Unit II: Introduction to Forces
The Invisible Push and Pull That Governs All Motion.
This is the first chapter of Unit II. It builds on our knowledge of acceleration from the previous unit.
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The Problem: Why Does Motion Change?
In Unit I, we became experts at *describing* motion. We can calculate velocity and acceleration. But we never asked the most important question: Why? Why does a ball start moving when you kick it? Why does a car slow down when you hit the brakes? Why does an apple fall from a tree?
The answer to all these questions is the same: a force. A force is simply a push or a pull. It's an invisible interaction that can change an object's state of motion. Understanding forces is the key to unlocking the "why" behind the "how" of motion.
The First Step: Classifying Forces
To make sense of the world, scientists love to categorize things. We can sort all forces into two main types:
- Contact Forces: These forces require physical touching. Examples include pushing a box (an applied force), the floor pushing up on your feet (the normal force), and the rope pulling a wagon (tension).
- Field Forces (or Action-at-a-Distance): These forces act across empty space without any physical contact. The most famous example is gravity. Magnetism and electrostatic forces are also field forces.
An Example: The FBD of a Falling Apple
Before you build your own, let's look at a simple example. Imagine an apple falling from a tree. If we ignore air resistance, what is the only force acting on it?
The answer is gravity. The Free-Body Diagram for the apple would look like this:
Notice how we represent the apple as a simple dot and draw one arrow pointing down to represent the force of gravity (F_g). Now, you're ready to try it yourself.
The "Aha!" Moment: The Free-Body Diagram
Drawing a full picture of a car, a person, and the ground is complicated. Physicists needed a simpler way to visualize only the forces acting on an object. The "Aha!" moment was the invention of the Free-Body Diagram (FBD).
An FBD simplifies a complex situation by representing the object as a single dot and drawing arrows (vectors) to represent all the forces acting *on* it. The direction of the arrow shows the force's direction, and its length can represent the force's magnitude (strength).
Your Mission: Select a scenario below and build its free-body diagram by dragging the correct forces from the palette onto the canvas.
Key Takeaways & The Concept of Net Force
In the interactive above, you discovered the two forces acting on the book: Gravity pulling it down and the Normal Force from the table pushing it up. Because the book isn't moving, these forces must be perfectly balanced.
This leads to the concept of Net Force (ΣF). The net force is the sum of all forces acting on an object. When the forces are balanced, the net force is zero, and the object's motion does not change. This is the core idea behind Newton's First Law of Motion, which we will explore in the next chapter.
- A force is a push or a pull that can change an object's motion.
- A Free-Body Diagram (FBD) is a simplified drawing used to visualize the forces acting on an object.
- The Net Force is the vector sum of all forces. If the net force is zero, the object is in equilibrium (not accelerating).
You Can Now Visualize Forces. What's Next?
You've learned how to identify and draw forces. The next step is to explore the three fundamental laws that Isaac Newton discovered, which precisely describe the relationship between force, mass, and acceleration.