matter and motion
Outline
- objects are lazy by nature
- opposing contact creates friction
- more time means less impact
- solids have stability
- faster fluid, lower pressure
- smart bodies self-regulate
- reaction is equal and opposite
- soft bubbles and crunchy crystals
- liquid surfaces have tension
objects are lazy by nature
Lazy Tomatoes, Stable Wheels, and an Old Magic Trick
While out grocery shopping, Ajay pulls on a large roll of plastic bags near the vegetables. The big roll at rest resists unrolling.
AJAY
When I pull slowly, the rolls unwinds. The bag stretches out and tears off unevenly.
Next time Ajay uses a quick snap-jerk. Snapped, the bag comes off the roll cleanly. A snap-jerk applies force along perforation holes between bags on the roll.
The vibration causes a couple tomatoes to roll off the table together. They hit the floor at the time. The heavier tomato is twice as heavy. It is also twice as lazy, which is to say it has double the inertia of the smaller tomato.
AJAY
If tossed off a diving board, likely a tomato and I would splat at the same moment.
He is right about that. More mass, more pull of gravity, but also more inertia. Gravity and inertia even out.
The steel side door of Ajay’s house closes by a spring. He pulls it open to enter. As he goes through, arms loaded with grocery bags, the door swings back and hits one hand. That doesn’t hurt, at least not when carrying paper towel or anything light.
Carrying a crate of potatoes, however, the door is painful as it strikes the back of his hand. The potato crate is more massive, so there is more force in the impact. More mass, more force.
Alan and Lana rode over to help prepare the meal. Their bicycle wheels help keep their bicycle stable since they resist tipping. They are lazy that way. Each wheel is a large gyroscope, like a spinning top.
Karl’s motorcycle is even more stable due to faster spin of the wheels. Simplified, a spinning wheel acts to keep spinning and resists tipping. The faster the spin, the greater the resistance to tipping.
Alan set the table using a silk table cloth with a checkered pattern. Glass rubbed against silk becomes positively charged and can create a static spark, but Alan has something else in mind for after dinner.
The potatoes will be boiled, but first must be washed, then cut. Lana opens the tap. As the stream of water falls from the faucet it becomes narrower and breaks into droplets just above the basin of the sink.
Water picks up speed as it falls, going further with each second. The same amount of water comes from the faucet each moment, so the water stretches, like stretching chewing gum becomes thinner.
LANA
Cutting potatoes into cubes works better with a cleaver than with a knife.
Both are sharp, but the cleaver is more massive and so it strikes with greater force. More mass, more force. It takes more effort to swing the cleaver, but once in motion it is easier to keep the clever in motion, splitting potatoes. The clever is lazy. Staying at rest or staying in motion.
Lana finishes by slicing the cubes so there is more surface area exposed to the boiling water. More cooking surface means less cooking time. It is like splitting wood into kindling for more surface area.
After dinner, Alan snaps the table cloth out from under dishes. The dishes are lazy, much like the roll of plastic bags back at the grocery. They stay put as the tablecloth is pulled out from under them.
ALAN
It helps to pull the cloth out and slightly down. If pulled upward, the cloth would tip over upright glasses.
It also helps that the cloth is slick and seamless and the glasses weighted down with water. More mass, more laziness.
opposing contact creates friction
Wrestlers, Yo-Yos, and Shooting Stars
Alan is headed to the gym with Clay and Lacy. The siblings consider themselves retro. Alan considers them nerdy. A skateboard made from actual skates. A bicycle with banana seat and butterfly handlebars.
Retro or not, Clay isn’t likely to slide off his skateboard. With the size of his foot, his sneakers make a lot of contact with the board. More contact means more friction, foot to board. More friction, less sliding.
The skateboard wheels have less contact with the sidewalk than a sneaker has. Much less. Each wheel touches the sidewalk along a thin line segment of contact. With little friction, wheel to ground, the wheels roll easily.
Clay has his skateboard, Alan and Lacy have bicycles. Hers has white-wall tires. Lacy over-inflates her tires for less contact with the road, like Clay’s steel skateboard wheels. Less contact means easier to roll, easier to skid.
CLAY
I have an idea for a skateboard that uses big ball bearings instead of wheels.
Spheres have even less surface contact than wheels. Less contact, less friction. His plan has three ball bearings. Two in front; one at the back. That gives stability with minimum contact.
Lacy is talented in gymnastics and yo-yo tricks. Alan and Clay call her Morpheus, after the Greek god of dreams, since she is a master of making a yo-yo sleep. Modern yo-yos, like the one Lacy uses, have ball bearing to reduce friction between the string and axle.
A basic yo-yo is two discs connected by a peg (axle) and string tied at one end around the peg. A yo-yo spins (“sleeps”) when there is little friction between the string and bottom of the peg—not enough for the yo-yo to wrap around the string.
Jerking the string up forces it to make more contact with the peg. The string sticks to the peg by friction and wraps around it. As it wraps, the string becomes shorter and up goes the yo-yo.
Alan, a wrestler on the school team, begins by rubbing his hands together, warming them up by friction. His hands resist sliding, producing heat.
Same thing happens when a tire hits the road. The molecules on one surface form links of attraction with another surface and resist sliding. Friction comes from the links between the tire and asphalt. Pulling them apart gives off heat, so tires that have travelled for a while are warm.
When rubbing his hands together, Alan tries to push harder on one hand than the other. The exercise never works. Each hand pushes equally on the other. The action on one is an equal action on the other.
Once warm, Alan applies powder to keep his hands dry. Wrestling is sweaty business and it is best to keep a grip on the opponent. Alan uses friction to grab his opponent, but lack of friction as a strategy.
Alan’s wrestling strategy is to get under his opponent and push upward, lifting his opponent off the floor. Lifting means the opponent doesn’t have ground to stand on and no friction to resist.
After the match, outside the gym, the trio spot a shooting star. It isn’t a star; more likely a grain of sand from outer space that burns up in the atmosphere by friction against the air. Bright, but brief.
As they take off for home, the two on their bikes pull up on the handlebars. The handlebars, in turn, pull down on them and this force is transmitted to the pedals for a quick takeoff. Home before dark.
more time, less impact
Gym Mats, Catcher’s Mitts, and Hammer Hard
Lacy’s gymnastic practice takes place on a basketball court. The court has hardwood floors, but a springy layer underneath the hardwood. This layer spreads the impact over a longer time, reducing the force.
The gymnasts have mats to lessen the impact. Impact spread over a longer time reduces the force, so less chance of injury. The gymnasts also bend their knees to lessen the impact, like compressing a spring in a mattress.
Padding to extend the time of impact is commonplace: boxing gloves; rubber bumpers; carpeting; pillows; automobile airbags, padded dashboards. It is even in automobile hoods that crumple on impact.
Some apartments, especially in older buildings, have an unwanted example of springy floors. The thud and thump sounds from the apartment above are made by people walking, causing the floor to vibrate like a drum.
Newer apartment buildings have a slab of concrete between the floor of one apartment and the ceiling of the apartment below. The slab has no spring to it and so it prevents the thud and thump effect.
When wrestling, Alan will roll with a lunge from his opponent. That’s what a boxer does: roll with the punch. Rolling increases the time of impact, which means less force in the punch.
Increasing time of impact to reduce the force is common in sports to prevent injury. Baseball catcher’s mitts, for instance, have more padding than the mitts of other players. Padding prolongs the time of the impulse to stop the baseball and lessen the force on the catcher’s hand.
Mountain climbers will often use nylon ropes since they stretch. Stretching increases the time to bring a falling climber to a halt. Less force means the rope is less likely to snap.
Bungee cords for jumping stretch a lot to reduce the time of impact. A steel bungee cord would likely snap. The stop time is too abrupt. Similarly, a concrete floor is more tiring than a wood floor.
Modern automobiles are made to crumple on impact, extending the time of impact, absorbing some force of the impact. Race cars are made to blow apart and so carry away the force of a crash.
As mentioned, an automobile hood that crumples can prevent serious injury to a pedestrian struck by the car. Likewise, roadway guard rails are designed to bend. Light poles on a highway are surrounded by barrels which crush to slow the impact.
ALAN
I can throw an egg against a bedsheet hanging on a clothesline and it will not break.
But throw it against a wall and splat! An egg that falls on carpet is less likely to break than if it falls onto a hardwood floor.
The more massive an object is, the greater the force it has when it hits. A cruise ship might not be fast, but it has a lot of mass. To dock safely, the engines stopped still far from dock.
Sometimes it helps to have more impact over less time, like hammering in a nail. Swinging a hammer gives it kinetic energy, so it strikes the nail with a blunt force, driving the nail into the wood.
CLAY
It is not possible to push in a nail and my hand is a poor and painful substitute for a hammer.
Skin is soft and cushions impact, unlike a hammer. The nail goes into Clay’s thumb rather than the wood.
solids have stability
Finding Balance, Slicing, and Rope Walking
The Wolf Pack is an outdoor adventure group. Members do a lot of hiking, camping, and woodcraft. Often their gear laden backpacks are heavy and awkward to carry. Noam, Lacy’s dad, is their pack leader.
NOAM
Lean into it. Bend forward when carrying a heavy load on your back.
Leaning forward shifts the centre of gravity of the load to above the feet. If the weight were shifted backward, a person would fall backward.
They set up camp on the edge of a forest, not far from a stream. Everyone is assigned chores, one of which is to fetch several buckets of water.
NOAM
Carry two buckets at a time, not just one. It is easier that way.
It is easier to carry the same amount of water in two buckets, one in each hand, than in a single bucket. A person can stand upright since the centre of gravity is between the feet so one does not have to lean.
To start a campfire, Noam tells them to think thin. Kindling, not logs of wood. Kindling has more surface area for the same amount of mass. It will reach a higher temperature in less time than the logs.
Thin applies to supper as well. Potato slices cook faster than wedges. Flatter burgers cook faster than thicker burgers with the same mass. Thinner and flatter expose more surface area for cooking.
While out picking mushrooms, some members see small animals curled up. Think thick, says Noam with a smile. Animals curl up into a ball since curling up exposes less surface area to the surroundings.
The troop made caramel apples. It takes less candy to make them using larger apples, one per member, than using smaller apples, two per member, since larger apples have less surface area per weight.
Morning stretches include sit-ups. Sit-ups with knees bent is easier than sit-ups with legs straight out. Straight out, your centre of gravity is farther away, so you exert more torque sitting up. Torque exerted at the base of the spine is more prone to injury. After stretches, the troop walk a tightrope tied between two trees.
NOAM
Shake a stick at it. By stick, I mean a pole.
A pole long enough that it droops at the ends lowers the centre of mass of the tightrope walker. With a drooping pole, centre of mass may be below the tightrope. The pole also resists rotating, keeping the tightrope walker stable on the rope.
The tightrope is twice as thick as ordinary rope. Twice as thick has four times the cross-sectional area, so is twice as wide and four times as strong. With the rope and pole, all members are successful.
faster fluid, lower pressure
Rocking, Rising, and Measure of Pressure
Clay’s mom, Mona, volunteers on the school bus when there is a gymnastics competition. She sits near the middle. On a bumpy ride, the front and back of the bus rock up and down more than the middle.
Rocking rotates the bus about its centre of mass, which is just in front of the middle. The engine weighs down the front, so the back of the bus is lively. Some kids choose the back for this reason. On a bumpy road the bus is like a ship in a choppy sea or airplane in turbulent air.
MONA
The farther I sit from the centre, the greater the see-saw up and down motion.
Other times no bus is available and Mona drives Lacy with a few other gymnasts to the competition. As they accelerate to highway speeds, she asks the girls to roll up the windows.
Open windows cause drag. Running the car’s air conditioner increases fuel consumption, but at highway speeds, open windows create air drag that uses more fuel. Drag could offset any saving from turning off the air conditioner.
On the highway, Mona’s car slightly leans toward a passing truck. Air between them is moving fast and the faster a fluid flows, the lower its pressure. With lower pressure between the car and truck, surrounding pressure pushes the vehicles toward each other.
Wind across the top of the chimney at Ajay’s cabin likewise lowers pressure inside the chimney, creating a better draw from air inside the room. That helps the fire burn, but also draws out warm air from the room.
It wasn’t until they stopped for lunch that Mona made the connection. Lacy, playing with a straw in a glass of lemonade, blew across the top of the straw. The fast air drew lemonade up the straw.
That’s why prairie dogs make mounds around their entrance and exit holes. As air is forced up and over the mound, air pressure over the hole lowers, pulling up air from the tunnel, providing prairie dogs with ventilation.
After lunch, Mona still has ice in her lemonade. The ice in Lacy’s drink has melted away. That’s because Mona had ice cubes and her daughter had crushed ice in their lemonade drinks.
Crushed ice melts faster than ice cubes since the surface area of crushed ice is greater, which gives more melting surface to the surroundings. It is the same principle as kindling or potato slices.
The glasses were poured to the brim, but neither over-flowed when the ice melted. When ice be melts, the water level is unchanged. The volume of water displaced by the weight of the cubes is the same.
Cloudy ice cubes make a slight sizzling sound as they melt. They release the small pockets of air that make the cubes cloudy. Clear ice cubes can make a cracking sound when placed in liquid.
Cracking occurs by a sudden change in temperature. Water is a poor conductor, so the surface of the cube and its cooler interior are under tension. The cube cracks, similar to Ajay’s “Hot Stuff” mug.
A woman enters the restaurant and takes off her high heels walking across the restaurant’s wood floor. If worn, her high heels could ruin the floor. Their pressure is in a smaller area than an elephant’s flat foot and so can have more pressure.
When the door opened and the woman entered, the girls caught a whiff of gasoline. In the lot, a car is leaking fuel from its gas cap. A tank filled to the brim can overflow if the car is then parked in direct sun. The gas expands with heat, bursting through the gas cap.
smart bodies self-regulate
Smart Home, Sweat, and Stinky Sneakers
Lacy likes gymnastics. Clay plays basketball. Both like coming home to the comfort of a smart home. Lights automatically on at dusk, off at dawn. They can give a voice command for a favourite playlist or open the garage door.
Heating and cooling are part of the smart system. The thermostat has a strip that is brass on one side, iron on the other. It bends as it gets warmer, breaking the circuit which kept the heater on. As the strip cools, it bends back. When it makes contact, completing an electrical circuit, then the heater switches back on. The dehumidifier like that, but with a belt instead of a bimetallic strip.
In a dehumidifier, a fan pushes air over a nylon belt. Higher humidity causes the belt to stretch and trip the ON switch. Lower humidity causes the belt to shrink and break off the ON connection. By use of feedback, the thermostat and dehumidifier keep the temperature and humidity inside the house within a set, comfortable range. The system isn’t smart as much a responsive to changes.
Clay and Lacy are healthy and maintain a stable internal temperature of 37 °C. Doing work causes food to be converted into motion and heat. Excess heat can be unhealthy, so they sweat to carry away heat.
Sweating works for people, but is not enough for a bulky animal because it has a large mass relative to a small surface area for cooling. That’s why warm-blooded animals are limited in size and dinosaurs were much larger.
An elephant has a lot of mass and relatively little surface area. Living in a hot climate, it needs a large surface area to dispose of extra body heat through radiation and evaporation. Large ears are large radiators.
Every day, Clay and Lacy eat about 2% of their own weight in food. The mouse in the attic eats 50% its own weight in a day. In other words, 25 times as much as people. Even so, there are no obese mice.
Compared to an elephant, a mouse as little mass and a lot of surface area. The elephant needs to lose heat. The mouse needs to make heat, so it requires extra food energy to maintain body temperature.
The heat loss in animals is mainly through their surface, so it varies in proportion to an animal’s surface area. Heat production, however, occurs in all cells and varies in proportion to an animal’s volume.
Babies have a larger surface area for their mass than adults do and so dissipate more heat. They feel colder at the same room temperature. Layers of bundling help insulate against excessive heat loss.
More surface, more heat loss. More heat loss, then proportionally more food is needed to maintain healthy body temperature. That’s why a mouse needs proportionally more food than an elephant.
Controlling fluids in a living body is another type of self-regulation. The entire blood supply is filtered by the kidneys 60 times each day and about 99% of the water filtered is reclaimed by the body.
In human beings, the lungs, skin, and urinary system work to expel wastes produced in metabolic activities. Lungs expel water and carbon dioxide. Sweat glands expel water, salts, and urea.
CLAY
I wish my basketball shoes were self-regulating. They stink.
Water, salts, and urea from the skin can become trapped, such as in an arm pit or inside a shoe. An accumulation of urea causes an unpleasant odour.
Walking barefoot or not washing causes an accumulation of bacteria, which contribute their own waste odours. Playing ball, Clay loses as much sweat as the cooling equivalent of a small air conditioner.
The body has more than two million sweat glands. For a healthy person, sweat is mostly water with little or no odour. Bacteria on the skin pollute sweat and give an odour to perspiration.
Only some apes (such as the gorilla), horses, and a few types of cows perspire for temperature control. Other animals pant. To cool off, the ostrich and emu urinate on their legs and let the liquid evaporate.
CLAY
I used to say that I sweat like a pig, but that isn’t true.
Pigs have no sweat glands. They cannot cool themselves by the evaporation of perspiration. They can lose heat by conduction; that is, by contact with something cool.
To cool off, pigs cover their skin with moisture. Mud is effective in both respects and protects the skin of a pig from pests and sunburn. To sweat like a pig, clay would cool off with a roll in the mud.
reaction is equal and opposite
Pond Launch, Walking through Walls, and Missing Matter
The pond froze over winter and now, with spring thaw, it is super slippery. Clay has an idea for free motion. The pond is not perfectly frictionless, but is far too slick to walk on.
Alex pushes Clay from shore, launching Clay to the middle of the pond. It is too slick to walk on; even too slippery to sit up. Clay takes off an ice skate and throws it hard toward the edge of the pond. It works—he slides in the opposite direction to shore. As the skate went one way, he went an equal and opposite direction courtesy of energy he supplied. Energy from food, ultimately from the sun.
Clay has another idea. He can see through ice. He can see through glass. Sight goes through. Perhaps he can jiggle his molecules to walk through a window or a wall like a comic book superhero.
This idea doesn’t work. The wall remains impenetrable. Atoms in the wall and Clay electrically repel one another. Matter might be mostly empty space, but these fields of repulsion are strong enough to make objects act solid.
Clay has a skateboard made of iron and an idea to make his iron board move with ease. Standing on the board, he holds a fishing pole with a large magnet attached where there would be bait. He hangs the magnet in front of the skateboard.
No go. The magnet pulls on the pole with the same force it pulls the skateboard. It is like trying to lift yourself off the ground by pulling up on your ankles.
Clay wants to use sand to make matter disappear. Perhaps 2%. Half a glass of water plus half a glass of alcohol does not amount to a full glass. It is missing 2%. Water and alcohol molecules fit into one another and take up less space combined than they do individually.
By analogy, if a volume of sand is mixed with an equal volume of marbles, the resulting volume will be less than the sum of their volumes. Some sand will fill spaces between the marbles.
CLAY
Water and alcohol. Sand and marbles. The sum doesn’t add up to its parts. That gives me an idea of how to make matter disappear, maybe the same two percent.
He put two sand hourglasses balancing on a scale. They balance. Then he turned one hourglass upside-down.
CLAY
That hourglass will be weightless during free fall, so the scale will tip to the other hourglass. What were equal weights are made unequal by motion in one hourglass.
Except he cannot do it with the hourglasses. Equal and opposite forces balance out with no net change. The two hourglasses weigh the same, in spite of the fact that some of the sand in one is in the air.
The additional force on one side is due to the impact force of the sand. This is somewhat like the ice skate thrown on the pond. In the end, Clay discovers that there is no disappearing matter and no free motion.
texture can be fabricated
Soft Bubbles, Crunchy Crystals, and Smooth as Cream
Occasionally, Clay will have ice cream in a bowl. Ice cream is mostly air and water.
CLAY
I stir it to make the ice cream smooth, even if it uses up some in stirring.
The stirred ice cream isn’t used up, but does have a smaller volume. Stirring releases air from the ice cream. There is still the same amount of ingredients, just not as thick.
Air is put into ice cream for a variety of reasons. One reason is to make it more profitable for the vendor. Air adds volume to the product. More volume, more profit since air is free. Soft serve ice cream has even more overrun air to make it serve smoothly.
Clay thinks his stirred ice cream tastes better than when scooped out, and a lot better than premium brands that do not have much overrun. Overrun is the name given to the amount of air added to ice cream.
Whether it tastes better, it does taste more. Softer ice cream spreads across taste buds more readily than ice cream in bulk. It melts faster in the mouth, adding to detection by taste buds.
Soft ice cream is also less likely to cause brain-freeze since it melts faster. Cold food, such as from hard ice cream or a slushy ice drink, causes blood vessels at the back of the mouth to rapidly constrict.
The brain sends blood to open the vessels and conflict of constriction and expansion causes pain (similar to the stress cause by putting hot water in Ajay’s “Hot Stuff” mug). Pain felt in the forehead is really in the outer covering of the brain, where the arteries at the base of the brain meet.
Cold numbs the sense receptors for sweetness, so more sugar is added to ice cream to compensate. So much so that some people find the taste of melted ice cream to be cloying, sickening sweet. The same thing is true of soda pop intended to be served cold. When chilled, the beverage tastes the right amount of sweet. Warmed, it tastes too sweet.
The smooth texture of ice cream comes from milk fats. Butter also comes from milk fats. Fat does not mix well with water (ice crystals in ice cream), so it will separate from the water as it melts. Even in frozen ice cream, fats exists as tiny globules that hold air in tiny pockets within the ice cream, like a foam. Higher fat ice cream stays on the tongue longer than ice milk and so offers more flavour.
Egg yolks keep milk fat and ice crystals together. The lecithin in eggs acts like an emulsifier so the fat globules can cluster together. Gelato uses cornstarch or tapioca starch instead of egg yolks as an emulsifier.
To keep ice crystals from forming, commercial ice cream contains stabilizers, often made from seaweed, to absorb excess water. Without stabilizers, ice cream would look like a milkshake.
liquid surfaces have tension
Serious Grime, Surface Tension, and Walking on Water
Dirt and grease together make grime, as Alex knows well, working on his bicycle. Grease is a fat. It does not come off easily by water alone. Turpentine works. Turpentine is frequently used to clean paint brushes. The solvent used for dry cleaning also works. Clothes that are dry-cleaned are washed in a container full of the chemical trichloroethane. It is said to be dry cleaning only in the sense that the tub does not contain water.
Soap also works to remove grime. Part of a soap molecule attaches to water and part to grease, surrounding a grease globule and lifting it out of clothing to be rinsed away and down the drain. Soap can be made with animal fats and caustic lye.
Detergents work like soap and are biodegradable. They break down harmlessly in the environment. There are detergents in toothpaste and dish-washing liquid. Some detergents make clothing whiter than white. A fluorescent dye turns ultraviolet light, which we cannot see, into blue light, which we can see. The clothing reflects blue, which appears as being whiter.
Soap makes suds, tiny bubbles. They are the same as the bubbles one might blow through a ring dipped into a soapy solution. A soap bubble, sudsy or blown, is a sphere of two layers of soap molecules. Air pressure makes a soap bubble round by pushing in equally on all sides. Of all shapes, a sphere has the greatest volume for its surface area.
Some extra-soapy bubbles are elongated toward the Earth due to gravity. Soap in a bubble drains by gravity, so the top thins first and is most likely to burst first. When water in a bubble evaporates, the bubble pops. Adding glycerine slows evaporation, so bubbles last longer.
Soap and detergent are less effective in hard water (water that has dissolved minerals). Clothes don’t come as clean or smell as fresh if washed in hard water. Hard water also makes it difficult to blow soap bubbles.
Water is said to be hard if it has a lot of calcium and magnesium dissolved in it. These chemicals attach to the detergent molecule and reduce how well the detergent can attach to grime.
Bubbles are held together by surface tension. Water molecules pull other molecules from all directions. But surface water molecules have no molecules above, so they pull harder on those beside and below.
A bug known as a water strider can walk on water due to surface tension and their long, flexible legs to distribute body weight. A water strider can row across the surface at a meter per second.
Surface tension is strong enough to gently float a razor blade or paper clip. Adding a drop of dish-washing detergent lowers the surface tension and both paperclip and razor blade will sink.
Sprinkle pepper evenly on the surface of a bowl of water, then add a drop of liquid detergent in the centre. The soap will weaken the surface tension and pull the pepper along toward the edge of the bowl.
Stepping out of a bath, a person has a thin a film of water with a mass of about half a kilogram. That’s not noticeable to a human being. A wet mouse, however, has to carry its own weight plus its weight in water.
A wet fly has to carry many times its weight, so it is likely to drown. An ant or other small bug may end up trapped inside a water droplet. Surface tension makes water cling in amounts they cannot carry.
