a meter to your left. Shifting your vantage point along some linear path—a process called translation—doesn’t affect the laws of physics. The laws of physics are said to be symmetric with respect to translations and, because translations of any size are possible, this symmetry is considered to be continuous in character (as opposed to mirror reflection, which is a discrete symmetry). Whenever the laws of physics exhibit a continuous symmetry of this sort, there is a related conserved quantity. The conserved quantity that accompanies translational symmetry is known as momentum. An isolated object’s momentum can’t change because momentum is a conserved quantity—it can’t be created or destroyed. Since momentum is related to motion, an isolated object that’s at rest and has no momentum must remain at rest with no momentum. And an isolated object that’s moving and has a certain momentum must remain in motion with that same momentum.

Incidentally, the laws of physics also exhibit rotational symmetry—meaning that turning your head doesn’t change the laws of physics—and this symmetry leads to the existence of a conserved quantity known as angular momentum. The laws of physics also don’t change with the passage of time, a temporal symmetry that leads to the existence of a conserved quantity known as energy.

If you have four carts of equal weights, one with small wheels, one with large w…

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If you have four carts of equal weights, one with small wheels, one with large wheels, one with small wheels in front and large wheels in back, and one with large wheels in front and small wheels in back, which cart will be easiest to move? — PK

The cart with the small wheels will be easiest to move. That’s because, as the cart starts moving, each kilogram of mass in the wheels acquires twice as much energy as each kilogram of mass in the cart itself. Keeping the mass of the wheels low by making the wheels small reduces the energy in the overall cart and makes it easier to start or stop.

Why does light travel slower in some media than in a vacuum? For example, in gla…

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Why does light travel slower in some media than in a vacuum? For example, in glass or other transparent media, visible light is not absorbed and yet it slows down. What’s going on? — FH, Waltham, MA

When a light wave enters matter, the light wave’s electric field causes charged particles in the matter to accelerate back and forth. That’s because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don’t retain the light’s energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened.

However, something has happened—the light wave has been delayed ever so slightly. This absorption and reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially.

Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more effectively absorbed and reemitted by the atoms in most transparent materials. That’s because when a high frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons’ motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower and the electrons’ motions are so large that they quickly reach the boundaries of the atom. As a result, those electrons aren’t able to jiggle back and forth as far as they should and they don’t slow the light wave down as well.

Can you suggest an experiment to prove that a helium balloon floats because heli…

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Can you suggest an experiment to prove that a helium balloon floats because helium is lighter than oxygen? — CR

If you have a balance scale, you can do a series of comparisons. First compare a cup of water to a cup of salad oil, using the balance, to show that the salad oil is less dense than the water. Then show that the salad oil floats on water. Then compare an air-filled balloon to an identical helium balloon, using the balance, to show that the helium is less dense than air. Then show that the helium floats on air. It’s just like the salad oil on water, but now it’s the helium on air. You can’t simply pour the helium on the air to show that it floats, because they’ll mix. So you leave the helium wrapped up in a rubber balloon and then let it float on air. It floats just fine!

How does the temperature of a fire correspond to its color. How hot is blue fire…

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How does the temperature of a fire correspond to its color. How hot is blue fire? How hot is yellow fire? — SF, Lake Almanor, CA

The hotter the fire, the more green and blue light it emits. The dimmest glow that you can see in a darkened room appears when a surface is about 400° C. The dull red of a heat lamp is about 500° C. A candle’s yellow glow is about 1700° C. A normal incandescent lamp is about 2500° C. And the sun is about 5800° C. Blue fire would be hotter still, except it’s usually colored artificially by the presence of excited atoms. Atomic emissions are colored because atoms can’t emit all colors in order to produce a normal spectrum of thermal radiation. Instead, they preferentially emit only specific colors. That’s why when you burn copper, you see blue-green light, even when the copper isn’t very hot. The copper atoms just can’t emit red or yellow light, even though those would be the more appropriate colors at the temperature of the burning copper.

When you walk on snow when it is cold (-20° C), the snow squeaks; but when i…

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When you walk on snow when it is cold (-20° C), the snow squeaks; but when it is relatively warm (-5° C) the snow doesn’t squeak. Why? — PW, Alberta, CA

Near ice’s melting temperature, the surfaces within warm snow become more and more liquid-like. These liquid-like surfaces not only allow the warm snow to stick together as firm snowballs, but they act as lubricants so that the snow is particularly slippery. At much lower temperatures, the snow’s surfaces are much more solid and they slide uneasily and noisily across one another. The cold snow squeaks because it hasn’t “been oiled.”

When an object is free falling, I understand that the earth’s gravity causes its…

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When an object is free falling, I understand that the earth’s gravity causes its velocity to increase at 10 meters/second2 in the downward direction. Is there a point at which this object would reach a “terminal velocity” in the earth’s atmosphere and cease to accelerate? — CS, Sykesville, MD

Yes, most objects will reach a terminal velocity and stop accelerating downward. The faster an object drops, the more air resistance it experiences. This air resistance pushes the object upward and at least partially cancels the downward force of gravity—the object’s weight. When the object’s downward speed becomes high enough, the upward air resistance force exactly cancels the object’s downward weight. At that point, the object experiences zero net force and it no longer accelerates. Instead, it descends at a constant downward velocity—its terminal velocity. This terminal velocity is determined partly by the object’s density and size and partly by its aerodynamics. Large, dense, and aerodynamic objects tend to have very large terminal velocities while small, low-density, non-aerodynamic objects tend to have very small terminal velocities.

I’ve heard that, technically speaking, our atmosphere is a fluid. Can you discus…

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I’ve heard that, technically speaking, our atmosphere is a fluid. Can you discuss this?

Since both gases and liquids are fluids, the earth’s atmosphere is certainly a fluid. Any material that flows in response to sheer stress (tearing) is considered a fluid. The earth’s atmosphere flows in responses to sheer stress—for example when you drive your car past another car, the air in between experiences this tearing and it flows in a complicated fashion. Winds are another important example of fluid flow in the earth’s atmosphere.

When raisins are added to a solution containing water, baking soda, and vinegar,…

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When raisins are added to a solution containing water, baking soda, and vinegar, why do the raisins dance? — RE, Troy, IL

Baking soda and vinegar react in water to release carbon dioxide molecules. If the chemicals are sufficiently dilute in the water, the carbon dioxide molecules may remain dissolved in the water almost indefinitely. But when the water has impurities in it, the carbon dioxide molecules tend to come out of solution as gas bubbles at those impurities. The impurities allow the molecules to form tiny gas bubbles—a process called nucleation. In the present case, the raisins serve as the impurities that nucleate gas bubbles. As the gas bubbles grow on the surfaces of the raisins, the raisins experience upward buoyant forces from the surrounding water. The bubbles float upward, carrying the raisins with them and causing the raisins “to dance.”

What is the difference between internal and external combustion engines?

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What is the difference between internal and external combustion engines?

External combustion engines burn a fuel outside of the engine and produce a hot working fluid that then powers the engine. The classic example of an external combustion engine is a steam engine. Internal combustion engines burn fuel directly in the engine and use the fuel and the gases resulting from its combustion as the working fluid that powers the engine. An automobile engine is a fine example of an internal combustion engine.