An atom's structure consists of a dense, positively charged nucleus at the center, made of protons and neutrons, and negatively charged electrons orbiting the nucleus in shells or orbitals. The number of protons in the nucleus determines the element's atomic number, while a neutral atom has an equal number of protons and electrons. Electrons exist in specific energy levels, governed by quantum mechanics, and their arrangement determines the atom's chemical properties.
A simple machine is a basic mechanical device that makes work easier by changing the direction or magnitude of a force. The six classical simple machines are the lever, inclined plane, wheel and axle, pulley, wedge, and screw. These machines reduce the effort needed to complete a task by trading force for distance, using a principle called mechanical advantage.
A cell is the basic structural and functional unit of all living organisms, acting as the smallest unit of life that can replicate on its own. Every living thing, from single-celled bacteria to complex animals and plants, is made of cells. Cells have three main components: a cell membrane, a nucleus, and cytoplasm. They also contain organelles, which are specialized structures that perform specific functions like creating energy, making proteins, and storing genetic material.
Earth is the third planet from the sun and the only known planet to support life, thanks to its liquid water, atmosphere, and moderate temperature. It is the fifth-largest planet in the solar system and is about 4.5 billion years old. Its surface is 71% water-covered, and the planet is a slightly flattened sphere called an oblate spheroid.
