Short answers to common questions.
1) Is the first law independent of the second?
Logically, the first law is a special case of the second (F = 0 → a = 0). But Newton stated them separately to define inertial frames.
2) If two people pull equally on a rope, is the force zero?
Net force is zero. Each person's force isn't. The object doesn't accelerate, but each person is doing work.
3) Why did Newton say inertial and gravitational mass are equal?
He didn't — it was assumed as an experimental fact. Einstein elevated it to a fundamental principle (equivalence) in general relativity.
4) If I drop a ball from a plane's window, what happens?
In the plane's frame: it falls straight down (assuming inertial frame). In Earth's frame: it traces a parabola — because it retains the plane's initial horizontal velocity.
5) How does a seatbelt save my life?
Without a belt, during hard braking your body keeps its initial forward velocity (inertia) — until it hits the windshield. With a belt, your body decelerates with the car — distributed force across the chest, not a concentrated impact.
6) Why does water leave the washing machine but clothes don't?
Water can pass through the drum holes (inertia, along its initial velocity). Clothes are too big — they press against the drum wall.
7) If four forces on an object sum to zero, what does the object do?
Equilibrium. Either at rest or at constant velocity. F_\text{net} = 0 does not necessarily mean at rest.
8) Third law — why don't both forces cancel?
Because they act on different objects. On one object they would cancel.
9) Then why does my hand hurt when I punch a wall?
Force by hand on wall = force by wall on hand (third law). But your hand is structurally weaker — internal stress breaks it.
10) Is centrifugal force real?
No — an illusion in a rotating frame. In an inertial frame, only centripetal force exists.
11) Is an ISS astronaut's weight truly zero?
No. W = mg still gives ~522 N with g ≈ 8.7 m/s². Weightlessness feeling comes from free-falling with the station.
12) What if friction didn't exist?
- Couldn't walk (feet slip)
- Cars wouldn't move on roads
- Structures would collapse (screws, bolts useless)
- Geology would differ (mountains collapse)
- But celestial motion would still work
13) Why is acceleration on a frictionless slope independent of mass?
From F = ma and F_\text{net} = mg\sin\theta: a = g\sin\theta. Mass cancels — because both force and inertia scale with mass.
14) What is normal force?
Surface force on an object resting on it — perpendicular to the surface. Never does work in parallel motion (perpendicular), but prevents falling.
15) Is rope tension always uniform?
In massless rope: yes, everywhere. In heavy rope: no — tension varies along its length (rope weight also acts).
16) What is a "frictionless" pulley?
Simplifying assumption: no mass, no friction. In reality, it reduces tension on both sides and spins with angular acceleration (Ch 10).
17) If a car cruises at constant speed on flat road, no net force. So why burn gas?
Gas produces engine force that cancels friction (air + rolling). Net force is zero — but individual forces are not.
18) What does the scale in a free-falling elevator read?
Zero. The person is weightless (no normal force from the scale). Briefly experienced in drop-towers.
19) Is F = ma valid at all speeds?
No — only far below c. Near light speed, effective mass grows (special relativity). Precise form: F = dp/dt where p = \gamma m v with \gamma = 1/\sqrt{1-v^2/c^2}.
20) What did this chapter add beyond Chapter 4?
Chapter 4 said "how position, velocity, acceleration relate". Chapter 5 said "why acceleration happens" — due to force. Kinematics → dynamics.
Chapter wrap-up
Three main ideas:
- First law — no force means constant velocity
- Second law —
\vec F = m\vec a, the bridge from force to motion - Third law — forces come in pairs, on different objects
Chapter 6 — advanced Newton's law applications: resistive forces, circular dynamics, non-inertial systems.
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