Terminal velocity
The steady top speed reached when drag grows until it equals the driving force, so the resultant force is zero.
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Analogy
Like running into stronger and stronger wind until you cannot go any faster.
Example
Falling: weight 800 N down.Drag grows as speed grows.At terminal velocity:drag 800 N = weight 800 Nresultant = 0, speed is steady
In practice
A skydiver stops speeding up at about 55 m/s. Opening a parachute gives a new, much lower terminal velocity.
Practice
Flashcards
- What is true about the forces at terminal velocity?They are balanced: drag equals weight, so the resultant is zero.
Interview questions
- Explain why a falling skydiver reaches a terminal velocity.
- Weight stays constant and pulls down
- Drag increases as speed increases
- When drag equals weight the resultant force is zero
- By Newton's first law, speed then stays constant
Questions people ask
Why does a falling skydiver stop accelerating?
As speed rises, air resistance rises too. Eventually the drag upwards equals the weight downwards. The resultant force is then zero, so by Newton's first law the skydiver falls at a steady speed. This steady speed is the terminal velocity.
Why does a parachute slow a skydiver down?
The open parachute has a huge area, so drag becomes much bigger than the weight. The resultant force points upwards and the skydiver slows. As speed drops, drag falls until it matches the weight again. The skydiver then falls at a new, much lower terminal velocity.