Unit II: Friction & Drag
The Unseen Forces That Oppose Motion.
This is the third chapter of Unit II. It builds on our understanding of Newton's Laws.
← Go back to Chapter 2: Newton's Laws of Motion.
The Problem: Why Do Things Stop?
Newton's First Law tells us an object in motion stays in motion. But in the real world, if you slide a book across a table, it quickly comes to a stop. Why? Because of an unseen force called friction.
Friction is a force that opposes motion (or attempted motion) between surfaces that are in contact. It's caused by microscopic imperfections on the surfaces that catch on each other.
There are two main types of friction: static (when an object is still) and kinetic (when an object is moving). Use the simulator below to discover the difference.
The "Aha!" Moment
As you slowly increase the applied force, notice that the friction force perfectly matches it, keeping the box still. This is static friction. It has a maximum value. Once you push harder than that maximum, the box starts to move, and the friction force drops to a lower, constant value. This is kinetic friction. It's always easier to keep something moving than it is to start it moving!
Air Resistance and Terminal Velocity
Friction doesn't just happen between solid surfaces. When an object moves through a fluid (like air or water), it experiences a similar resistive force called drag. We often call it air resistance.
Unlike kinetic friction, drag is not constant. It increases as an object's speed increases. This leads to a fascinating phenomenon called terminal velocity.
When an object is dropped, it accelerates due to gravity. As its speed increases, the upward force of air resistance also increases. Eventually, the force of air resistance becomes equal to the downward force of gravity. At this point, the net force is zero, the object stops accelerating, and it falls at a constant maximum speed. This is its terminal velocity.
Use the simulator below to drop a bowling ball and a parachute. Which one reaches terminal velocity faster? Why?
The "Aha!" Moment
The parachute has a much larger surface area, so it "catches" more air. This means the force of air resistance becomes equal to the force of gravity at a much lower speed. As a result, the parachute has a much lower terminal velocity than the bowling ball, allowing for a safe landing.
You've Mastered the Forces of Resistance. What's Next?
You now understand the forces that govern motion and the forces that resist it. The next logical step is to explore the concepts of Work and Energy, which provide a powerful new way to analyze motion.