به انگلیسی

Falling against the wind

Science · Forces & Motion · ages 18-19

Name ______________________ Date ____________

Tilting your hand edge on to the flow reduces the drag you feel.
Circle one: True False
Which way does the drag force point?
- Opposite the motion
- Along with the motion
- Straight down always
For a fast cyclist or car, how does drag grow with speed?
- It stays the same at all speeds
- It shrinks as speed grows
- It grows with speed squared
A skydiver opens into spread position and the facing area grows. What happens to terminal speed?
- It drops to a lower value
- It climbs to a higher value
- It stays exactly the same
How do you find terminal speed from the equation of motion?
- Set the velocity itself to zero and solve
- Set the acceleration to zero and solve
- Delete the drag term and solve
What does the speed against time curve look like for a falling diver?
- It rises forever in a straight line
- It rises steeply, then flattens just below terminal speed
- It jumps straight to terminal speed at once
Dust settles in still air at a slow, steady pace. Which drag rule fits it?
- The tiny, slow rule where drag grows with speed itself
- The fast rule where drag grows with speed squared
- No drag rule, since dust has no weight
A student draws the speed curve crossing above the terminal line and coming back. What is wrong?
- Nothing, overshoot is normal for divers
- The curve can never cross the terminal value from below
- The curve should start flat and end steep

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Less area faces the flow, so the fluid pushes back less hard.
Opposite the motion · Drag always resists the motion through the fluid, so it points back against it.
It grows with speed squared · At high speed through air, drag follows half C rho A v squared.
It drops to a lower value · More area means drag matches weight sooner, so the steady speed is lower.
Set the acceleration to zero and solve · Steady speed means no acceleration, so weight equals drag at that point.
It rises steeply, then flattens just below terminal speed · Early on drag is small so speed climbs fast, then growing drag flattens the curve.
The tiny, slow rule where drag grows with speed itself · Small, slow drifters live in the linear regime, so they settle quickly to a gentle terminal pace.
The curve can never cross the terminal value from below · Past balance drag would beat weight and slow the fall, so crossing cannot happen.