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Flip to reveal answersWhat is drag (fluid resistance)?
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All 11 Flashcards — Drag force & terminal velocity
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Question
What is drag (fluid resistance)?
Answer
A **resistive force** a fluid (air or liquid) exerts on an object moving through it. It points **against the motion** and **grows with speed**.
Question
What is terminal velocity?
Answer
The **steady (constant) speed** a falling object reaches when the **drag balances the weight**, so the net force — and the acceleration — is zero.
Question
What is 'viscosity'?
Answer
How **thick or sticky** a fluid is (symbol η, unit Pa s). Honey has high viscosity; water has low viscosity.
Question
Stokes' law for drag on a small sphere?
Answer
$F_d = 6\pi\eta r v$ — drag grows with viscosity η, radius r and speed v. **Given** in the data booklet.
Question
Force condition at terminal velocity?
Answer
**Weight = drag**: $mg = 6\pi\eta r v$ (net force zero, so steady speed).
Question
Acceleration just after release?
Answer
About **g** — there's no drag yet because the speed is zero.
Question
How does acceleration change as an object falls through air?
Answer
It **starts near g and decreases to zero** as drag builds up — it is **not** constant.
Question
What does the flat part of a v–t graph for a falling object show?
Answer
The **terminal velocity** — speed constant, acceleration zero, drag = weight.
Question
How does terminal velocity scale with radius (same material, same fluid)?
Answer
**v ∝ r²** — weight ∝ r³ and Stokes drag ∝ r, so doubling the radius gives **4×** the terminal velocity.
Question
Common drag/terminal-velocity trap?
Answer
Assuming the acceleration is **constant** while falling. It isn't — it falls from ≈ g to zero as drag grows.
Question
Why does an oil drop falling at constant speed have weight = drag?
Answer
Constant speed ⇒ no acceleration ⇒ net force = 0, so the upward drag exactly balances the downward weight.
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Forces and momentum
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