FIZZIX

A new way to appreciate basic science* by thinking seriously– like Einstein did — about questions you may not have thought of. Click the + for the answer.

1. Squealing chalk
Why does a piece of chalk produce a hideous squeal if you hold it incorrectly? Why do squeaky doors squeak?

The squeal results from ‘stick and slip’. The chalk first sticks on the chalkboard, but then slips and vibrates periodically. Similarly with squeaky door hinges.

2. Banjo versus harp
Why does the banjo produce a twangy sound and the harp a soft mellow sound?

The banjo is plucked with a hard pick or fingernail but the harp is plucked with a soft finger. More of the higher frequency harmonics are excited when the banjo is plucked with a sharp pick of the fingernail than when the harp string is plucked with a soft finger. These higher frequencies give us the characteristic twangy banjo sound.

3. Whip crack
What makes the sound when a whip is cracked?

The whip crack may be due to the slap of the tip against itself…or from the shock wave created as the tip exceeds the speed of sound. Airplanes produce such shock waves (“sonic booms”) as they cross the threshold into supersonic flight. 

4. Locking brakes
If you must stop your car in a hurry, should you slam on the brakes and lock them?

On new cars with anti-skid brakes, you should press your foot hard on the brake pedal. The anti-skid feature allows the wheels to turn slightly less than what will lock them. This stops the car quicker because static friction is greater than locking of brakes produced by sliding or slipping friction.

5. Falling Cat
It is common knowledge that if you drop a cat upside down it will land on its feet. Is its angular momentum constant during the fall? How does the cat turn itself through a full 180 degrees?

The net angular momentum of the cat is constant throughout the free fall because there are no external torques on it. By extending or retracting its legs, the cat can make the front half of its body have a different  moment of inertia about its body axis than the rear half. Thus, if it extends its front legs and retracts its hind legs and then rotates the rear half, the front half will rotate in the opposite direction but not as far. So there is a net rotation in the direction that the rear half rotated. The cat then extends the rear legs and retracts the front legs and repeats the process  to gain a further net rotation in that direction sufficient to right itself completely.

6. Freezing hot and cold water
In cold regions like Canada and Iceland, it is common knowledge that water left outside will freeze faster if it is originally hot. While this may seem completely wrong, it is not an old wives tale. Why does this happen

The critical feature is the increased evaporation from the initially warmer water. The evaporation reduces the remaining mass in the container.  With less mass to cool, the water in a hot container can overtake the water in a cool container. People in warmer climates find it mysterious, but the result is well known in Canada. In fact, it explains why at very low outside temperatures, the droplets in a hot cup of coffee when emptied by flinging it upwards will freeze before the droplets reach the ground. You can lose money betting against this cold-climate fact.

7. Life and the Second Law of Thermodynamics
In thermodynamics one learns that entropy, a measure of the disorder in a system, always increases in an irreversible process. Isn’t this violated by the creation and growth of a human being – and the facts of species evolution over millions of years?

The question overlooks an important fact: you and the evolutionary process are biological systems, which are not isolated thermodynamic systems. They are not in thermodynamic equilibrium since they must constantly require energy input to maintain themselves. The energy that flows through an organism or a species reduces its disorder or entropy, but increases the net disorder of the world — which preserves the Second Law of Thermodynamics. And saves physics professors from any potential conflicts of interest or embarrassment.

8. Time to turn on a light
When you turn on a light switch, how long does it take for the light to come on?

The electrons move through the circuit relatively slowly, but the signal—the change in the electric field along the wire—moves at nearly the speed of light. It is the signal, rather than the actual electrons from the switch, that must reach the light and this may take a nano-second (a billionth of a second). But the filament must be heated to several thousand degrees Kelvin before it can emit light. This typically takes about a hundredth to a tenth of a second.

9. Boat sinking in a pool
You hold a rock in a rowboat that’s floating in a pool, and you drop the rock overboard. Does the level of the pool rise, fall, or stay the same?

While in the boat, since the rock is supported by the boat (as if it’s floating), it displaces a volume of water equal to the weight of the rock. Since the rock is denser than water, it displaces more than its own volume of water. When the rock is on the bottom of the pool, it displaces only its own volume of water, which is less than it displaced when floating in the boat. So the boat rises and the pool level falls. The same result occurs when the person in the rowboat steps out onto dry land.

10. The season lag
Why is it cold in winter and warm in summer?

The Northern Hemisphere winters are cold because the tilt of the Earth’s axis shortens the days and lowers the Sun in the sky. These two factors reduce the amount of heat deposited on the surface during the day. But the ground and the atmosphere take about a month to cool, so there’s a month’s lag before winter kicks in. They take a month to heat up, so summer lags behind the lengthened days and the higher Sun.

11. The Crapper
How does a flush toilet work?

You can flush a toilet by merely pouring a bucket of water into it because there is a siphon between the toilet bowl and the plumbing to the sewer. When enough water accumulates in the toilet bowl, the inlet side of the siphon, the water spills out through the siphon to the outlet side and siphoning begins. The extra hole at the bottom of the bowl is a water jet that entrains the fluid from the bowl and increases the speed and vigor of the siphoning.

*The Flying Circus of Physics, Jearl Walker, John Wiley & Sons, 1977

FIZZIX

A new way to appreciate basic science* by thinking seriously– like Einstein did — about questions you may not have thought of. Click the + for the answer.

1. Squealing chalk
Why does a piece of chalk produce a hideous squeal if you hold it incorrectly? Why do squeaky doors squeak?

The squeal results from ‘stick and slip’. The chalk first sticks on the chalkboard, but then slips and vibrates periodically. Similarly with squeaky door hinges.

2. Banjo versus harp
Why does the banjo produce a twangy sound and the harp a soft mellow sound?

The banjo is plucked with a hard pick or fingernail but the harp is plucked with a soft finger. More of the higher frequency harmonics are excited when the banjo is plucked with a sharp pick of the fingernail than when the harp string is plucked with a soft finger. These higher frequencies give us the characteristic twangy banjo sound.

3. Whip crack
What makes the sound when a whip is cracked?

The whip crack may be due to the slap of the tip against itself…or from the shock wave created as the tip exceeds the speed of sound. Airplanes produce such shock waves (“sonic booms”) as they cross the threshold into supersonic flight. 

4. Locking brakes
If you must stop your car in a hurry, should you slam on the brakes and lock them?

On new cars with anti-skid brakes, you should press your foot hard on the brake pedal. The anti-skid feature allows the wheels to turn slightly less than what will lock them. This stops the car quicker because static friction is greater than locking of brakes produced by sliding or slipping friction.

5. Falling Cat
It is common knowledge that if you drop a cat upside down it will land on its feet. Is its angular momentum constant during the fall? How does the cat turn itself through a full 180 degrees?

The net angular momentum of the cat is constant throughout the free fall because there are no external torques on it. By extending or retracting its legs, the cat can make the front half of its body have a different  moment of inertia about its body axis than the rear half. Thus, if it extends its front legs and retracts its hind legs and then rotates the rear half, the front half will rotate in the opposite direction but not as far. So there is a net rotation in the direction that the rear half rotated. The cat then extends the rear legs and retracts the front legs and repeats the process  to gain a further net rotation in that direction sufficient to right itself completely.

6. Freezing hot and cold water
In cold regions like Canada and Iceland, it is common knowledge that water left outside will freeze faster if it is originally hot. While this may seem completely wrong, it is not an old wives tale. Why does this happen

The critical feature is the increased evaporation from the initially warmer water. The evaporation reduces the remaining mass in the container.  With less mass to cool, the water in a hot container can overtake the water in a cool container. People in warmer climates find it mysterious, but the result is well known in Canada. In fact, it explains why at very low outside temperatures, the droplets in a hot cup of coffee when emptied by flinging it upwards will freeze before the droplets reach the ground. You can lose money betting against this cold-climate fact.

7. Life and the Second Law of Thermodynamics
In thermodynamics one learns that entropy, a measure of the disorder in a system, always increases in an irreversible process. Isn’t this violated by the creation and growth of a human being – and the facts of species evolution over millions of years?

The question overlooks an important fact: you and the evolutionary process are biological systems, which are not isolated thermodynamic systems. They are not in thermodynamic equilibrium since they must constantly require energy input to maintain themselves. The energy that flows through an organism or a species reduces its disorder or entropy, but increases the net disorder of the world — which preserves the Second Law of Thermodynamics. And saves physics professors from any potential conflicts of interest or embarrassment.

8. Time to turn on a light
When you turn on a light switch, how long does it take for the light to come on?

The electrons move through the circuit relatively slowly, but the signal—the change in the electric field along the wire—moves at nearly the speed of light. It is the signal, rather than the actual electrons from the switch, that must reach the light and this may take a nano-second (a billionth of a second). But the filament must be heated to several thousand degrees Kelvin before it can emit light. This typically takes about a hundredth to a tenth of a second.

9. Boat sinking in a pool
You hold a rock in a rowboat that’s floating in a pool, and you drop the rock overboard. Does the level of the pool rise, fall, or stay the same?

While in the boat, since the rock is supported by the boat (as if it’s floating), it displaces a volume of water equal to the weight of the rock. Since the rock is denser than water, it displaces more than its own volume of water. When the rock is on the bottom of the pool, it displaces only its own volume of water, which is less than it displaced when floating in the boat. So the boat rises and the pool level falls. The same result occurs when the person in the rowboat steps out onto dry land.

10. The season lag
Why is it cold in winter and warm in summer?

The Northern Hemisphere winters are cold because the tilt of the Earth’s axis shortens the days and lowers the Sun in the sky. These two factors reduce the amount of heat deposited on the surface during the day. But the ground and the atmosphere take about a month to cool, so there’s a month’s lag before winter kicks in. They take a month to heat up, so summer lags behind the lengthened days and the higher Sun.

11. The Crapper
How does a flush toilet work?

You can flush a toilet by merely pouring a bucket of water into it because there is a siphon between the toilet bowl and the plumbing to the sewer. When enough water accumulates in the toilet bowl, the inlet side of the siphon, the water spills out through the siphon to the outlet side and siphoning begins. The extra hole at the bottom of the bowl is a water jet that entrains the fluid from the bowl and increases the speed and vigor of the siphoning.

*The Flying Circus of Physics, Jearl Walker, John Wiley & Sons, 1977