Welcome to the World of Real-World Motion!
In your previous physics lessons, you might have spent a lot of time imagining objects moving in a perfect "vacuum" where nothing slows them down except gravity. But look around! In the real world, we have air, water, and rough surfaces.
In this chapter, we are going to look at Non-uniform motion. We will explore why objects don't just keep accelerating forever when they fall and how "hidden" forces like air resistance change the way things move. Whether you’re a future skydiver or just curious about why raindrops don't hit the ground like bullets, this section is for you!
1. The Forces That Slow Us Down: Friction and Drag
When an object moves, there is almost always a force trying to stop it. We call these resistive forces. According to the syllabus, you need to understand two main types:
Frictional Forces
Friction happens when two solid surfaces slide (or try to slide) over each other. Think of pushing a heavy box across a carpet—the carpet "grips" the box, creating a force in the opposite direction of your push.
Viscous and Drag Forces
When an object moves through a fluid (which means a liquid or a gas, like air), it experiences drag.
Example: When you stick your hand out of a moving car window, you feel the air pushing your hand back. That is air resistance, a type of drag force.
The Golden Rule of Drag:
In a vacuum, there is no drag. But in a fluid, the amount of drag depends on how fast you are going.
Important Point: As the speed of an object increases, the drag force acting on it also increases.
Quick Analogy: Imagine walking through a swimming pool. If you walk slowly, it's easy. If you try to sprint through the water, the water pushes back much harder! This is exactly how drag works.
Quick Review:
- Resistive forces always act in the opposite direction to motion.
- Drag increases as speed increases.
2. Falling with Style: Motion in a Gravitational Field
Now, let's look at what happens when you drop an object (like a ball) through the air. This is a classic exam topic! Don't worry if it seems complex; we can break it down into three simple steps.
Step 1: The Moment of Release
At the very start (\( t = 0 \)), the object is not moving yet (\( v = 0 \)).
- Drag: Since speed is zero, drag is zero.
- Resultant Force: The only force is Weight (\( W \)) acting downwards.
- Acceleration: The object starts accelerating at \( 9.81 \text{ m/s}^2 \).
Step 2: Gaining Speed
As the object falls, it gets faster.
- Drag: Because speed is increasing, the drag force increases upwards.
- Resultant Force: Since the upward drag is fighting the downward weight, the total (resultant) force decreases.
- Acceleration: According to \( F = ma \), if the force decreases, the acceleration decreases. (The object is still speeding up, but it's speeding up more slowly).
Step 3: Reaching Terminal Velocity
Eventually, the object goes so fast that the Drag force becomes exactly equal to the Weight.
- Resultant Force: The forces are now balanced, so the resultant force is zero (\( W - D = 0 \)).
- Acceleration: The acceleration becomes zero.
- Velocity: The object stops speeding up and travels at a constant speed called terminal velocity.
Did you know? A human skydiver usually reaches a terminal velocity of about 120 mph (54 m/s) when falling "belly-to-earth"! If they pull their arms in and dive headfirst, they become more "streamlined," which reduces drag and allows them to reach a much higher terminal velocity.
Key Takeaway: When an object reaches terminal velocity, it doesn't stop moving! It just stops accelerating. It continues to fall at a steady, maximum speed.
3. Summary of Motion Graphs
To do well in your AS Physics exams, you should be able to visualize these changes on a graph.
Velocity-Time (\( v-t \)) Graph for Falling with Air Resistance:
1. The graph starts at the origin (0,0).
2. It starts with a steep, constant gradient (representing acceleration \( g \)).
3. The curve starts to flatten out as drag increases.
4. Finally, the graph becomes a horizontal line. This horizontal line represents the terminal velocity.
Acceleration-Time (\( a-t \)) Graph:
1. Starts at \( 9.81 \text{ m/s}^2 \).
2. Curves downwards as the drag builds up.
3. Ends at zero when terminal velocity is reached.
Common Mistake to Avoid: Many students think that when the resultant force is zero, the object stops. Remember Newton’s First Law! If the forces are balanced (resultant force = 0), the object keeps moving at a constant velocity.
4. Memory Aids and Tips
The "D.A.V." Trick
If you get confused about the steps of a falling object, remember what happens to these three things in order:
1. Drag increases.
2. Acceleration decreases.
3. Velocity becomes constant (Terminal Velocity).
Quick Check List:
- Does drag increase with speed? Yes.
- Is acceleration \( 9.81 \text{ m/s}^2 \) at terminal velocity? No, it is 0.
- Are forces balanced at terminal velocity? Yes (Weight = Drag).
Encouraging Note: Non-uniform motion is all about telling a story. If you can describe the "story" of a falling object from the moment it's dropped until it hits a steady speed, you’ve mastered this chapter!
Key Takeaway for the Chapter:
Non-uniform motion occurs when resistive forces like drag change as an object moves. This leads to a decreasing acceleration until the resistive force equals the driving force (like weight), resulting in a constant terminal velocity.