Cats and dogs can't sweat like we do to keep cool. Sweating is an effective way of cooling down the body. It utilizes the difference in thermal energy contained in a liquid versus that contained in a gas. A gas contains more thermal energy (because the gas molecules move fast) than a liquid. Therefore turning a liquid into a gas requires an input of energy.
When it's hot, our body wants to expend some thermal energy so it can cool to a comfortable temperature. When sweating, our body takes some of our water and puts it out on the skin, where it comes in contact with air. In air, the sweat evaporates. The water turns into water vapor, from its liquid form to its gas form. This requires an input of thermal energy, which our body is happy to provide. The water vapor floats away, carrying that extra energy with it and helping us keep cool.
The principle for cats and dogs is exactly the same, except instead of turning sweat into gas, they turn their drool into gas. Like us, they get dehydrated when hot, and so must be provided with plenty of water. Dogs also tend to overheat more easily than cats. When a cat pants, it's really hot.
Saturday, September 14, 2013
Friday, July 12, 2013
Why is it deadly to hit concrete when falling from a high place?
This one goes back to a central concept in physics called the impulse-momentum theorem. The impulse momentum-theorem relates a change in velocity with the force that causes this change in velocity.
Related post: How do ducklings get away with falling from great heights without getting hurt?
Warning: What follows is a little graphic.
Say suicidal Johnny leaps off a tall building. On his way down, Johnny speeds up and gains a lot of momentum. When he hits the ground below, he stops suddenly. His momentum goes from a large amount to zero. This happens because the ground exerts a large impulse on Johnny. That impulse is equal to the force the ground exerts on Johnny times the amount of time it takes for the ground to stop Johnny. The latter is very short, as unlike a mattress, concrete is hard. Johnny stops over the course of a few milliseconds. The force from the ground has to be very large in order to compensate for the tiny time interval and still provide the impulse needed to stop Johnny's fall. The force is so large that it causes all kind of damage to suicidal Johnny's body.Related post: How do ducklings get away with falling from great heights without getting hurt?
Monday, July 1, 2013
Why do golf balls have dimples?
Golf balls have dimples so that they are less sensitive to air resistance and go further when hit. You might think air would flow more easily around a smooth ball and so such a ball would cut through the air more easily. But that is not the case.
A smooth ball carries around itself a pocket of still air. Around that, there is turbulent air. A small bug on the surface of a smooth ball is actually protected by this thin layer of thin air and will not feel a wind as the ball flies. This layer of thin air increases the effective size of the ball and a larger ball has a harder time cutting through air. It will not be able to fly as fast and won't go as far.
A dimpled ball creates air turbulences around itself as it cuts through the air and so it doesn't have this handicap of looking like a bigger ball. It cuts through air more easily and goes further.
Reference:
The Physics of Baseball: Third Edition, Revised, Updated and Expanded, 2002, Robert Adair
Friday, June 14, 2013
Why is it so much more miserable in the summer when it is humid?
"It's not the heat, it's the humidity." Well, really, it's both. The heat makes it necessary for the body to cool off, while the humidity makes the process difficult. The inability to cool off is what makes it feel so uncomfortable when it's hot and humid. In a related post, I talk about how sweating is an efficient way to draw heat out and away from the body. The mechanism works best when the sweat can evaporate off of the skin quickly. The evaporation process is what takes the heat away from the body. When it is humid, the sweat has a hard time evaporating and instead sits on the skin. It no longer helps us cool, or at least not very much. All it does is make us feel gross. Meanwhile the body still wastes water producing sweat (more of it in fact, in a doomed effort to speed up the cooling process), so one has to drink a lot, even if it may feel like simply breathing should supply all the water we need.
An aggravating factor on hot an humid days is that the water vapor in the air traps the heat from the Sun. Water vapor is an often ignored, but effective green house gas. When the air is dry, the temperature drops quickly at sunset and nights are cool. Hot and humid days tend to stay hot even after the sun goes down (love those muggy nights!). The lack of relief in the evening and overnight makes it that much more miserable.
An aggravating factor on hot an humid days is that the water vapor in the air traps the heat from the Sun. Water vapor is an often ignored, but effective green house gas. When the air is dry, the temperature drops quickly at sunset and nights are cool. Hot and humid days tend to stay hot even after the sun goes down (love those muggy nights!). The lack of relief in the evening and overnight makes it that much more miserable.
Saturday, June 1, 2013
Why do we sweat when we are hot?
While it feels gross, sweating is an effective way of cooling down the body. It utilizes the difference in thermal energy contained in a liquid versus that contained in a gas. A gas contains more thermal energy (because the gas molecules move fast) than a liquid. Therefore turning a liquid into a gas requires an input of energy.
When it's hot, our body wants to expend some thermal energy so it can cool to a comfortable temperature. When sweating, our body takes some of our water and puts it out on the skin, where it comes in contact with air. In air, the sweat evaporates. The water turns into water vapor, from its liquid form to its gas form. This requires an input of thermal energy, which our body is happy to provide. The water vapor floats away, carrying that extra energy with it and helping us keep cool.
To replenish the sweat, we need water and that is why when it's hot, we feel thirsty and have to drink a lot of water.
When it's hot, our body wants to expend some thermal energy so it can cool to a comfortable temperature. When sweating, our body takes some of our water and puts it out on the skin, where it comes in contact with air. In air, the sweat evaporates. The water turns into water vapor, from its liquid form to its gas form. This requires an input of thermal energy, which our body is happy to provide. The water vapor floats away, carrying that extra energy with it and helping us keep cool.
To replenish the sweat, we need water and that is why when it's hot, we feel thirsty and have to drink a lot of water.
Tuesday, May 21, 2013
Why does water at 95 degrees Farenheit (35 degrees Celsius) feel cold when air at the same temperature feels hot?
The simple, somewhat dumb answer to that is that water is more effective at cooling the body than air.
95 degrees Farenheit is lower than body temperature so heat will naturally flow from the body into the surroundings, resulting in a cooling of the body. Both in air and in water, it is movement of the surrounding medium that helps the transport of heat from our skin to the surrounding air or water. In water for instance, 95 degree water comes in contact with our 99 degree skin. There is a heat exchange where the water warms up and the skin cools. The heated water then moves away and gets mixed with the rest of the cooler water, while heat from within our body rushes to the skin to warm it back up again. The cycle then begins again.
The difference between water and air is that the process is much more rapid in water than in air. Warmed air sits on the skin for a lot longer than the warmed water. So it takes a lot more energy from our body to maintain the 99 degree temperature in 95 degree water than it does in air. As a matter of fact, the cooling process in air is so inefficient that with 95 degree surrounding air, our body fails to cool if we exert ourselves and moving around feels uncomfortable. We start sweating in an attempt to remove heat from our body faster.
95 degrees Farenheit is lower than body temperature so heat will naturally flow from the body into the surroundings, resulting in a cooling of the body. Both in air and in water, it is movement of the surrounding medium that helps the transport of heat from our skin to the surrounding air or water. In water for instance, 95 degree water comes in contact with our 99 degree skin. There is a heat exchange where the water warms up and the skin cools. The heated water then moves away and gets mixed with the rest of the cooler water, while heat from within our body rushes to the skin to warm it back up again. The cycle then begins again.
The difference between water and air is that the process is much more rapid in water than in air. Warmed air sits on the skin for a lot longer than the warmed water. So it takes a lot more energy from our body to maintain the 99 degree temperature in 95 degree water than it does in air. As a matter of fact, the cooling process in air is so inefficient that with 95 degree surrounding air, our body fails to cool if we exert ourselves and moving around feels uncomfortable. We start sweating in an attempt to remove heat from our body faster.
Wednesday, May 15, 2013
Isn't there a better way to pick reading glasses?
OK, this one is not a question a 3-year old would ask, but more like a 40 year old with (until then) 20/20 vision would ask. It is a question that cropped up in my life very recently for which some simple physics comes VERY handy.
Most people develop presbyopia somewhere in their late thirties to early forties. The eye loses its flexibility and the ability to adjust the lens for near vision. Seeing far is still OK, as that is the eye's relaxed position. That is when one has to go shopping for reading glasses. Reading glasses, like prescription lenses, are rated by their power, which is just 1 over the focal length of the lenses.
I found lots of websites with reading charts to help me pick the power of my reading glasses. I found these charts completely useless. The issue with them is that I can still focus, it just takes effort. It is only recently that I realized that the chronic fatigue and headaches I have been living with for a few months now were likely partly due to my declining eyesight. It's hard for me to tell which line of small characters on these charts I can no longer read. I can read all of them (especially when it is an easily recognizable sentence), it just takes increasing effort. How much effort is too much effort? I found that very subjective and could not decide.
When faced with a problem, a physicist goes back to the underlying principles. In this case, the thin lens equation:
f is the focal length of the lens and 1/f is the power of the lens (in diopters). Stronger lenses are more curved, have shorter focal lengths, and higher power. s is the distance between the lens and the object we are looking at, and s' is the distance between the lens and the image formed. Distances must be measured in meters. Translated to our problem:
The left hand side is the power of the lens, what we are trying to determine. s is the distance between our eyes and whatever it is that we are looking at: the book lying on the desk, the computer screen, a project in our hands. s' is a little more abstract, but translated to our problem, it is the distance at which we wish the thing we are looking at were. It is the distance that is comfortable for us. The book held at fully stretched arms length for instance.
In the case of reading glasses, s' must be entered as a negative number.
To find the power lenses that you need, do the following:
1) Determine s'. That is the toughest part. The goal is to find the minimum distance of comfortable focus. That is the s' we are looking for. To find it, stand at increasing distance from an object that has fine details on it. Start at reading distance. Relax your eye completely. You will see two objects. Slowly bring the object into focus until there is just one object. Go slow. Resist the temptation to bring the object completely into focus if it doesn't do that without extra effort. You will have to tune into your newly discovered lack of ability to focus and practice. If you do it right, you will discover with dismay that at reading distance, the object is all blurred. Progressively move away from the object until you can bring it into a single object and in sharp focus without strenuous effort.
That's your minimum distance of comfortable focus. Translate that distance to meters (or measure it with a metric ruler), put a negative sign in front of it, and stick it into the thin lens equation.
2) Determine s. s is I think pretty standard for everybody. You may need different power glasses for different purposes. For instance, if you are using your glasses to look at a computer screen on your desk at work, it's probably something like 0.50 meters away from your face (measure it). On the other hand, if you are working on crafts or reading a book, you are holding your work closer to 0.30 meters away from your face.
In my case, for instance, I have discovered that my presbyopia is more serious than I would have thought for something I was not noticing or at least misinterpreting as tiredness. My minimum distance of comfortable focus is around 1.5 meters. That is to say, holding things at arms length don't make it much more comfortable to read. In the thin lens equation, my 1/s' is -0.67. If your near point is greater than 5 meters (that is you can't find a point where it starts to be comfortable to focus), use 0 for 1/s'. It means your minimum distance of comfortable focus has moved to "infinity" and 1/infinity is zero (in physicist's math).
So for a computer screen fairly far from my face, the power I need is
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Such glasses would be rated "+1.33" at the store. Ready-made reading glasses go in increments of 0.25, so I am somewhere between a +1.25 and a +1.50. I went with +1.25. The slightly lower power can be compensated for by moving my computer screen back some.
For close work, the power I need is
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That is, glasses rated as close to "+2.33" as possible.
When you shop for reading glasses, you will notice that the maximum power is +4.00. This corresponds to someone who has a near point at infinity (the worse it can get), trying to look at work 0.25 meter (25 cm) from their face (the closest you would normally be looking at things). In many states, you can't buy reading glasses over the counter when they are much stronger than 2.75 diopters.
To summarize, the first term on the right hand side of the thin lens equation depends on what you are trying to do, while the second term depends on the quality of your eyesight. While determining s' is still subjective (that's the nature of the problem), I still prefer this method. Determining s' is a traceable source of uncertainty, while the reading chart is just complete black magic. I did find a use for the reading chart at the store, but not the way they suggested using it. I stood a reading distance away from the chart and tried the "comfort focus" method detailed above while wearing different strengths reading glasses. The ones that were the right ones brought the chart into sharp focus with the minimum of effort.
Most people develop presbyopia somewhere in their late thirties to early forties. The eye loses its flexibility and the ability to adjust the lens for near vision. Seeing far is still OK, as that is the eye's relaxed position. That is when one has to go shopping for reading glasses. Reading glasses, like prescription lenses, are rated by their power, which is just 1 over the focal length of the lenses.
I found lots of websites with reading charts to help me pick the power of my reading glasses. I found these charts completely useless. The issue with them is that I can still focus, it just takes effort. It is only recently that I realized that the chronic fatigue and headaches I have been living with for a few months now were likely partly due to my declining eyesight. It's hard for me to tell which line of small characters on these charts I can no longer read. I can read all of them (especially when it is an easily recognizable sentence), it just takes increasing effort. How much effort is too much effort? I found that very subjective and could not decide.
When faced with a problem, a physicist goes back to the underlying principles. In this case, the thin lens equation:
f is the focal length of the lens and 1/f is the power of the lens (in diopters). Stronger lenses are more curved, have shorter focal lengths, and higher power. s is the distance between the lens and the object we are looking at, and s' is the distance between the lens and the image formed. Distances must be measured in meters. Translated to our problem:
The left hand side is the power of the lens, what we are trying to determine. s is the distance between our eyes and whatever it is that we are looking at: the book lying on the desk, the computer screen, a project in our hands. s' is a little more abstract, but translated to our problem, it is the distance at which we wish the thing we are looking at were. It is the distance that is comfortable for us. The book held at fully stretched arms length for instance.
In the case of reading glasses, s' must be entered as a negative number.
To find the power lenses that you need, do the following:
1) Determine s'. That is the toughest part. The goal is to find the minimum distance of comfortable focus. That is the s' we are looking for. To find it, stand at increasing distance from an object that has fine details on it. Start at reading distance. Relax your eye completely. You will see two objects. Slowly bring the object into focus until there is just one object. Go slow. Resist the temptation to bring the object completely into focus if it doesn't do that without extra effort. You will have to tune into your newly discovered lack of ability to focus and practice. If you do it right, you will discover with dismay that at reading distance, the object is all blurred. Progressively move away from the object until you can bring it into a single object and in sharp focus without strenuous effort.
That's your minimum distance of comfortable focus. Translate that distance to meters (or measure it with a metric ruler), put a negative sign in front of it, and stick it into the thin lens equation.
2) Determine s. s is I think pretty standard for everybody. You may need different power glasses for different purposes. For instance, if you are using your glasses to look at a computer screen on your desk at work, it's probably something like 0.50 meters away from your face (measure it). On the other hand, if you are working on crafts or reading a book, you are holding your work closer to 0.30 meters away from your face.
In my case, for instance, I have discovered that my presbyopia is more serious than I would have thought for something I was not noticing or at least misinterpreting as tiredness. My minimum distance of comfortable focus is around 1.5 meters. That is to say, holding things at arms length don't make it much more comfortable to read. In the thin lens equation, my 1/s' is -0.67. If your near point is greater than 5 meters (that is you can't find a point where it starts to be comfortable to focus), use 0 for 1/s'. It means your minimum distance of comfortable focus has moved to "infinity" and 1/infinity is zero (in physicist's math).
So for a computer screen fairly far from my face, the power I need is
Such glasses would be rated "+1.33" at the store. Ready-made reading glasses go in increments of 0.25, so I am somewhere between a +1.25 and a +1.50. I went with +1.25. The slightly lower power can be compensated for by moving my computer screen back some.
For close work, the power I need is
That is, glasses rated as close to "+2.33" as possible.
When you shop for reading glasses, you will notice that the maximum power is +4.00. This corresponds to someone who has a near point at infinity (the worse it can get), trying to look at work 0.25 meter (25 cm) from their face (the closest you would normally be looking at things). In many states, you can't buy reading glasses over the counter when they are much stronger than 2.75 diopters.
To summarize, the first term on the right hand side of the thin lens equation depends on what you are trying to do, while the second term depends on the quality of your eyesight. While determining s' is still subjective (that's the nature of the problem), I still prefer this method. Determining s' is a traceable source of uncertainty, while the reading chart is just complete black magic. I did find a use for the reading chart at the store, but not the way they suggested using it. I stood a reading distance away from the chart and tried the "comfort focus" method detailed above while wearing different strengths reading glasses. The ones that were the right ones brought the chart into sharp focus with the minimum of effort.
Saturday, May 11, 2013
Why is it not my birthday today?
I heard this one today at a birthday party. The conversation went like this (the little girl was watching the birthday girl unwrap her presents).
Little girl: I want a present too!
Mom: Not today.
Little girl: Why not?
Mom: Because it's not your birthday.
Little girl: Why is it not my birthday today?
Mom: ....
Yeah, I'm stumped too.
Saturday, April 20, 2013
When boiling water, where does the air that fills up the never-ending bubbles come from?
Aha! How do you know it's air that fills these bubbles? Isn't there a cloud of steam coming from your boiling pot of water, with the bursting bubbles at the surface? Doesn't the water level go down in your pot if you leave it boiling for a long time? See where I am going?
The answer to the question is that it is not air that fills the water bubbles, but water! Water vapor to be exact. Water in the form of gas. Water turns to vapor when it is heated. The vapor is trapped in bubbles that form at the bottom of the pot. The bubbles filled with vapor rise to the surface of the pot, where they burst, releasing the water vapor into the air above the pot. There, the water vapor cools and turns into the steam you see coming off the pot.
The steam dissipates into your kitchen (sometimes ending up as water condensation on cold surfaces), and that much water is lost to the pot. Over time, the water level in the pot goes down.
The answer to the question is that it is not air that fills the water bubbles, but water! Water vapor to be exact. Water in the form of gas. Water turns to vapor when it is heated. The vapor is trapped in bubbles that form at the bottom of the pot. The bubbles filled with vapor rise to the surface of the pot, where they burst, releasing the water vapor into the air above the pot. There, the water vapor cools and turns into the steam you see coming off the pot.
The steam dissipates into your kitchen (sometimes ending up as water condensation on cold surfaces), and that much water is lost to the pot. Over time, the water level in the pot goes down.
Wednesday, April 10, 2013
How do parachutes make it safer to fall?
In my last post, I talked about terminal velocity and how it was different according to the situation. Ducklings have a smaller terminal velocity than humans because they are a lot lighter. Another way to reduce the terminal velocity is to increase the cross sectional area of the falling object. You can think of cross sectional area as what you get if you light up the object from straight above and look at the shadow on the ground. The shadow of a parachute is a lot larger than the shadow of a human. The large cross sectional area of the parachute reduces the terminal velocity of the human attached to it and makes for a much gentler landing.
Sunday, March 31, 2013
How do ducklings get away with falling from great heights without getting hurt?
The thing I like most about this video is watching my husband watch it. There is something endearing about a fully grown man giggling at the sight of falling ducklings.
Now to the question. How can the ducklings get away with falling from a tall tree without getting hurt? Even with a cushion of leaves, if we as people attempted something like this, we would get badly hurt if not killed. So how do the ducklings get away with it?
The short answer is that 1) they don't hit the ground as hard as we would and 2) they are sturdy little animals.
1) is in the realm of physics and deserves some more explanation. Why is it that the ducklings hit the ground less hard than we would? It's a question of terminal velocity and also a matter of how large a force the ground exerts on the ducklings to stop their fall.
When something falls toward the ground, it speeds up. At the same time, air slows down the fall. The drag due to air gets stronger and stronger as the speed of the object increases. Eventually, an equilibrium situation is reached, where the air drag exactly balances out the downward pull of gravity. The object quits accelerating and falls at a steady, maximum speed, called terminal velocity.
Terminal velocity depends on a number of things and can be higher or lower depending upon the situation. For ducklings, terminal velocity is not as high as it is for humans, because ducklings are a lot lighter than humans. If a duckling and a human jump off a tall tree, they will both reach their own terminal velocity. For the human, the higher terminal velocity means that they will hit the ground first, and also harder.
Because of their smaller speed and smaller mass, the ducklings have less momentum than a person would. Having less momentum when they land, they require less force from the ground to come to rest below the tree. Less force means a softer landing.
Now your 3-year old may ask "How do the ducklings know they will be fine?". I can't help with that.
Now to the question. How can the ducklings get away with falling from a tall tree without getting hurt? Even with a cushion of leaves, if we as people attempted something like this, we would get badly hurt if not killed. So how do the ducklings get away with it?
The short answer is that 1) they don't hit the ground as hard as we would and 2) they are sturdy little animals.
1) is in the realm of physics and deserves some more explanation. Why is it that the ducklings hit the ground less hard than we would? It's a question of terminal velocity and also a matter of how large a force the ground exerts on the ducklings to stop their fall.
When something falls toward the ground, it speeds up. At the same time, air slows down the fall. The drag due to air gets stronger and stronger as the speed of the object increases. Eventually, an equilibrium situation is reached, where the air drag exactly balances out the downward pull of gravity. The object quits accelerating and falls at a steady, maximum speed, called terminal velocity.
Terminal velocity depends on a number of things and can be higher or lower depending upon the situation. For ducklings, terminal velocity is not as high as it is for humans, because ducklings are a lot lighter than humans. If a duckling and a human jump off a tall tree, they will both reach their own terminal velocity. For the human, the higher terminal velocity means that they will hit the ground first, and also harder.
Because of their smaller speed and smaller mass, the ducklings have less momentum than a person would. Having less momentum when they land, they require less force from the ground to come to rest below the tree. Less force means a softer landing.
Now your 3-year old may ask "How do the ducklings know they will be fine?". I can't help with that.
Introduction
The idea of this blog is to post questions a 3-year old would ask and the simplest answers I can think of. I will not hide the fact that this is a blog about physics. It has to be, as many of the questions 3-year olds ask have their answers in physics. The reason is that young children wonder about two things: 1) What makes people tick and 2) How nature works. How nature works is the realm of physics.
I was surprised and distraught to realize, as a young professor, that most people in the US perceived physics as being very much disconnected from their world. That surprised me because I have been thinking about my environment in physics term from very early on. To me, it's not a connection, it's completely ingrained.
Teaching physics is a lot of fun, because it allows me to recall being a child and wondering about everything. I find answers in our intro physics books that I did not know or hadn't thought of, and find many more questions. In this blog, I am going to share these questions and answers. You may find the answer to the question your 3-year old asked just the other day. If not, I hope you will relay the questions your 3-year old asks. There may be some I can answer. Evidently, I am defining "3-year old" very broadly.
Wisdom from a 3 year old: Always ask WHY?
I was surprised and distraught to realize, as a young professor, that most people in the US perceived physics as being very much disconnected from their world. That surprised me because I have been thinking about my environment in physics term from very early on. To me, it's not a connection, it's completely ingrained.
Teaching physics is a lot of fun, because it allows me to recall being a child and wondering about everything. I find answers in our intro physics books that I did not know or hadn't thought of, and find many more questions. In this blog, I am going to share these questions and answers. You may find the answer to the question your 3-year old asked just the other day. If not, I hope you will relay the questions your 3-year old asks. There may be some I can answer. Evidently, I am defining "3-year old" very broadly.
Wisdom from a 3 year old: Always ask WHY?
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