Earth & Space Science

Earth & Space Science

  • About This Project
    • Preface/About
    • Author/Contributors
    • For Investors/Donors
    • Teaching Guide
  • Ch 1 – Our Place in the Universe
    • Chapter Introduction
    • 1.1 Our Cosmic Address
    • 1.1.1 Overview
    • 1.1.2 What do we mean when we say “Earth is a planet”?
    • 1.1.3 What is our solar system?
    • 1.1.4 What is a galaxy?
    • 1.1.5 What is the universe?
    • 1.1 Review: Our Cosmic Address
    • 1.2 The Scale of Space
    • 1.2.1 Overview
    • 1.2.2 How Big is the Earth–Moon System?
    • 1.2.3 How Big is our Solar System?
    • 1.2.4 How far are the stars?
    • 1.2.5 How big is the Milky Way Galaxy?
    • 1.2.6 How big is the universe?
    • 1.2 Review: The Scale of the Universe
    • 1.3 Spaceship Earth
    • 1.3.1 How is Earth moving in our solar system?
    • 1.3.2 How is our solar system moving in the Milky Way Galaxy?
    • 1.3.3 How does our galaxy move relative to other galaxies in the universe?
    • 1.3 Review
  • Ch 2 – Understanding the Sky
    • Chapter Introduction
    • 2.1 Our Everyday View of the Universe
    • 2.1.1 What do we see in the local sky?
    • 2.1.2 What is the celestial sphere?
    • 2.1.3 Why do stars rise and set?
    • 2.1.4 Why do we see different constellations at different times of year?
    • 2.1 Review
    • 2.2 Seasons
    • 2.2.1 What causes the seasons?
    • 2.2.2 How do seasons differ around the world?
    • 2.2.3 Does the orientation of Earth’s axis ever change?
    • 2.2 Review
    • 2.3 Viewing the Moon: Phases and Eclipses
    • 2.3.1 Why do we see phases of the Moon?
    • 2.3.2 When do we see different phases of the Moon in our sky?
    • 2.3.3 Why do we always see the same face of the Moon?
    • 2.3.4 What are eclipses?
    • 2.3 Review
    • 2.4 Planets in the Night Sky
    • 2.4.1 How do we recognize planets in the sky?
    • 2.4.2 Why do the planets “wander”?
    • 2.4 Review
  • Ch 3 – How Science Discovered the Earth
    • Chapter Introduction
    • 3.1 The Ancient View of Earth
    • 3.1.1 How did the ancient Greeks learn that Earth is round?
    • 3.1.2 Why didn’t the ancient Greeks realize that Earth orbits the Sun?
    • 3.1 Review
    • 3.2 The Copernican Revolution
    • 3.2.1 How did the idea of Earth as a planet gain favor?
    • 3.2.2 How did Galileo seal the case for Earth as a planet?
    • 3.2 Review
    • 3.3 The Nature of Modern Science
    • 3.3.1 How does science work?
    • 3.3.2 What is a “theory” in science?
    • 3.3.3 What is the value of science?
    • 3.3 Review
    • 3.4 The Fact and Theory of Gravity
    • 3.4.1 What is gravity?
    • 3.4.2 How does gravity hold us to the ground and make objects fall?
    • 3.4.3 Why does gravity make planets round?
    • 3.4.4 How does gravity govern motion in the universe?
    • 3.4 Review
  • Chapter 4 – Planet Earth
    • Chapter Introduction
    • 4.1 A Planetary Overview
    • 4.1.1 What does Earth look like on the outside?
    • 4.1.2 What does Earth look like on the inside?
    • 4.1.3 How has Earth changed through time?
    • 4.1.4 How do we study the Earth?
    • 4.1 Review
    • 4.2 Earth System Science
    • 4.2.1 What are Earth’s four major systems?
    • 4.2.2 What drives Earth system changes?
    • 4.2.3 What IS energy and how do we measure it?
    • 4.2 Review
    • 4.3 Earth In the Context of Other Worlds
    • 4.3.1 How does Earth compare to other worlds of our solar system?
    • 4.3.2 Could there be life on other worlds?
  • Chapter 5 – Earth Through Time
    • Chapter Introduction
    • 5.1 Learning from Rocks and Fossils
    • 5.1.1 How do rocks form?
    • 5.1.2 What are fossils?
    • 5.1.3 How do we learn the ages of rocks and fossils?
    • 5.1 Review
    • 5.2 Shaping Earth’s Surface
    • 5.2.1 How do continents differ from oceans?
    • 5.2.2 What processes shape continents?
    • 5.2.3 What dangers do geological changes pose?
    • 5.2 Review
    • 5.3 Plate Tectonics — The Unifying Theory of Earth’s Geology
    • 5.3.1 What evidence led to the idea that continents move?
    • 5.3.2 How does the theory of plate tectonics explain Earth’s major features?
    • 5.3 Review
    • 5.4 A Brief Geological History of Earth
    • 5.4.1 What major changes mark Earth’s fossil record?
    • 5.4.2 What killed the dinosaurs?
    • 5.4.3 Have we humans started a new geological epoch?
    • 5.4 Review
  • Chapter 6 – Air and Water
    • Chapter Introduction
    • 6.1 Atmosphere and Hydrosphere
    • 6.1.1 What exactly is the atmosphere?
    • 6.1.2 How is water distributed on Earth?
    • 6.1.3 How does water cycle through the hydrosphere and atmosphere?
    • 6.1 Review
    • 6.2 Global Winds and Currents
    • 6.2.1 What drives global winds and currents?
    • 6.2.2 What is the general pattern of winds on Earth?
    • 6.2.3 What is the general pattern of ocean currents?
    • 6.2 Review
    • 6.3 Weather and Climate
    • 6.3.1 What is the difference between weather and climate?
    • 6.3.2 How and why does climate vary around the world?
    • 6.3.3 How do we measure and predict the weather?
  • Chapter 7 – Human Impact on the Climate
    • Chapter Introduction
    • 7.1 The Basic Science of Global Warming
    • 7.1.1 What is the greenhouse effect?
    • 7.1.2 How is human activity strengthening Earth’s greenhouse effect?
    • 7.1.3 How do we know that global warming is really happening and is human-caused?
    • 7.1.4 How does human-caused climate change compare to natural climate change?
    • 7.1 Review
    • 7.2 Consequences of Global Warming
    • 7.2.1 What are the major consequences of global warming?
    • 7.2.2 How do scientists predict future consequences of global warming?
    • 7.2.3 How will climate changes affect you and others around the world?
    • 7.2 Review
    • 7.3 Solutions to Global Warming
    • 7.3.1 What existing technologies could solve the problem of global warming?
    • 7.3.2 What future technologies might help even more?
    • 7.3.3 What does it take to implement a solution?
    • 7.3.4 What will your world look like AFTER we solve global warming?
    • 7.3 Review

I was wondering...

How exactly does the ground “push back” against gravity?

As you can see in Figure 3.25, the ground prevents you from falling because it pushes upward on you, so that the force of gravity pulling you downward is precisely balanced by the upward force from the ground. But how exactly does the ground push upward? The answer is that the atoms and molecules in solid objects resist being pushed together, which leads to balance as follows:

  • The downward force of gravity is your weight. (Mathematically, this force is your mass times the acceleration of gravity, or mg.)
  • The greater your weight, the harder you are pushing down on the ground, and therefore the more your weight is trying to push the atoms and molecules in the ground together.
  • Those atoms and molecules therefore push back, which in this case means it is pushing upward on you. As long as the ground does not crack or open up, the upward push will automatically balance your weight pushing down.

In other words, as gravity pulls you downward against the ground (or any other solid object, like a chair or table or floor in a tall building) , the atoms and molecules of the ground automatically push back against you. That is the reason you can stand on solid ground.

Show Teachers Notes

This box explains the basic reason why the ground pushes back with what, in physics, is called the “normal force.” If you want to go a bit deeper into it, you can talk to students about what is sometimes called the “illusion of solidity,” as follows:

Consider pushing down on a table with your hand. The table will feel solid, but in fact, it is almost entirely made of empty space. To understand why, remember that nearly all the mass of the table is contained in the nuclei of its atoms, while the volume of each atom (essentially the volume occupied by the electrons) is more than a trillion times the volume of its nucleus. This means that the nuclei of adjacent atoms are nowhere near to touching one another, which is why we say the table is almost entirely made of empty space. If we could somehow pack all the table’s nuclei together, the table’s mass would fit into a microscopic speck. (Note: Although we cannot pack matter together in this way, nature can and does—in the strange objects called neutron stars, in which a mass greater than that of the Sun is compressed down to a ball just a few kilometers across.)

So why does the table feel solid? The solidity comes from the way the atoms push back, just as described for the ground in the box. More specifically, this push back comes from a combination of electrical interactions between the charged particles in its atoms and the strange quantum laws governing the behavior of electrons.

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