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Studying how objects fall through the atmosphere and the concept of terminal velocity.
If you dropped a bowling ball and a feather at the same time, which would hit the ground first? The answer depends entirely on whether you are standing in your backyard or on the surface of the Moon!
Air might feel like 'nothing,' but it is actually a fluid made of gas molecules. When an object falls through the atmosphere, it must push these molecules out of its way. This interaction creates air resistance, a type of fluid friction that acts in the opposite direction of motion. Unlike gravity, which is constant near Earth's surface, air resistance is dynamic. It depends on two main factors: the surface area of the object and its speed. The faster an object moves, or the wider it is, the more air molecules it hits every second, increasing the upward force pushing against it.
Quick Check
If an object is falling downward, in which direction does the force of air resistance act?
Answer
Upward (opposite to the direction of motion).
Imagine a skydiver with a weight () of . 1. Right after jumping, is , so they accelerate quickly. 2. As they speed up, increases to . The net force is now downward. 3. Finally, reaches . Since , they have reached terminal velocity.
Quick Check
What is the acceleration of an object once it has reached terminal velocity?
Answer
The acceleration is because the forces are balanced.
A vacuum is a space that contains no matter—no air, no gas, nothing. In a vacuum, there is no air resistance to oppose gravity. This means that all objects, regardless of their mass or shape, fall with the exact same acceleration of . On Earth, a feather falls slowly because its large surface area and small weight allow it to reach terminal velocity almost instantly. In a vacuum chamber, however, that same feather will drop like a stone, hitting the ground at the same time as a bowling ball.
Consider two identical pieces of paper: one flat and one crumpled into a ball. 1. Both have the same mass, so gravity () pulls on them equally. 2. The flat paper hits more air molecules because of its large surface area, creating high quickly. 3. The crumpled ball hits fewer molecules, so it accelerates for a longer time before balances gravity. 4. Result: The crumpled ball hits the ground first because it has a higher terminal velocity.
A box is falling. Calculate the net force if the air resistance is currently . 1. Calculate the force of gravity: . 2. Identify the direction: is downward (), is upward (). 3. Subtract the forces: downward.
Which of the following would increase the air resistance acting on a falling object?
When an object reaches terminal velocity, the net force is:
In a vacuum chamber on Earth, a feather will fall slower than a lead ball.
Review Tomorrow
In 24 hours, try to explain to a friend why a crumpled piece of paper falls faster than a flat one using the term 'surface area'.
Practice Activity
Find two identical plastic bags. Crumple one into a tight ball and leave the other open. Drop them from the same height and observe the difference in their fall times.