4. A person jumped in water and swimming. The water disturbed and after few minutes
stops the water movement. Why ? Explain the reason.
Answers
Answer:
Humans evolved from sea creatures but—looking at our bodies—you'd never know it. We couldn't be less well suited to moving through water if we tried. We don't float too well, can't breathe for long beneath the surface, and rapidly tire as we thrash through the waves trying to move ourselves along; in a straight race with a dolphin or a shark, you'll always come last! But there's one big advantage we humans do have: we know about science. We understand how forces work and how to use them to our advantage. If you've never thought about swimming as a science, now's the time to start. Apply some scientific thinking and you'll find you can swim much more effectively. If you're a nervous nonswimmer, thinking about the solid science that keeps people afloat can give you enough confidence to break through your fear. So what are we waiting for? Let's take the plunge—with a closer look at the science of swimming!
Photo: Swimming takes humans back from the land to the ocean—or the pool! You have to apply forces to move yourself through the water and other forces slow you down. Understand those forces and you can swim much more effectively. Photo by R. Jason Brunson courtesy of US Navy.
Contents
What is swimming?
Newton's laws of swimming
Minimizing your drag
Swimming efficiently
Floating and buoyancy
Question: Does swimming warm the pool?
Find out more
What is swimming?
That sounds like a trivial question, but it helps to be clear. Swimming is moving your body through water (a moderately viscous fluid) that's either still (as in a swimming pool), turbulent (as in the ocean), or somewhere in between. If you're swimming completely under the surface (for example, scuba diving), you're moving through relatively still water; other times, you're going to be moving along at the more turbulent interface between air and water, with your legs, arms, head, and body moving from one element to the other and back again, speeding up or slowing down as they cross the border.
Photo of a swimmer breathing to the side.
Photo: Even the best swimmers have to move along the choppy interface between air and water. It's the most inefficient place to swim, but the only place you can do it if you need to breathe air. Photo by Jennifer A. Villalovos courtesy of US Navy.
Water versus air
Before we can understand the science of swimming, it helps to remember that air (a gas) is very different from water (a liquid). The biggest difference is that water is much more dense (the same volume of it weighs much more) and viscous (in other words, thicker—in the same way that treacle is more viscous than water).
The difference between air and water makes a huge difference to how we can move on air and land. When you walk on land, the main thing your body has to do is work against gravity (lifting your legs, swinging your arms, and keeping you from toppling over through constant adjustments of your balance) and a little bit of friction where your shoes meet the ground. If you move more quickly (say, on a bicycle), air resistance becomes a more important force than gravity; unless you're walking into a really strong wind, you barely notice the air while you're walking. When you're in the water, gravity is much less important because your buoyancy (tendency to float) largely cancels it out. The main force you have to think about as a swimmer is drag—water resistance. We'll come to that in a moment.
Other differences between water and air are important if you swim outdoors, particularly in the winter months: because water is much more dense than air (more precisely, because it contains many more molecules per unit of volume, and those molecules are bonded together), it removes heat from your body about 25–40 times faster than air at the same temperature. (That's why surfers and "wild" outdoor swimmers tend to wear wetsuits to avoid hypothermia, the very dangerous cooling of the body's core that can kill you.) Because water is so much denser than air, it takes a much longer time to warm up. That's why the ocean temperature typically lags behind the land temperature by 2–3 months in countries such as the East Coast of the United States and the UK (where the ocean is often warmest in September).
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Newton's laws of swimming
If you love science but swimming scares you, you'll find it very helpful—as I did when I was learning to swim—to think about Newton's three laws of motion. Among the most fundamental rules of physics, these three basic principles are enough to explain completely the movement of almost every single object you're ever likely to come across.
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