When watching skydivers jump from an airplane, you may notice that after a while, they stop accelerating and seem to fall at a constant speed. This constant speed is known as terminal velocity. But what causes a parachutist to reach terminal velocity? Understanding terminal velocity helps us learn more about physics concepts like gravity, air resistance, and balance of forces. In this topic, we will explain why a parachutist reaches terminal velocity in simple language that’s easy for everyone to understand.
What Is Terminal Velocity?
Terminal velocity is the maximum constant speed an object reaches when falling through a fluid, such as air. At terminal velocity, the downward force of gravity is exactly balanced by the upward force of air resistance (also called drag). This balance means the parachutist no longer accelerates and continues falling at a steady speed.
Forces Acting on a Falling Parachutist
1. Gravity
Gravity is a force that pulls the parachutist toward the Earth. It is constant and depends on the parachutist’s mass. The more massive the person, the stronger the gravitational pull.
2. Air Resistance (Drag)
As the parachutist falls, air resistance pushes upward against them. The faster the parachutist falls, the stronger the air resistance becomes. Air resistance depends on speed, body position, and surface area.
How Terminal Velocity Occurs
When a parachutist first jumps out of a plane, gravity pulls them down, and they start accelerating. At this point, air resistance is small. But as the speed increases, air resistance grows larger. Eventually, air resistance becomes equal to the gravitational force. At this point, acceleration stops, and the parachutist continues to fall at a constant speed – this is terminal velocity.
The Formula for Terminal Velocity
Terminal velocity can be calculated with the formula:
Vt = â((2 à mass à gravity) / (air density à drag coefficient à cross-sectional area))
Where:
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Vt = terminal velocity
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mass = mass of the parachutist
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gravity = acceleration due to gravity (9.8 m/s²)
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air density = density of the air
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drag coefficient = a number that measures the shape’s resistance
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cross-sectional area = the surface area facing the airflow
Factors Affecting Terminal Velocity
1. Mass of the Parachutist
Heavier parachutists have higher terminal velocities. This is because they experience a stronger gravitational force, requiring more air resistance to balance out that force.
2. Surface Area
The larger the surface area, the more air resistance. When a parachutist spreads out their body or deploys a parachute, they increase their surface area, greatly reducing terminal velocity.
3. Body Position
A parachutist can control terminal velocity by adjusting body posture. Falling head-first reduces surface area and increases terminal velocity. Spreading arms and legs out increases air resistance and lowers terminal velocity.
4. Air Density
Air density also plays a role. Higher altitude has thinner air, reducing air resistance and increasing terminal velocity. Closer to the ground, air is denser, and terminal velocity decreases slightly.
What Happens When the Parachute Opens?
Before the parachute opens, the parachutist is falling at terminal velocity, which is around 190 km/h (120 mph) in the belly-to-earth position. When the parachute deploys, it dramatically increases the surface area and drag coefficient. This reduces terminal velocity to a much safer speed of around 15-30 km/h, allowing the parachutist to land gently.
Real-Life Example of Terminal Velocity
A skydiver jumps out of a plane from 4,000 meters. At first, gravity pulls them downward and speed builds up quickly. Within 10 to 15 seconds, air resistance becomes strong enough to balance gravity, and terminal velocity is reached. The skydiver then falls at a constant speed until the parachute is deployed. When the parachute opens, air resistance becomes massive, and the parachutist’s speed decreases to a new, much lower terminal velocity.
Why Terminal Velocity Is Important
1. Safety in Skydiving
Skydivers need to understand terminal velocity to know when to open their parachutes and what speed they will experience in freefall. Training helps them position their bodies to control their descent and reduce risk.
2. Designing Parachutes
Engineers use terminal velocity calculations to design parachutes that can slow descent to safe speeds. Different parachute designs cater to different body weights and activities, such as military jumps or sport skydiving.
3. Understanding Other Phenomena
Terminal velocity isn’t just for parachutists. It also applies to raindrops falling from clouds, leaves drifting from trees, and space capsules re-entering Earth’s atmosphere.
How to Control Terminal Velocity
1. Body Position Control
Skydivers learn to control their fall speed by adjusting body posture. Tucking arms and legs reduces drag and increases terminal velocity. Spreading out increases drag and reduces speed.
2. Parachute Deployment
Deploying the parachute is the most effective way to reduce terminal velocity. The canopy increases drag so much that the descent speed slows down dramatically.
3. Specialized Equipment
Some jumpers use special suits, like wingsuits, that add surface area and help control both speed and direction. These suits allow for longer flight times and lower terminal velocity.
Fun Facts About Terminal Velocity
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A human body falling head-first has a terminal velocity of approximately 210 km/h (130 mph).
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The fastest terminal velocity recorded in a skydive without a parachute (in a controlled environment) exceeded 1,300 km/h during Felix Baumgartner’s jump from the stratosphere.
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Raindrops also reach terminal velocity, which is around 9 meters per second (32 km/h). That’s why rain doesn’t hurt when it falls.
The Science Behind Balance of Forces
The reason a parachutist reaches terminal velocity is the balance between two forces: gravity pulling downward and air resistance pushing upward. As speed increases, air resistance rises until it matches gravity. At this moment, no net force remains, and acceleration stops. This state of balance is a classic example of Newton’s first law of motion, where an object continues at constant speed unless acted upon by a net external force.
Can Terminal Velocity Be Avoided?
In a freefall situation, terminal velocity cannot be avoided – it’s a natural outcome of gravity and air resistance. However, it can be controlled or modified. By changing body position or using parachutes and wingsuits, a skydiver can manipulate terminal velocity to ensure safety and control.
Terminal Velocity in Different Environments
1. Water
In water, terminal velocity is reached much more quickly and at a much lower speed due to the higher density of water compared to air. This is why diving into water requires careful technique to avoid injury.
2. Spacecraft Re-entry
Spacecraft experience terminal velocity when entering Earth’s atmosphere. Engineers design heat shields and control systems to manage extreme speeds and reduce the craft’s terminal velocity to safe landing speeds.
So, why does a parachutist reach terminal velocity? The answer is simple yet fascinating: it happens when the upward force of air resistance balances the downward pull of gravity, causing the parachutist to stop accelerating and fall at a constant speed. Factors such as mass, surface area, body position, and air density all influence this terminal velocity.
The science behind terminal velocity is not only important for skydiving but also for understanding how objects move through air and other fluids. Whether it’s designing parachutes, developing safe landing systems for spacecraft, or simply watching raindrops fall, the concept of terminal velocity is at play everywhere.
In essence, terminal velocity is the moment when nature finds balance – where speed, gravity, and air resistance meet in harmony, ensuring that parachutists can enjoy the thrill of freefall and still land safely on the ground.
