Why is it easier to float in salt water?

The short answer is "because salt water is denser than fresh water". That begs the question "so how does that help floating?"

To answer that question, we need to talk about Archimede's Principle. Archimede's Principle determines the strength of buoyancy. When you float in water, there are two main forces at work: gravity pulling you downward and buoyancy, holding you up so you don't sink. To be able to float, the buoyancy has to counteract the gravity. And in salt water, buoyancy is stronger.

Archimede's principle states that the strength of buoyancy is equal to the weight of the water you displaced by being in it. Consider a boat floating on water (forgive my art):

The boat has displaced a certain amount of water. The weight of that volume of water is equal to the buoyancy force that allows the boat to float. The more water the boat displaces, the higher the buoyancy force, because a larger volume of water weighs more. The boat will sink until there is enough of it under water to increase the buoyancy to a level where it can support the boat against gravity.

This, by the way is why boats need large empty spaces in order to be able to float. You want the boat to be able to displace a lot of water, while not being too heavy. Otherwise, even a large buoyancy becomes insufficient to counteract the downward pull of gravity.

Back to our question. How does the higher density of salt water help with the whole floating deal? In Archimede's Principle, what sets the strength of the buoyancy is the weight of the displaced water. Weight can be increased in two ways: 1) having a bigger volume, as discussed above, or 2) being denser. If the water is dense, then a given volume of it will be heavier and therefore, the buoyancy force will be larger. This is why it is easier to float in salt water than in fresh water.

Quiz: Is it easier to float in oil or in water?

Dad, I'm hungry! Why is the spaghetti taking so long to cook?

Growing up in Switzerland, pasta came with a special set of instructions on the box for cooking at higher elevation.

Pasta is cooked by immersing it in boiling water for some amount of time. At sea level, the boiling point of water is 100 degrees Celsius. At higher elevations, the boiling point is a few degrees lower. With decreasing atmospheric pressure, it is easier for the water to evaporate and it requires less energy (lower heat).

At 1000 meters above sea level, where a good number of Swiss towns are found, the boiling point is down to 96 degrees Celsius. At 2000 meters, were the lower ski resorts are, the boiling point is 93 degrees. And at 3000 meters, were the top of the slopes and mountain cabins are found, the boiling point is down to 90 degrees.

If you are hiking and you are attempting to cook pasta, instead of immersing it in water that is at 100 degrees Celsius, you are immersing it in water that is 10 degrees cooler. You will have to compensate by allowing the pasta to cook for longer.

Why do dogs (and cats) pant when they are hot?

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.

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.

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?

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

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.

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.