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These three short online lessons discuss asteroids, comets and meteors.

Introduction to cell theory--the idea that 1) all living things are made of one or more cells, 2) cells are the basic unit of life and 3) all cells come from other cells. Explore the roles that Hooke, Leeuwenhoek and others played in developing cell theory.

Hooke and Leeuwenhoek were two of the first scientists to use microscopes to study the microscopic world of cells. Hooke coined the term "cell" after observing the tiny compartments in cork, while Leeuwenhoek discovered a variety of living creatures in pond water, blood, and other samples. They contributed to the cell theory by suggesting that cells are the fundamental units of life and structure, and that all living things consist of one or more cells that originate from other cells by division. 

All living things are made up of cells, which is the smallest unit that can be said to be alive. An organism may consist of one single cell (unicellular) or many different numbers and types of cells (multicellular).

Plant cells have a cell wall in addition to a cell membrane, whereas animal cells have only a cell membrane. Plants use cell walls to provide structure to the plant. Plant cells contain organelles called chloroplasts, while animal cells do not. Chloroplasts allow plants to make the food they need to live using photosynthesis.

• Identify when forces are balanced vs unbalanced.
• Determine the sum of forces (net force) on an object with more than one force on it.
• Predict the motion of an object with zero net force.
• Predict the direction of motion given a combination of forces.

Determining how fast something will be traveling upon impact when it is released from a given height. 

Introduction to the cell.

In this chapter, we’ll use vectors to expand our understanding of forces and motion into two dimensions. Most real-world physics problems (such as with the game of pool pictured here) are, after all, either two- or three-dimensional problems and physics is most useful when applied to real physical scenarios. We start by learning the practical skills of graphically adding and subtracting vectors (by using drawings) and analytically (with math). Once we’re able to work with two-dimensional vectors, we apply these skills to problems of projectile motion, inclined planes, and harmonic motion.

Other objects in the solar system are discussed in this online textbook viz. asteroids, meteorites, comets and dwarf planets.

Overview of animal and plant cells. Topics include cell walls, vacuoles, chloroplasts, peroxisomes, lysosomes, mitochondria, etc.