Summary
Highlights
Air resistance is the force exerted by air on moving objects, directly proportional to the object's velocity. It always acts opposite to the direction of motion, causing objects to slow down. Initially, a fast-moving object experiences significant air resistance, leading to rapid deceleration. However, as velocity decreases, air resistance also lessens, meaning the rate of slowing itself diminishes over time.
Without air resistance, a projectile follows a perfect, symmetrical parabolic path, where horizontal displacement in the first half equals that in the second half. With air resistance, the path is altered; the object doesn't reach the same height or travel as far. Importantly, the horizontal displacement in the first half of its motion is greater than in the second half because air resistance continuously slows the object's forward velocity.
The graphs for position, velocity, and acceleration primarily focus on the y-direction for objects experiencing air resistance. For a free-falling object, acceleration starts at 9.81 m/s² (due to gravity alone) because there's no initial velocity, hence no air resistance. As velocity increases, air resistance grows, reducing the net downward force and thus decreasing acceleration. This continues until air resistance balances gravity, at which point acceleration becomes zero, and the object reaches terminal velocity – its maximum possible speed.
The velocity graph shows a rapid increase in velocity initially, then a gradual leveling off as air resistance takes effect. The slope of the velocity graph represents acceleration. As acceleration decreases, the slope of the velocity graph lessens until it becomes flat, indicating constant velocity—this flat part signifies terminal velocity, where acceleration is zero.
When an object falls from air into water, it first reaches terminal velocity in the air. Upon entering the water, the resistance (buoyant force) significantly increases because water exerts a much stronger opposing force than air. This causes a sudden negative acceleration, slowing the object down. As the object slows, the buoyant force decreases until it balances gravity again, establishing a new, lower terminal velocity in the water.
The principles apply to skydivers: they initially accelerate at 9.81 m/s² until air resistance builds up, leading to a terminal velocity without a parachute. When the parachute opens, its large surface area dramatically increases air resistance, creating a massive upward force (negative acceleration). This rapidly decreases the skydiver's velocity until a much smaller, safer terminal velocity is achieved for landing.