Grade 7 General Science: Units 6 & 7 - Earth in Space, Motion, Force, and Energy

Unit 6: Earth in Space – Introduction and Shape of the Earth

Definition of Earth: Earth is one of the eight planets in our Solar System and is the only known planet to support life. Geologists study its materials, processes, products, physical nature, and history.
Ancient Local Ideas (Flat Earth Assumption):
- Early Cultures: Ancient Greeks, Sumerians, Babylonians, Egyptians, and Vikings believed Earth was a flat disc or plane surrounded by water, based on immediate visual observation.
- Ancient Chinese: Believed Earth was a flat square shape surrounded by a round, egg-shaped heaven.
- Flat Earth Society: Claims Antarctica is an ice wall surrounding a disc-shaped Earth.
- Vantage Point Fallacy: Ancient people believed Earth was flat because it looks flat from the ground due to its enormous size.
Historical Shift to Spherical Assumption:
- Pythagoras ( $500\,B.C.$ ): First proposed a spherical Earth based on aesthetic grounds rather than physical evidence.
- Aristotle ( $384-322\,B.C.$ ): Provided physical evidence for a sphere, including:
  - Ships disappearing "hull first" when sailing over the horizon.
  - Different constellations being visible at different latitudes.
True Shape: Oblate Spheroid:
- Scientists describe the shape as an "Oblate spheroid."
- Oblate: Refers to a slight elliptical appearance.
- Spheroid: Means almost a sphere but not perfectly round.
- Physical Characteristics: Earth is flattened at the poles and bulges in the middle (equatorial bulge).

Evidences Supporting the Spherical Shape of the Earth

Circumnavigation: Traveling across the world in a straight path (e.g., in a fast plane) leads back to the starting point.
Earth’s Curved Horizon: Seen from ships, planes, or high cliffs. The horizon widens and becomes circular as altitude increases.
Ship’s Visibility: When two ships approach at a distance, the front ship is seen before the back ship. On a flat surface, both would appear simultaneously.
Sunrise and Sunset: Times vary by location. As Earth rotates West to East, Eastern locations see the sun earlier. On a flat Earth, everyone would see sunrise at the same time.
Lunar Eclipses: The shadow cast by Earth on the moon is always circular. Only a sphere can consistently cast a circular shadow at all angles.
Space Photographs: Rockets and satellites provide the most modern and convincing proof by capturing the curved edge of Earth.
Changing Altitude of the Sun: The sun appears at low levels in the morning/evening and high levels at noon due to the curved surface.

Dimensions of the Earth

Astronomical Unit (AU): The average distance from Earth to the Sun is approximately $1\,AU$ , which equals $1.496 \times 10^8\,km$ .
Mass of Earth: Approximately $5.974 \times 10^{24}\,kg$ .
Diameter Details:
- Equatorial Diameter: $12,756\,km$ .
- Polar Diameter: $12,714\,km$ .
- Equatorial Bulge: The diameter is wider at the Equator.
Circumference Details:
- Equatorial Circumference: $40,075\,km$ .
- Polar (Meridional) Circumference: $40,008\,km$ .
Orbital and Rotational Periods:
- Rotational period: $23.9345\,hours$ .
- Orbital period: $365.256\,days$ .
Angular Distance: Also called angular separation; it is the angle between two sightlines or point objects as viewed from an observer.

Structure of the Earth: External Structure (Atmosphere)

Atmosphere Definition: A thin layer of gases held by gravitational attraction, containing abiotic (non-living) and biotic (microscopic organisms like bacteria) matter.
Atmospheric Composition:
- Nitrogen ( $N_2$ ): $78\%$ .
- Oxygen ( $O_2$ ): $21\%$ .
- Carbon dioxide ( $CO_2$ ): $0.03\%$ .
- Other gases: $0.97\%$ .
Layers of the Atmosphere:
1. Troposphere: The lowest layer where humans live. Contains $75\%$ of all air and almost all water vapor. Weather (clouds, rain, snow) occurs here. Temperature decreases with height. The boundary layer is the bottom; the top is the Tropopause ( $7-10\,km$ at poles, $17-18\,km$ at the equator).
2. Stratosphere: Extends to $50\,km$ . Contains the Ozone layer, which absorbs ultraviolet (UV) radiation, causing temperature to increase with height. Upper boundary is the Stratopause.
3. Mesosphere: Extends from $50\,km$ to $85\,km$ . Temperature decreases with height. Contains the coldest temperatures (approx. $-90^{\circ}C$ ) at the Mesopause (boundary with thermosphere).
4. Thermosphere: Extends from $90\,km$ to $500-1,000\,km$ . Temperature increases with height due to absorption of X-rays and UV radiation. Boundary is the Thermopause.
5. Exosphere: The highest layer, located between $700\,km$ and $10,000\,km$ above the surface.

Structure of the Earth: Internal Structure

The Crust: The coldest, outermost layer made of solid rock (basalt and granite).
- Oceanic Crust: Denser, $5-10\,km$ thick, mainly basalt.
- Continental Crust: Less dense, thicker, mainly granite.
The Mantle: The thickest layer between the crust and core. Composed of oxygen, silicon, and magnesium. Acts as a viscous liquid. The upper portion is the Asthenosphere.
- Lithosphere: The solid outer part consisting of the crust and the brittle upper portion of the mantle.
The Core: The hot, dense center composed of iron and nickel alloy. Temperatures reach up to $12,000^{\circ}F$ .
- Outer Core: Liquid state due to high heat.
- Inner Core: Solid state despite higher temperatures because of extreme pressure.

Earth Movements and Their Effects

Rotation: Motion of Earth around its axis (imaginary line through North and South poles) from West to East.
- Duration: $23\,hours$ , $56\,minutes$ , and $4\,seconds$ (Standard day length: $24\,hours$ ).
- Effects: Diurnal cycle (day and night); time differences ( $1\,hour$ per $15^{\circ}$ of longitude); deflection of winds and currents (Coriolis effect); daily rise and fall of tides.
Revolution: Motion of Earth around the Sun.
- Duration: $365.24\,days$ .
- Orbital Path: Elliptical.
- Perihelion: Closest point to Sun ( $147\,million\,km$ ) on January 3rd.
- Aphelion: Furthest point from Sun ( $152\,million\,km$ ) on July 4th.
- Speed: Average $27\,km/s$ (faster at perihelion, slower at aphelion).
- Seasons: Caused by Earth's tilt of $23.5^{\circ}$ . Solstices and Equinoxes define seasonal transitions.

Earth Systems and Cycles

Four Main Spheres:
1. Geosphere (Lithosphere): Land, rocks, minerals, and soil.
2. Atmosphere: Blanket of gases surrounding the planet.
3. Hydrosphere: All water (liquid, vapor, and ice).
4. Biosphere: All living organisms.
Biogeochemical Cycles:
- Water Cycle: Evaporation (liquid to gas), Condensation (gas to liquid/clouds), Precipitation (rain, snow, hail), and Transpiration (water movement through plants).
- Carbon Cycle: Carbon atoms travel from atmosphere to Earth and back via photosynthesis, consumption, ocean intake, decay, and respiration.
- Nitrogen Cycle: Maintenance of nitrogen balance via Nitrogen fixation, Nitrification, Assimilation, Ammonification, and De-nitrification. Bacteria complete most steps except assimilation.
Measurement of Large Quantities:
- GPS (Global Positioning System): Network of satellites to determine precise locations and measure continental drift.
- Laser Ranging (Laser Radar): Optical sensing technology for measuring Earth's size and plate motions.

Unit 7: Motion, Force, and Energy

Definition of Motion: Continuous change of position from a reference frame over time.
Types of Motion:
1. Rectilinear Motion: Moving in a straight line (e.g., walking on a straight road, a falling ball).
2. Curvilinear Motion: Moving along a curved path (e.g., throwing a basketball, moon orbiting Earth). Circular motion is a special case.
3. Rotatory (Rotational) Motion: An object spinning on its own axis (e.g., spinning top, Earth’s rotation, clock hands).
4. Oscillatory Motion: To-and-fro, back-and-forth, or up-down motion (e.g., pendulum, swinging swing, boat tossing on a river).

Force and Gravitational Force

Definition of Force: A push or pull resulting from interaction with another object. An external agent that changes a body's state of rest or motion.
Categories of Force:
1. Contact Forces: Physical touch (e.g., frictional force, spring force).
2. Non-contact Forces: Action at a distance (e.g., gravity, magnetism).
Gravitational Force (Gravity):
- Discovered by Sir Isaac Newton ( $1642–1727$ ).
- Attractive force between masses; always a pull, never a push.
- Weight: The measure of the pull of Earth's gravity on an object.
Effects of Force:
- Move a stationary object.
- Change the speed of a moving object (acceleration/deceleration).
- Stop a moving object.
- Change the direction of motion.
- Change the shape or size of an object (deformation).

Measuring Force and Energy Concepts

Measuring Device: Newton meter or Spring balance.
- Unit: Newton ( $N$ ).
- Components: Hook (to hang body), Graduated scale (measurement units), Pointer (indicates weight), Spring ( $stretches proportional to force$ ).
Energy: A property of matter that can be transferred or converted but not created or destroyed.
Forms of Energy: Chemical, Elastic, Electrical, Gravitational Potential, Heat/Thermal, Kinetic, Light, Sound, Nuclear/Atomic.
Energy Conversion: Changing one form to another.
- Transducers: Devices that transform energy (e.g., Battery: Chemical to Electrical; Generator: Mechanical to Electrical; Motor: Electrical to Mechanical).
Energy Sources:
- Renewable: Naturally replenished, inexhaustible (Solar, Wind, Geothermal, Hydropower, Biomass).
- Non-renewable: Finite, take millions of years to form, higher carbon footprint (Coal, Petroleum, Natural Gas, Nuclear).

Conservation and Environmental Issues

Strategies for Conservation (The 3 R's):
1. Reduce: Use less energy and natural resources.
2. Reuse: Use products more than once.
3. Recycle: Process used materials into new items.
Resource Depletion: Exhaustion of raw materials (deforestation, overconsumption, mining).
Environmental Degradation: Deterioration of the environment (air, water, soil) through depletion of resources, habitat destruction, and pollution.
- Human Causes: Urbanization, industrialization, overpopulation.
- Natural Causes: Floods, droughts, fires.

Questions & Discussion

Activity 7.1 Question: When moving in a bus, are you at rest or in motion?
- Response: You are at rest with respect to the bus seat but in motion with respect to the ground.
Exercise 6.1, Question 1: The Earth's actual shape is most correctly described as?
- Answer: An oblate sphere.
Exercise 6.1, Question 2: Which object best represents a scale model of Earth?
- Answer: A Ping-Pong ball (due to being nearly spherical).
Exercise 6.4, Question 2: Which layer of the earth is liquid?
- Answer: The outer core.
Exercise 7.7, Question 1: Which is a nonrenewable energy resource?
- Answer: Coal.
Review Exercise Question: Why does an object thrown upwards come down?
- Response: Because of the force of gravity pulling it toward the center of the Earth.