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Hold a rubber band with your index fingers, and stretch it. There are four conceptually distinct forces at play here:
Already in this simple example we have found several forces. They are distinct because each has a different answer to the questions "What is exerting the force? What is the force being exerted upon?" The forces will have different effects; they even act in different directions.
The four forces are related to each other by two conditions. One is that the rubber band isn't going anywhere; then the forces on it must be balanced (we will discuss this more in the next chapter). The other relationship is Newton's Law of Interaction (often called Newton's Third Law). The Law states that for any two objects A and B, the force of A on B is equal and opposite to the force of B on A.
This law is completely general: it holds for all kinds of forces. For another example, find the magnets in the kit, and hold two in each hand. Depending on how you are holding them, they might attract, or repel. Don't try to figure it out! Just observe what is the force of one stack of magnets on the other, and how that is related to the force of the second stack back on the first.
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Mass, weight, and density