Friction, drag and terminal velocity
| English | Chinese | Pinyin |
|---|---|---|
| terminal velocity | 收尾速度 | shōu wěi sù dù |
| air resistance | 空气阻力 | kōng qì zǔ lì |
| friction | 摩擦力 | mó cā lì |
| drag | 阻力 | zǔ lì |
| viscous | 黏性 | nián xìng |
| fluid | 流体 | liú tǐ |
| thermal energy | 热能 | rè néng |
Why a skydiver stops speeding up
- Jump from a plane and you accelerate — but not forever.
- Soon you fall at a steady speed, the terminal velocity 收尾速度, even though gravity still pulls.
- The reason is air resistance 空气阻力 growing with speed.
Friction 摩擦力 and drag 阻力
- Friction acts between two solid surfaces, opposing the sliding.
- Drag (a viscous 黏性 force) is the resistance from a fluid 流体 — a liquid or gas.
Newton's third-law pair: R on the book (up) and R-prime on the table (down)
Stopping a car
Friction is what brakes a car. Set a speed and brake — the car keeps moving while the driver reacts, then friction slows it. Double the speed and watch the braking distance quadruple.
Which best describes a drag (viscous) force?
Drag is the resistance a fluid (liquid or gas) exerts on something moving through it. Friction is the solid-on-solid case.
Drag grows with speed
- At rest there is no drag.
- The faster you go, the bigger the drag force becomes.
Velocity-time graph for an object falling through air
Reach terminal velocity
Jump and watch the air-resistance arrow grow until it balances the weight — then the speed is constant. Open the parachute and the much bigger drag drops the diver to a slow, safe terminal velocity.
The drag force on a falling object grows as its speed grows.
Yes — at rest the drag is zero, and it increases with speed. That is what eventually balances the weight.
Falling to terminal velocity
- Start: only weight acts, so you accelerate at $g$.
- Middle: drag grows, the resultant force shrinks, so the acceleration falls.
- End: drag equals weight → zero resultant force → constant speed.
Put the stages of a fall through air in order, from the moment of release.
Drag starts at zero and builds with speed until it cancels the weight, ending the acceleration.
At terminal velocity, the drag force is equal to the ____.
Drag = weight, so the resultant force is zero and the speed stays constant.
Energy at terminal velocity
- The kinetic energy is now constant, but the object keeps losing height.
- That lost gravitational PE turns mainly into thermal energy 热能 of the air — not into extra speed.
Free-body diagram showing all forces on a block being pulled at an angle
As a parachutist falls at terminal velocity, the lost gravitational PE turns mainly into:
The kinetic energy is constant, so the falling PE cannot become KE — it heats the air (and the parachutist) through drag.
Cruising at constant speed
- A car at steady speed has zero resultant force: driving force = total resistance.
- Going faster means more drag, so more driving force — and more power.
A car cruising at a higher steady speed needs more power because the drag is larger.
Higher speed → larger drag → a larger driving force is needed → more power (power = force × velocity).
You've got it
- drag grows with speed; friction is between solids, drag is from a fluid
- terminal velocity: drag = weight, so zero resultant force and constant speed
- the lost gravitational PE becomes thermal energy of the air, not kinetic energy