Short answers to common questions.
1) Is "centripetal force" a new kind of force?
No. It's a role played by an existing force (friction, rope, gravity).
2) Why can you take a banked turn without friction?
The surface's normal force has a horizontal component — providing the centripetal force. The road is banked so that at the design speed, N alone suffices.
3) At the top of a vertical loop, why don't I fall?
Because you're moving on a circle with centripetal acceleration. Gravity is part of the centripetal force — the rest from the normal force of the seat.
4) If my speed at the top of a loop is less than \sqrt{gr}, what happens?
You leave the track — the coaster is no longer in contact, becomes a projectile.
5) Is centrifugal force real?
Only in a rotating frame. From an inertial frame, it doesn't exist — centripetal explains everything.
6) Why do northern storms rotate counterclockwise?
Coriolis force. Wind heading toward a low-pressure center deflects to the right in the northern hemisphere — resulting in counterclockwise rotation.
7) Southern hemisphere storms rotate clockwise?
Yes. Coriolis is reversed in the southern hemisphere.
8) Does Coriolis cause water to swirl in a sink?
No. Way too weak at that scale. Sink swirl direction depends on the basin shape and water entry angle, not Coriolis.
9) What sets a skydiver's terminal velocity?
Mass, cross-sectional area, drag coefficient (shape), air density. "Free" ~50 m/s, with parachute ~6 m/s.
10) Why does a small bag fall slower than a ball despite having more mass?
If the bag has a larger area, its terminal velocity is lower. More mass, but more drag. At the very start, all fall at g — differences show at higher speeds.
11) Without friction, what happens on a horizontal curve?
You can't stay on the curve — you go straight. Centripetal force is needed; no friction, no force to provide it.
12) In non-uniform circular motion, how many components of acceleration are there?
Two: radial (toward center, v²/r) and tangential (along motion, dv/dt).
13) What's the most conclusive experiment for Earth's rotation?
Foucault's pendulum (1851). Its swing plane rotates with Earth — observable and unambiguous.
14) At terminal velocity, acceleration is zero — so no force?
Net force is zero. But two equal and opposite forces act: gravity and drag. Balance, not absence.
15) Where do nuclear forces show up in everyday physics?
They don't — too short-range. Only inside the nucleus.
16) So everyday physics is just gravity and electromagnetism?
Yes. All contact forces (friction, normal, spring) are electromagnetic at microscopic level.
17) Why is ice slippery?
\mu_s and \mu_k on ice are very small. Also, human pressure creates a thin water film that further reduces friction.
18) Why are ISS astronauts weightless?
Not because gravity is zero (it isn't). Because they're in free fall — both astronaut and station accelerate identically. Zero normal force from the station.
19) When do §2.4 kinematics fail?
When acceleration varies. Air drag, non-uniform circular motion, springs — all require differential equations.
20) What's new here vs Chapter 5?
Chapter 5 gave the three laws. Chapter 6 applied them to more complex scenarios: circular dynamics, non-inertial frames, velocity-dependent forces. Same laws, more realistic settings.
Chapter wrap-up
Three main ideas:
- Centripetal force is a role, not a type — always sourced from an existing force
- Non-inertial frames — fictitious forces to keep
F = ma - Air drag depends on velocity — terminal velocity is a beautiful consequence
Chapter 7 — work and energy: another way to analyze motion, one step removed from forces.
🎓 You've finished Chapter 6!
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